# Some Optical Poperties of Eyes

Our eyes have interesting optical properties. These can be demonstrated through following activities (games):

## Advantages of having two eyes

Having two eyes provides a sense of depth perception, known as stereoscopic vision. The brain combines the slightly different images from each eye to create a three-dimensional perception of the world. The following demonstration (game) clearly shows this.

Keep a glass of water on a table. Ask your friend to sit about 5 metre from the table. Hold a coin in your hand and move it slowly near the glass. Ask your friend to cover one eye and say when the coin is just above the glass. When s(he) say so, drop the coin. You will find that in most cases your friends judge the position of the coin wrongly, and the coin falls outside the glass. But if s(he) look through both the eyes, s(he) will be able to judge the position of the coin correctly most of the time, and the coin will drop into the glass. This happens because we are unable to judge depth, or the relative distance between objects, with one eye, especially when the objects are at some distance from the eye. Two eyes are required to perceive depth. Also, we can see a wider area with two eyes than with one eye.

## Blind spot of eyes

There are millions of photo sensors at the surface of the retina. These sensors are at the tips of nerves which take the voltage pulse generated to the brain. All these nerves leave the eyeball from a small area of retina say about $$2\,\mathrm{mm^2}$$. This particular part of the retina has no sensors in it and is called the blind spot of the eye. If the image of a small object falls on this part of the retina, the object will not be seen.

You need a paper (preferably ruled one), and a pen.

1. On a horizontal line of the paper, make two marks using the pen. Each mark may be of 2-3 mm size and the separation between the marks may be say 10 cm to start with. Write the distance in the table.
2. Keep the paper at a distance of about 40 cm from the eyes. Bring the left spot in front of the right eye. Close the left eye. Focus on the left spot.
3. Very slowly bring the paper closer to yourself. All the time keep looking at the left spot. At a certain distance from the eye, you will find that the right spot has become invisible.
4. Measure the distance of the paper from the eye and write in the table.
5. Repeat with two more separations between the spots, say 12 cm and 16 cm.
6. Close your right eye. Bring the left spot in front of your left eye. Try changing the distance of the paper from the eye. Are you able to find a position of the paper where the right spot becomes invisible? Explain.

When you look at a particular spot, the position and focal length of the eye-lens gets fixed. Now different objects create images at different portions of the retina. The object that gives image at the blind spot is not visible. If you are looking at the left spot with left eye, the right spot will make image on the retina, left to the center. That is on the side opposite to the nose or outer side. But if you are looking at the left spot using the right eye, the other spot makes image on the inner side. Using your observations, which case the spot becomes invisible, you can determine whether the blind spot is on the inner side or the outer side of the eye.

## Resolving power of eyes

Our eyes have a limited resolving power due to diffraction. This experiment illustrates this in a simple manner. Take a plain paper on which closely spaced rectangles are printed (see figure).

1. Hold the paper in your hands so that the rectangles are vertical. Ask your friend to stand at a distance of about 30 ft.
2. Ask him/her to tell how many black rectangles he/she can see. The expected answer is 2.
3. Ask the person to slowly come closer. At some distance the person will be able to recognize that there are 8 rectangles. Do it with several of your friends (including yourself) and write the minimum distances needed to resolve the rectangles.
4. Question: Does the diffraction takes place at the pupil or at the eye lens?

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