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Ray Optics and Optical Instruments

Optical Instruments

How do see the objects around us? Yes, with our eye. It is a most important organ of the human body which enables us to view all things around us. An eye is one of the most important optical instrument. Let us study in detail about other optical instruments used by us.

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The Eye

The human eye is an optical instrument that enables us to view all the objects around us is a very complex organ. Let us study the structure of the human eye. The white protective membrane seen when looked into the eye directly is a Sclera. It is tuff, opaque and fibrous outer layer of the eyeball.

The circular part is the Iris. The color of the eye is determined by the color of the iris. The center transparent area of the iris is the Pupil. The iris works like the shutter of the camera. It absorbs most of the light falling on it and allows it to pass through the pupil.

The amount of light that enters the inner part of the eye depends on the size of the pupil. In bright light, the iris contracts the pupil to restrict the light, whereas in low light it widens the pupil to emit more light into the eye. The eyeball is spherical in shape. The retina of the eye is able to detect the light and its color because of the presence of senses known as rods and cones.

Light entering the human eye is first refracted by the cornea. The refracted light is then incident on an iris. The lens is just behind the iris and light after refracted through the pupil falls on it and forms a sharp image. Image formation exactly on the retina enables us to see the object clearly.

Defects in Human Eye

Like the ability to focus diminishes with the age of a person and this defect is Presbyopia. This defect is corrected by using converging lenses. The other defect is Hypermetropia. It is observed in the people of any age. The person suffering from this defect will have a normal vision while looking at the far object. But the vision is blurred for nearby objects. This is corrected by using a convex lens.

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The Microscope

As we all know Microscope is an optical instrument used to view small object. Let us first talk about the simple microscope.

A simple microscope is an optical instrument, we use for the magnification of small objects to get a clear image or vision. It is a convex lens having a short focal length. This microscope is at a small distance from the object for the magnification and hence this forms a virtual image. The simple microscope enables us to view very small letters and figures. Watchmakers also make use of these. Now let us see what compound microscope is.

With a compound microscope, we get very large values of magnification. We use this microscope to see microscopic objects like microorganisms. It comprises of two convex lenses and magnification occurs in both of these lenses.  the components of a compound microscope are eyepiece, objective lens, fine and rough adjustment screw.

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The telescope is of two types. One is the reflecting type and another one is the refracting type. Reflecting telescopes are the ones which do not use lenses at all. They use mirrors to focus the light together. The type of mirror used is a concave mirror.

Mirrors also bend the light together, except that they do it by reflecting the light instead of bending it. Refracting telescopes work by using two lenses to focus the light and make it look like the object is closer to you than it really is. Both the lenses are in a shape of ‘convex’. Convex lenses work by bending light inwards.

Question For You

Q1. In a compound microscope, the intermediate image is:

  1. Virtual, erect and magnified
  2. Real, erect and magnified
  3. Real, inverted and magnified
  4. Virtual, erect and reduced.

Answer: C. As shown in the figure, the intermediate image P’Q’ is real, inverted and magnified.

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