Lenses

Lenses

Lens: - 

A lens is a transparent medium bound by two surfaces.

There are mainly two types of lens. 1) Convex lens    2) Concave lens

Convex lens: - 

Convex  Lens

The lens which has two Spherical surfaces which are puffed up outwards is called a convex or double convex lens. This lens is thicker near the centre as compared to the edges.

Concave lens: -

Concave Lens

 The lens with both surfaces spherical on the inside is called a concave or double concave lens. This lens is thinner at the centre as compared to its edges.

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Human eye and working of its lens

Human Eye

There is a very thin transparent cover (membrane) on the human eye. This is called

Cornea. Light enters the eye through it. Maximum amount of incident light is refracted inside the eye at the outer surface of the cornea.

There is a dark, fleshy screen behind the cornea. This is called the Iris. The colour of the Iris is different for different people.

There is a small hole of changing diameter at the centre of the Iris which is called the pupil. The pupil controls the amount of light entering the eye. If the light falling on the eye is too bright, pupil contracts while if the light is dim, it extend.

On the surface of the iris, there is bulge of transparent layers. There is a double convex transparent crystalline lens, just behind the pupil. 

The lens provides small adjustments of the focal length to focus the image. This lens creates real and inverted image of an object on the screen inside the eye. This screen is made of light sensitive cells and is called the retina.

The capacity of the lens to change its focal length as per need is called its power of accommodation.



 Defects of Vision and their corrections

Some people can not see things clearly due to loss of accommodation power of the lenses in their eyes. Because of defective refraction by the lenses their vision becomes faint and fuzzy. In general, there are three types of refraction defects.

       1)  Near-sightedness ( Myopia )

       2)  Farsightedness or hypermetropia

       3)  Presbyopia           

1)   Near-sightedness (Myopia)

In this case, the eye can see nearby objects clearly but the distant objects appear indistinct.

In near-sightedness, the image of a distant object forms in front of the retina

There are two reasons for this defect.

          1)      The curvature of the cornea and the eye lens increases. The muscles near the lens cannot relax so that the converging power of the lens remains large.

         2)     The eyeball elongates so that the distance between the lens and the retina increases.

This defect can be corrected by using spectacles with concave lens of proper focal length.

This lens diverges the incident rays and these diverged rays can be converged by the lens in the eye to form the image on the retina.

The focal length of concave lens is negative, so a lens with negative power is required for correcting near-sightedness.

The power of the lens is different for different eyes depending on the magnitude of their near-sightedness.

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2) Farsightedness or hypermetropia

In this defect the human eye can see distant objects clearly but cannot see nearby objects distinctly.

This means that the near point of the eye is no longer at 25 cm but shifts farther away the images of nearby objects get formed behind the retina.

There are two reasons for farsightedness.

        1)      Curvature of the cornea and the eye lens decreases so that, the converging power of the lens becomes less.

        2)     Due to the flattening of the eye ball the distance between the lens and retina decreases.

This defect can be corrected by using a convex lens with proper focal length.

Convex lens converges the incident rays before they reach the lens. The lens then converges them to form the image on the retina.

The focal length of a convex lens is positive thus the spectacles used to correct farsightedness has positive power.

The power of these lenses is different depending on the extent of farsightedness.

      

    3)  Presbyopia

Generally, the focusing power of the eye lens decreases with age.

The muscles near the lens lose their ability to change the focal length of the lens.

The near point of the lens shifts farther from the eye. Because of this old people cannot see nearby objects clearly.

Sometimes people suffer from near-sightedness as well as farsightedness.

 In such a case bifocal lenses are required to correct the defect. In such lenses, the upper part is concave lens and corrects near-sightedness

 while the lower part is a convex lens which corrects the farsightedness.



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Use of concave lenses

      1)   Uses of concave lenses are Medical equipment’s, scanner, CD player – These instruments use laser light. For proper working of these equipment’s concave lenses are used.

      2)   The peep hole in door- This is a small safety device which helps us see a large area outside the door. This uses one or more concave lenses.

     3)   Spectacles- Concave lenses are used in spectacles to correct nearsightedness.

     4)   Torch- Concave lens is used to spread widely the light produced by a small bulb inside a torch.

     5)   Camera, telescope and microscope- These instruments mainly use convex lenses. To get good quality images a concave lens is used in front of the eyepiece or inside it.

Use of convex lenses

. Convex lenses are used in Simple microscope, Compound microscope and Telescope.

Optical instrument

Convex lenses are used in various other optical instruments like camera, projector, spectrograph etc.

Spectacles

Convex lenses are used in spectacles for correcting farsightness.

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