In activity-1, you learned that least distance for distinct vision is about 25 cm. It varies from person to person. If you are not able to see both end of the stick at this distance (25 cm), adjust the vertical stick on the horizontal rod till you are able to see both ends of the stick at the smallest possible distance from your eye. Fix the vertical stick at this position with the help of the clamp.

Without changing the position of the clamp on the horizontal rod, replace this stick of 30 cm length with other sticks of various lengths one by one and try to see the top and bottom of the stick simultaneously without any change in the position of eye either upwards downwards or side ways.

• Are you able to see both ends of the sticks in all these cases? If not why?
Let us know.

Observe the following fig.-2. You can see the whole object AB which is at a distance of 25 cm (least distance of distinct vision) because the rays coming from the ends A and B of the object AB will enter the eye. Similarly you can also see whole object CD with eye as explained above. Let us assume that AB moves closer to the eye to a position AI BI as shown in fig.2

• Will you be able to see the whole object now?

From the fig.-2, you notice that you will be able to see only the part (EF) of the object A^I B^I because the rays coming from E and F enter your eye. The rays coming from A and B^I cannot enter your eye.




The rays coming from the extreme ends of an form an angle at the eye. If this angle is below 600 , we can see the whole object. If this angle is above 600 , then we can see only the part of the object.

This maximum angle, at which we are able to see the whole object is called angle of vision. The angle of vision for a healthy human being is about 600 . It varies from person to person with the age.

You have learnt that the value of least distance of distinct vision is about 25 cm and the value of angle of vision of human beings is about 600 . You also learnt that these values change from person to person and with age of person.

• Why do the values of least distance of distinct vision and angle of vision change with person and age?
To answer the above question, we need to understand the structure of eye and its functioning.

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