Chapter 10 Solutions — The Human Eye and the Colourful World

The ability of the eye lens to adjust its focal length to focus on objects at different distances is called the power of accommodation of the eye.

How it works:

- The eye lens is a convex lens made of a fibrous, jelly-like material. Its curvature can be changed by the ciliary muscles attached to it.

- When we look at a distant object, the ciliary muscles relax, making the eye lens thinner. This increases the focal length, allowing a clear image of the distant object to form on the retina.

- When we look at a nearby object, the ciliary muscles contract, making the eye lens thicker (more curved). This decreases the focal length, so that the image of the nearby object is formed exactly on the retina.

The power of accommodation has limits — the focal length of the eye lens cannot decrease below a certain minimum value. This is why there is a least distance of distinct vision (near point), which is about 25 cm for a normal eye.

This is due to the power of accommodation of the eye.

The ciliary muscles attached to the eye lens can change the shape (curvature) of the lens:

- For distant objects: ciliary muscles relax ⇒ lens becomes thin ⇒ focal length increases ⇒ parallel rays from distant objects are focused on the retina.

- For nearby objects: ciliary muscles contract ⇒ lens becomes thick ⇒ focal length decreases ⇒ diverging rays from nearby objects are focused on the retina.

This ability of the eye lens to adjust its focal length is called accommodation. It allows us to see objects clearly at various distances, from the near point (25 cm) to the far point (infinity) for a normal eye.

The angle of deviation is the angle between the direction of the incident ray and the direction of the emergent ray when light passes through a prism.

When a ray of light enters a prism:

1. It bends towards the normal at the first surface (enters the denser medium — glass).

2. It travels through the prism and hits the second surface.

3. It bends away from the normal at the second surface (exits into the rarer medium — air).

The net effect of these two refractions is that the emergent ray is not parallel to the incident ray — it has been deviated from its original path. The angle between the original direction of the incident ray (extended forward) and the emergent ray is called the angle of deviation.

The angle of deviation depends on:

- The angle of the prism (angle of the prism = angle between the two refracting surfaces).

- The refractive index of the prism material.

- The angle of incidence.

- The wavelength of light (violet light deviates more than red light).