You have studied refraction of light through lenses in the previous
chapter. You have learnt about nature, position and relative size of image
formed by lenses for various distances of objects. In class IX, chapter VI
on sense organs in Biological science text book, explained the structure
of the human eye. The human eye functions on the principle of sensation
of vision. We see objects because the light scattered from them falls on
the eye. The eye has a lens in its structure.

In the previous chapter, you learned that focal length of lens and
object distance determine the nature,position and size of image.

• What is the function of lens in human eye?
• How does it help to see objects at long distances and short distances?
• How is it possible to get the image at the same distance on the retina?
• Are we able to see all objects in front of our eye clearly?
• How do the lenses used in spectacles correct defects of vision?

To answer these questions, you need to understand the structure and
functioning of the human eye.

Let us do the following activities to know about some interesting facts
about our vision.

Least distance of distinct vision

	Activity 1

Take a text book and hold it with your hands in front of you at a
certain distance. Now try to read the contents on the page. Slowly move
the book towards your eye till it is very close to your eyes.

• What changes do you notice?