CBSE Class 10 Science NCERT Solutions: The Human Eye and Colourful World

The human eye possesses a remarkable ability to adjust its focal length, allowing it to focus on objects situated at different distances. When we look at distant objects, the ciliary muscles in the eye are relaxed, causing the eye lens to become thinner. A thinner lens has a longer focal length, which is necessary to form a clear image of distant objects on the retina.

Conversely, when we look at nearby objects, the ciliary muscles contract. This contraction makes the eye lens thicker, reducing its focal length. A shorter focal length is required to focus the light from nearby objects clearly onto the retina.

This capacity of the eye lens to change its focal length to obtain a clear image of objects at various distances is known as the power of accommodation of the eye.

The condition described, where a person cannot see objects beyond a certain distance (1.2 m in this case) distinctly, is known as myopia or short-sightedness. In a myopic eye, the image of distant objects is formed in front of the retina instead of on the retina itself. This is because either the eyeball has become too long, or the eye lens has too much converging power (too short a focal length).

To correct this defect, a lens is needed that can diverge the incoming light rays slightly before they enter the eye. This divergence will effectively shift the point of focus back onto the retina. A concave lens is a diverging lens and is therefore used to correct myopia. By placing a concave lens of appropriate focal length in front of the eye, the light rays from distant objects are diverged, and the eye's lens then focuses these rays correctly on the retina, restoring clear vision.

For a human eye with normal vision, there are two important points related to its focusing ability:

• Near Point: This is the minimum distance at which an object can be seen clearly without any strain. For a normal human eye, the near point is approximately 25 cm. This means you need to hold reading material at least 25 cm away to see it comfortably.
• Far Point: This is the maximum distance at which an object can be seen clearly. For a normal human eye, the far point is considered to be infinity. This is why we can see very distant objects like stars and the moon clearly (assuming no atmospheric interference).

If a student faces difficulty in reading the blackboard while seated in the last row of a classroom, it indicates that they are unable to see distant objects clearly. This vision defect is known as myopia (or short-sightedness).

In myopia, the eye focuses the image of distant objects in front of the retina, rather than directly on it. This can be due to the eyeball being too long or the eye lens having excessive curvature (too much power).

To correct myopia, a lens is required that can reduce the converging power of the eye system. A concave lens is used for this purpose. When a concave lens of appropriate focal length is placed in front of the eye, it diverges the incoming light rays from the distant blackboard. This allows the eye's natural lens to then focus the image correctly on the retina, enabling the student to see the blackboard clearly.