REFRACTION :
Put a pencil in a jam jar of water and look at it carefully .From some angles the pencil will bent . If you have a pencil torch which produces a narrow beam of light , try shinning it into a jar . In a darkened room you can see that the light bends when it passes from air into water .The bending of light ,which occurs in both these experiments, is called refraction.
Light travels at different speeds in different transparent materials. Because of this, when light passes from one material to another its direction changes slightly.Refraction causes the shimmering effect just above the ground on a hot day. Light travels through the hot air rising from the ground faster than it travels through the cooler air above, and so a heat haze is produced.You can produce the same effect at home by shining a powerful torch beam at a plain wall just above a candle flame. Dancing patterns and shadows will appear on the wall behind the candle.
Try dissolving about one teaspoonful of sugar in a very small amount of hot water.Pour this solution into water in a jam jar, you will see a swirling effect as the sugar solution sinks to the bottom . This is because light travels more slowly through the sugar solution than through pure water.
Light travels more slowly through denser materials.Glass is denser than air .If light travels from air to glass it is bent or refracted towards the normal. The normal is a imaginary line perpendicular to the refracting surface .Light passing from glass to air is refracted away from the normal .The light always bends by a definite amount .This amount depends on the angle of incidence (The angle at which the light strikes the surface of the material), and on the material itself.
REFRACTIVE INDEX:
The refractive index of the material tells us how slowly light will travel through the material and hence how much it will be bent.The refractive index equals the speed of light in the material .For glass it equals to 1.5, for water it is 1.3. The denser the material,The higher the refractive index.
Drop a coin into jam jar of water and look into it from above .Put a chalk mark on the jar where you think the coin is much lower down than you had expected .This too is a result of refraction.Swimming pools and rivers may appear very shallow when you look in, but may really much deeper .
A coin in a glass of water will appear much nearer to the surface than it really is , due to the refraction of light ray from the coin to your eye .The eye sees the image as if the ray has traveled straight to it.The real depth divided by apparent depth gives the refractive index of water.
Prisms of water can split light into the colors of the spectrum .This is because the different colors in white light are refracted by the glass or water by different amounts. When white light passes from air into water or glass , violet light is refracted most. and red light least.
In a rainbow , the sunlight is refracted when it goes into the droplets of water .It is reflected inside them and refracted again as it passe out. The color seen by an observer will depend on the angle at which he or she looks at each droplet
3D Angler Animation
Due to refraction, The fish can get a good view of the river bank and the angler. The angler can see the fish, but it appears to be much further from the bank than if actually is - fortunately for the fish.
LENSES:
A lens is a piece of glass or other transparent solid .It is cut so that it has at least one surface shaped like part a large sphere. Lenses are used to spread out (diverge) light, or bring light closer together (converge light). They do this by refraction.When light passes from air into glass it is bent towards the normal (imaginary line perpendicular to the glass surface). When the light emerges it is bent away from the normal.
CONVEX AND CONCAVE LENSES:
The path of the light rays through a lens depends on the shape of the lens. The two main kind of lenses are convex lenses,which bulge out on each side , and concave lenses which cave in towards the center.
Convex lenses make parallel light rays falling on them converge to a single point called the focus of the lens.Concave lenses make parallel light rays spread out or diverge as though they came from a point called the virtual focus. In each case the distance between the focus and the center of the lens is called the focal length of the lens
A convex lens makes a good magnifying glass, if an object is placed between the lens and its focus. From the other side of the lens , the object appears larger , and further from the lens than it really is. if the object is beyond the focus , you will not see an image at all .However you can form a real inverted (upside down) image by holding a piece of paper on the opposite side of the lens to the object , specially if the object is as bright as a light bulb.For a concave lens ,It does not matter where the object is.The image is always on the same side of the lens as the object,and is smaller than the object.
3D Animation for Biconvex and Biconcave Lens
FORMULA OF BICONVEX LENS:
Here PQ is the object. From point Q a ray QA come parallel with the main axis and after refraction with biconvex lens
it bent and go through line F .Then another ray QO after refraction go toward line F. In case of biconvex lens both rays are joined together on point Q1 , so here Q1 is the reflection of Q. A perpendicular line P1Q1 over the main axis from point Q1 is the reflection of PQ.
so :- P1 Q1 O similar with P Q O
P1Q1 OP1
--------- = ----------
P Q OP
so :- P1Q1 F similar with OAF
P1Q1 P1F
---------- = -----------
OA OF
PQ = OA
OP1 P1F
-------- = --------
OP OF
OP = U and OP1 = V and OF = f
P1F = OP1 - OF = (-V) - (-f) = F-V
V f - V
- ---- = ---------- or Uf - UV = Vf
U -f
1 1 1
---- - ---- = ------
V U f
This formula is also found in case of biconcave lens
A normal eye lens focuses light on the retina.If a person is short sighted, light is focused in front of the retina. Concave lenses in spectacles diverge light so that the eye lens can focus it on the retina.If a person is long sighted, light is focused beyond the retina.Convex lenses in spectacles help the eye to focus the light on the retina.
OPTICAL INSTRUMENTS:
In order to see small object clearly ,we bring it closer to the eye .This gives a wider angle of view of the object and increases it apparent size ,However somebody with normal vision can bring the object no nearer than 25 cm from the eye .Closer than that , the object looks blurred.
To increase the size of tiny or distant object , without straining the eye , a microscope or telescope must be used .These are optical instruments which use lenses or mirrors to help us see small or distant objects more clearly .The mirror is the oldest optical instrument.Mirrors have been used for over 2000 years .A mirror changes the direction of beam of light by reflecting the light from a shiny metal surface .
MAGNIFICATION:
The lens is the main device used in optical instruments.It changes the direction of a light beam by bending or reflecting rays as they pass through it .When lenses are in certain position they can produce a larger or magnified image of the object .The magnification of an optical instrument is equal to the image size divided by the object size.A magnifying glass , consisting of single convex lens, forms an image several times larger than the object .
The magnification of an instrument can be greatly increased by using more than one lens .Both the simple telescope and the microscope use two convex lenses separated by a set of distance , enabling them to form a greatly enlarged image of the object .Other lenses are used in the instrument to improve the quality of the image.
The camera is different type of optical instrument .It produces a permanent image of an object or scene by focusing light from the object on to a photographic film. One or more lenses are used to for this and the lens system can be extremely
complicated.
POLARIZED LIGHT:
Some optical instruments contain an important substance called a polarized .Light normally vibrates in all directions perpendicular to the direction of the travel .It is said to be un polarized .Polarized light contains vibration in only one direction . A polarizer transmits polarized light. This cut down glare from scattered or reflected light. Polarizers are used in sunglasses .Filters from cameras and many other optical instruments.
A Single-lens reflex camera. One lens system is used both for viewing and focusing .The photographer views the subject through the viewfinder.This is linked optically to the lens system by a penta prism and a mirror.The mirror is hinged so that when the shutter is opened to expose the film, the mirror lifts up.
MICROSCOPES:
Some objects are so small that they cannot be seen clearly by eye. A microscope is used to form an enlarged picture or image of the object .The magnification of a microscope is the size of the image divided by that of the object .If the magnification is * 100.The image is 100 times larger than the object.
An optical microscope uses glass lenses to produce the enlarged image .The simplest type is a magnifying glass .This is a converging or convex lens which bulges out in the center It is held a short distance above the object and moved towards the eye until the image can be clearly seen .The image is now in focus.A microscope used in scientific laboratory is more complicated.It contains a number of different lenses. Only two are used in actual magnification.The tiny object or specimen is placed on a thin glass plate and strongly lit from below .The convex lens just above the object is called the objective .It produce a magnified image in the same way as the simple magnified glass .The image is formed between the objective and second convex lens ,called the eyepiece.The eyepiece magnifies the first image even more.The total magnification of the instrument is the magnification of the objective multiplied by that of the eyepiece.The final image can be over 1000 times the size of the object .Tiny details of objects such as the part of the plants, creatures in pond water, or hairs can be seen.
LIMIT OF MAGNIFICATION:
It is not possible to go on increasing the magnification of an instrument simply by improving its optical system .This is because there is a limit of magnification set by laws of physics.To view an object under a light microscope it must be illuminated by a beam of light.The magnification limit is determined by the wavelength of the light .Since the wavelength is a fixed quantity , the maximum magnification of a light microscope or telescope is also fixed ,So to view very tiny objects, an electron microscope is used .This uses a beam of tiny particles called electrons ,Instead of a beam of light.
A compound microscope with a multiple nos eyepiece.Three different objective lenses can be interchanged by swinging the nose piece round.This provides different magnifications. The cutaway objective lens system shown on the right gives a magnification of * 100 . The cutaway Huygenian eyepiece shown above it is composed of two lenses. Eyepieces usually have a magnification of about *5 or *10 the total magnification of a compound microscope is found by multiplying together the magnifications of the eyepiece and the objective.
REFRACTING TELESCOPES:
Telescopes are optical instruments used for observing distant objects. It is believed that the first telescope was made by Hans Lippershey in Holland. in 1608.He was a spectacle maker .Whilst examining a pair of lenses one behind the other, he accidentally discovered that objects appeared closer.By 1610, an Italian named Galileo had made a much improved telescope called Galilean telescope.It had a magnification of 33 times .Many improvements by other scientists and astronomers gradually followed.
Telescopes that use lenses to collect light are called refracting telescopes. The light is bent by refraction through the lenses. Galileo used his telescope for making astronomical observations. He was able to discover the moons of the planet Jupiter.He also observed that there are hill and valleys on the moon .Galileo`s telescope had two lenses .There was a convex lens at the front called the object lens or objective.The other lens is called the eye lens or eyepiece.This was a concave lens in Galileo`s telescope .With this lens arrangement the image and object are the same way up.
Later telescopes are used two convex lenses. The object lens brings the light to a focus between the two lenses.Another convex lens, the eyepiece , magnifies this image to produce a larger image seen by the eye. The image formed by this type of telescope is upside down or inverted.This does Not really matter in astronomy and these telescopes were used for many inventions of planets and stars.
ASTRONOMICAL TELESCOPE:
Refracting telescope are often called astronomical telescopes.The largest one in existence is the Yerkes telescope in Wisconsin, USA.It is over 18 meter long and the objective lens has a diameter of 1 meter (40 inches).
The early refracting telescopes had the disadvantage that their images had colored fringes around them.This is because a lens focuses light of different colors at slightly different positions, so it breaks up the different color that make up white light into colored fringes .This is called chromatic aberration.It is possible to make lenses that do not have this defect .They are used in all modern refracting telescopes, and are called achromatic lenses.
TERRESTRIAL TELESCOPES:
Telescopes with two convex lenses are unsuitable for land viewing because they give an inverted image. If a third convex lens is placed in the tube of the telescope the final image is upright .It can then be used for land viewing and is known as a terrestrial telescope. Galileo`s telescope was used for astronomical work .It can also be a terrestrial telescope because it gives an upright image.
Lippershey , who invented the telescope is also said to have invented the first binocular telescope in 1608. Unfortunately the idea of using an instrument with both eye caused little interest.
In 1823 the binocular telescope was reinvented by another Dutchman called J.Voigtlaender.Opera glasses and inexpensive field glasses consist of two telescopes of the Galilean type.Their magnification is only small .More powerful binoculars use convex lenses and eyepieces .Prisms turn the image the right way up
In a Galilean telescope the eyepiece is a concave lens. The image is erect. A convex lens is used for the eyepiece of astronomical telescopes. They produce an image that is upside down.
Binoculars are made up of two telescopes side by side .As you can see , the light rays are bent around inside by prisms.This means that the binoculars do not have to be very long. The prisms also turn the image the right way up.
3D Animation of Binocular
REFLECTING TELESCOPE:
The early refracting telescopes did not give clear , sharp images .This was because it was difficult to make good lenses .In 1668 . Sir Isaac Newton invented reflecting telescope. This telescope had no object lens at all .the light came down an open tube to fall on to a curved mirror placed at an angle. This directed the rays to the side of the tube on to a second flat mirror at the bottom .The mirror reflected the light back up the tube on to second flat mirror placed at an angle .This directed the rays to the side of the tube on to the magnifying convex lens of the eyepiece. Newton`s telescope produce clearer image free from chromatic aberration.
The world`s largest telescope s are of the reflecting type because mirrors can give very sharp images free from color fringes, also , it is much easier to make large mirrors than large lenses .
Most of the objects studied by astronomers are extremely distant and faint. They can be seen only in telescopes which collect a lot of light. Large modern reflecting telescopes gather enormous amounts of light and reveal fine detail .Large numbers of very faint ,very distant stars can be seen .
Cameras are often fitted to these telescopes. With long exposures ,They can gather a tremendous amount of extra information about stars and galaxies.No telescope yet built is able to show a star as anything more than a dot of light. Telescopes , however , can still tell us much about the stars. They can show the different star colous.Double and quadruple stars (groups of two or four stars close together) can be seen.
One of the largest telescopes is on Mount Palomer in California.It is housed in this observatory.The dome weighs 1000 tonnes.It has a shutter through which the telescope point at the sky.The dome can be revolved so that the telescope can point in different direction.
PHOTOGRAPHING GALAXIES:
Galaxies or star systems have thousands of millions of stars in them . Outside our own galaxy there are over 1000 million other separate galaxies .Only three can be seen with the naked eye.
Many galaxies have been photographed using telescopes.Astronomer can only learn a limited amount about the stars with the visual telescopes.Other instruments have been developed which can be tell us much more about the make up of the stars. One of these instruments is the spectroscope.
A large reflecting telescope is found in nearly every major observatory . Throughout the world there are over 300 observatories.They are built far from the bright lights of cities.Often in areas which are usually fine and clear.
One of the largest reflecting telescopes in the world is at Mount Palomer Observatory , California USA.There the climate and atmosphere enable the telescope to be used , on average , 300 days every year .The concave mirror is made of glass , coated with a thin layer of Aluminium to give it high reflecting power .It is 200 inches in diameter and the telescope can be used in Newtonian or Cassegrain form.
The Cassegrainian telescope. The light is first collected with the large concave mirror. It is then reflected back through a hole by a small convex mirror.
PHOTOGRAPHY:
A Single-lens reflex camera. One lens system is used both for viewing and focusing .The photographer views the subject through the viewfinder.This is linked optically to the lens system by a penta prism and a mirror.The mirror is hinged so that when the shutter is opened to expose the film, the mirror lifts up.
Film development using a light proof tank, in total darkness the film is fed into the spiral groove of the reel by turning the top. The reel is put inside the tank the chemicals are poured in and out.first developer, then water and then fixer. After washing in water for about 45 minutes, the film is taken out and dried.
An enlarger. This instrument contains a glass convex lens acting as a magnifying glass. The negative is placed above this lens and illuminated by a flash of light from above. An enlarged image of the negative is formed on the photographic paper placed beneath it.
The negative and final photo of a subject taken with black-and white or color film.
 |
| The angler can see the fish, but it appears to be much further from the bank than if actually is - fortunately for the fish. |
Put a pencil in a jam jar of water and look at it carefully .From some angles the pencil will bent . If you have a pencil torch which produces a narrow beam of light , try shinning it into a jar . In a darkened room you can see that the light bends when it passes from air into water .The bending of light ,which occurs in both these experiments, is called refraction.
Light travels at different speeds in different transparent materials. Because of this, when light passes from one material to another its direction changes slightly.Refraction causes the shimmering effect just above the ground on a hot day. Light travels through the hot air rising from the ground faster than it travels through the cooler air above, and so a heat haze is produced.You can produce the same effect at home by shining a powerful torch beam at a plain wall just above a candle flame. Dancing patterns and shadows will appear on the wall behind the candle.
Try dissolving about one teaspoonful of sugar in a very small amount of hot water.Pour this solution into water in a jam jar, you will see a swirling effect as the sugar solution sinks to the bottom . This is because light travels more slowly through the sugar solution than through pure water.
Light travels more slowly through denser materials.Glass is denser than air .If light travels from air to glass it is bent or refracted towards the normal. The normal is a imaginary line perpendicular to the refracting surface .Light passing from glass to air is refracted away from the normal .The light always bends by a definite amount .This amount depends on the angle of incidence (The angle at which the light strikes the surface of the material), and on the material itself.
REFRACTIVE INDEX:
The refractive index of the material tells us how slowly light will travel through the material and hence how much it will be bent.The refractive index equals the speed of light in the material .For glass it equals to 1.5, for water it is 1.3. The denser the material,The higher the refractive index.
 |
| Drop a coin into jam jar of water and look into it from above .Put a chalk mark on the jar where you think the coin is much lower down than you had expected |
Drop a coin into jam jar of water and look into it from above .Put a chalk mark on the jar where you think the coin is much lower down than you had expected .This too is a result of refraction.Swimming pools and rivers may appear very shallow when you look in, but may really much deeper .
A coin in a glass of water will appear much nearer to the surface than it really is , due to the refraction of light ray from the coin to your eye .The eye sees the image as if the ray has traveled straight to it.The real depth divided by apparent depth gives the refractive index of water.
Prisms of water can split light into the colors of the spectrum .This is because the different colors in white light are refracted by the glass or water by different amounts. When white light passes from air into water or glass , violet light is refracted most. and red light least.
In a rainbow , the sunlight is refracted when it goes into the droplets of water .It is reflected inside them and refracted again as it passe out. The color seen by an observer will depend on the angle at which he or she looks at each droplet
Due to refraction, The fish can get a good view of the river bank and the angler. The angler can see the fish, but it appears to be much further from the bank than if actually is - fortunately for the fish.
 |
| Position of Fish Respective to Angler and Position of Angler Respect to Fish |
|
| Position Of Angler Respective of Fish |
 |
| Position Of Fish Respective ofAngler |
LENSES:
A lens is a piece of glass or other transparent solid .It is cut so that it has at least one surface shaped like part a large sphere. Lenses are used to spread out (diverge) light, or bring light closer together (converge light). They do this by refraction.When light passes from air into glass it is bent towards the normal (imaginary line perpendicular to the glass surface). When the light emerges it is bent away from the normal.
CONVEX AND CONCAVE LENSES:
The path of the light rays through a lens depends on the shape of the lens. The two main kind of lenses are convex lenses,which bulge out on each side , and concave lenses which cave in towards the center.
 |
| Biconvex Lense |
Convex lenses make parallel light rays falling on them converge to a single point called the focus of the lens.Concave lenses make parallel light rays spread out or diverge as though they came from a point called the virtual focus. In each case the distance between the focus and the center of the lens is called the focal length of the lens
 |
| Biconcave Lense |
A convex lens makes a good magnifying glass, if an object is placed between the lens and its focus. From the other side of the lens , the object appears larger , and further from the lens than it really is. if the object is beyond the focus , you will not see an image at all .However you can form a real inverted (upside down) image by holding a piece of paper on the opposite side of the lens to the object , specially if the object is as bright as a light bulb.For a concave lens ,It does not matter where the object is.The image is always on the same side of the lens as the object,and is smaller than the object.
FORMULA OF BICONVEX LENS:
Here PQ is the object. From point Q a ray QA come parallel with the main axis and after refraction with biconvex lens
it bent and go through line F .Then another ray QO after refraction go toward line F. In case of biconvex lens both rays are joined together on point Q1 , so here Q1 is the reflection of Q. A perpendicular line P1Q1 over the main axis from point Q1 is the reflection of PQ.
so :- P1 Q1 O similar with P Q O
P1Q1 OP1
--------- = ----------
P Q OP
so :- P1Q1 F similar with OAF
P1Q1 P1F
---------- = -----------
OA OF
PQ = OA
OP1 P1F
-------- = --------
OP OF
OP = U and OP1 = V and OF = f
P1F = OP1 - OF = (-V) - (-f) = F-V
V f - V
- ---- = ---------- or Uf - UV = Vf
U -f
1 1 1
---- - ---- = ------
V U f
 |
| Formula in Biconvex Lense |
This formula is also found in case of biconcave lens
 |
| Formula of Biconcave Lense |
A normal eye lens focuses light on the retina.If a person is short sighted, light is focused in front of the retina. Concave lenses in spectacles diverge light so that the eye lens can focus it on the retina.If a person is long sighted, light is focused beyond the retina.Convex lenses in spectacles help the eye to focus the light on the retina.
 |
| If a person is short sighted, light is focused in front of the retina. |
 |
| Concave lenses in spectacles diverge light so that the eye lens can focus it on the retina |
 |
| If a person is long sighted, light is focused beyond the retina. |
 |
| Convex lenses in spectacles help the eye to focus the light on the retina. |
OPTICAL INSTRUMENTS:
In order to see small object clearly ,we bring it closer to the eye .This gives a wider angle of view of the object and increases it apparent size ,However somebody with normal vision can bring the object no nearer than 25 cm from the eye .Closer than that , the object looks blurred.
To increase the size of tiny or distant object , without straining the eye , a microscope or telescope must be used .These are optical instruments which use lenses or mirrors to help us see small or distant objects more clearly .The mirror is the oldest optical instrument.Mirrors have been used for over 2000 years .A mirror changes the direction of beam of light by reflecting the light from a shiny metal surface .
MAGNIFICATION:
The lens is the main device used in optical instruments.It changes the direction of a light beam by bending or reflecting rays as they pass through it .When lenses are in certain position they can produce a larger or magnified image of the object .The magnification of an optical instrument is equal to the image size divided by the object size.A magnifying glass , consisting of single convex lens, forms an image several times larger than the object .
The magnification of an instrument can be greatly increased by using more than one lens .Both the simple telescope and the microscope use two convex lenses separated by a set of distance , enabling them to form a greatly enlarged image of the object .Other lenses are used in the instrument to improve the quality of the image.
The camera is different type of optical instrument .It produces a permanent image of an object or scene by focusing light from the object on to a photographic film. One or more lenses are used to for this and the lens system can be extremely
complicated.
POLARIZED LIGHT:
Some optical instruments contain an important substance called a polarized .Light normally vibrates in all directions perpendicular to the direction of the travel .It is said to be un polarized .Polarized light contains vibration in only one direction . A polarizer transmits polarized light. This cut down glare from scattered or reflected light. Polarizers are used in sunglasses .Filters from cameras and many other optical instruments.
A Single-lens reflex camera. One lens system is used both for viewing and focusing .The photographer views the subject through the viewfinder.This is linked optically to the lens system by a penta prism and a mirror.The mirror is hinged so that when the shutter is opened to expose the film, the mirror lifts up.
MICROSCOPES:
 |
| 3D Microscope Picture |
Some objects are so small that they cannot be seen clearly by eye. A microscope is used to form an enlarged picture or image of the object .The magnification of a microscope is the size of the image divided by that of the object .If the magnification is * 100.The image is 100 times larger than the object.
 |
| Different Parts of Compound Micrscope |
An optical microscope uses glass lenses to produce the enlarged image .The simplest type is a magnifying glass .This is a converging or convex lens which bulges out in the center It is held a short distance above the object and moved towards the eye until the image can be clearly seen .The image is now in focus.A microscope used in scientific laboratory is more complicated.It contains a number of different lenses. Only two are used in actual magnification.The tiny object or specimen is placed on a thin glass plate and strongly lit from below .The convex lens just above the object is called the objective .It produce a magnified image in the same way as the simple magnified glass .The image is formed between the objective and second convex lens ,called the eyepiece.The eyepiece magnifies the first image even more.The total magnification of the instrument is the magnification of the objective multiplied by that of the eyepiece.The final image can be over 1000 times the size of the object .Tiny details of objects such as the part of the plants, creatures in pond water, or hairs can be seen.
 |
| Path of Light Within Compound Microscope |
LIMIT OF MAGNIFICATION:
It is not possible to go on increasing the magnification of an instrument simply by improving its optical system .This is because there is a limit of magnification set by laws of physics.To view an object under a light microscope it must be illuminated by a beam of light.The magnification limit is determined by the wavelength of the light .Since the wavelength is a fixed quantity , the maximum magnification of a light microscope or telescope is also fixed ,So to view very tiny objects, an electron microscope is used .This uses a beam of tiny particles called electrons ,Instead of a beam of light.
A compound microscope with a multiple nos eyepiece.Three different objective lenses can be interchanged by swinging the nose piece round.This provides different magnifications. The cutaway objective lens system shown on the right gives a magnification of * 100 . The cutaway Huygenian eyepiece shown above it is composed of two lenses. Eyepieces usually have a magnification of about *5 or *10 the total magnification of a compound microscope is found by multiplying together the magnifications of the eyepiece and the objective.
REFRACTING TELESCOPES:
Telescopes are optical instruments used for observing distant objects. It is believed that the first telescope was made by Hans Lippershey in Holland. in 1608.He was a spectacle maker .Whilst examining a pair of lenses one behind the other, he accidentally discovered that objects appeared closer.By 1610, an Italian named Galileo had made a much improved telescope called Galilean telescope.It had a magnification of 33 times .Many improvements by other scientists and astronomers gradually followed.
Telescopes that use lenses to collect light are called refracting telescopes. The light is bent by refraction through the lenses. Galileo used his telescope for making astronomical observations. He was able to discover the moons of the planet Jupiter.He also observed that there are hill and valleys on the moon .Galileo`s telescope had two lenses .There was a convex lens at the front called the object lens or objective.The other lens is called the eye lens or eyepiece.This was a concave lens in Galileo`s telescope .With this lens arrangement the image and object are the same way up.
 |
| Telescopes that use lenses to collect light are called refracting telescopes |
Later telescopes are used two convex lenses. The object lens brings the light to a focus between the two lenses.Another convex lens, the eyepiece , magnifies this image to produce a larger image seen by the eye. The image formed by this type of telescope is upside down or inverted.This does Not really matter in astronomy and these telescopes were used for many inventions of planets and stars.
ASTRONOMICAL TELESCOPE:
Refracting telescope are often called astronomical telescopes.The largest one in existence is the Yerkes telescope in Wisconsin, USA.It is over 18 meter long and the objective lens has a diameter of 1 meter (40 inches).
The early refracting telescopes had the disadvantage that their images had colored fringes around them.This is because a lens focuses light of different colors at slightly different positions, so it breaks up the different color that make up white light into colored fringes .This is called chromatic aberration.It is possible to make lenses that do not have this defect .They are used in all modern refracting telescopes, and are called achromatic lenses.
TERRESTRIAL TELESCOPES:
Telescopes with two convex lenses are unsuitable for land viewing because they give an inverted image. If a third convex lens is placed in the tube of the telescope the final image is upright .It can then be used for land viewing and is known as a terrestrial telescope. Galileo`s telescope was used for astronomical work .It can also be a terrestrial telescope because it gives an upright image.
Lippershey , who invented the telescope is also said to have invented the first binocular telescope in 1608. Unfortunately the idea of using an instrument with both eye caused little interest.
In 1823 the binocular telescope was reinvented by another Dutchman called J.Voigtlaender.Opera glasses and inexpensive field glasses consist of two telescopes of the Galilean type.Their magnification is only small .More powerful binoculars use convex lenses and eyepieces .Prisms turn the image the right way up
In a Galilean telescope the eyepiece is a concave lens. The image is erect. A convex lens is used for the eyepiece of astronomical telescopes. They produce an image that is upside down.
 |
| Binoculars are made up of two telescopes side by side 4 Prisms 2 Big and 2 Small lenses in Binocular |
 |
| Through Binocular Tree`s Reflection much closer |
Binoculars are made up of two telescopes side by side .As you can see , the light rays are bent around inside by prisms.This means that the binoculars do not have to be very long. The prisms also turn the image the right way up.
REFLECTING TELESCOPE:
The early refracting telescopes did not give clear , sharp images .This was because it was difficult to make good lenses .In 1668 . Sir Isaac Newton invented reflecting telescope. This telescope had no object lens at all .the light came down an open tube to fall on to a curved mirror placed at an angle. This directed the rays to the side of the tube on to a second flat mirror at the bottom .The mirror reflected the light back up the tube on to second flat mirror placed at an angle .This directed the rays to the side of the tube on to the magnifying convex lens of the eyepiece. Newton`s telescope produce clearer image free from chromatic aberration.
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| The Cassegrainian telescope |
The world`s largest telescope s are of the reflecting type because mirrors can give very sharp images free from color fringes, also , it is much easier to make large mirrors than large lenses .
Most of the objects studied by astronomers are extremely distant and faint. They can be seen only in telescopes which collect a lot of light. Large modern reflecting telescopes gather enormous amounts of light and reveal fine detail .Large numbers of very faint ,very distant stars can be seen .
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| Hooker Telescope |
Cameras are often fitted to these telescopes. With long exposures ,They can gather a tremendous amount of extra information about stars and galaxies.No telescope yet built is able to show a star as anything more than a dot of light. Telescopes , however , can still tell us much about the stars. They can show the different star colous.Double and quadruple stars (groups of two or four stars close together) can be seen.
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| Otto Struve Telescope |
One of the largest telescopes is on Mount Palomer in California.It is housed in this observatory.The dome weighs 1000 tonnes.It has a shutter through which the telescope point at the sky.The dome can be revolved so that the telescope can point in different direction.
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| Palomar Observatory and Hale Telescope dome |
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| Hubble Telescope |
PHOTOGRAPHING GALAXIES:
Galaxies or star systems have thousands of millions of stars in them . Outside our own galaxy there are over 1000 million other separate galaxies .Only three can be seen with the naked eye.
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| Nasa Picture of the_andromeda_galaxy,_m31,_spyral_galaxy |
Many galaxies have been photographed using telescopes.Astronomer can only learn a limited amount about the stars with the visual telescopes.Other instruments have been developed which can be tell us much more about the make up of the stars. One of these instruments is the spectroscope.
A large reflecting telescope is found in nearly every major observatory . Throughout the world there are over 300 observatories.They are built far from the bright lights of cities.Often in areas which are usually fine and clear.
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| The Triangulum Galaxy |
One of the largest reflecting telescopes in the world is at Mount Palomer Observatory , California USA.There the climate and atmosphere enable the telescope to be used , on average , 300 days every year .The concave mirror is made of glass , coated with a thin layer of Aluminium to give it high reflecting power .It is 200 inches in diameter and the telescope can be used in Newtonian or Cassegrain form.
The Cassegrainian telescope. The light is first collected with the large concave mirror. It is then reflected back through a hole by a small convex mirror.
PHOTOGRAPHY:
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| Still-Camera |
A Single-lens reflex camera. One lens system is used both for viewing and focusing .The photographer views the subject through the viewfinder.This is linked optically to the lens system by a penta prism and a mirror.The mirror is hinged so that when the shutter is opened to expose the film, the mirror lifts up.
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| Different Lenses and Pentaprism Inside The camera |
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| Path of Light Go through Camera Lenses |
Film development using a light proof tank, in total darkness the film is fed into the spiral groove of the reel by turning the top. The reel is put inside the tank the chemicals are poured in and out.first developer, then water and then fixer. After washing in water for about 45 minutes, the film is taken out and dried.
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| Film development on Photographic Paper |
An enlarger. This instrument contains a glass convex lens acting as a magnifying glass. The negative is placed above this lens and illuminated by a flash of light from above. An enlarged image of the negative is formed on the photographic paper placed beneath it.
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| Film development |
The negative and final photo of a subject taken with black-and white or color film.
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Really informative knowledge about optics, impressive.
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