CBSE Class 10 Science The Human Eye and the Colourful World Notes Set 04

The human eye and how it works

The eye is a camera-like organ. Light enters through the transparent cornea, passes through the pupil, is focused by the eye lens and forms an inverted real image on the retina. The retina has light-sensitive cells that convert the image into electrical signals, and the optic nerve carries these signals to the brain, which interprets the image as erect. Most of the bending of light happens at the outer surface of the cornea, while the lens makes fine adjustments to focus objects at different distances.

1. Cornea: the transparent front surface that refracts most of the incoming light.
2. Iris: the dark muscular diaphragm that controls the size of the pupil.
3. Pupil: the opening through which light enters; it becomes smaller in bright light and larger in dim light.
4. Lens: a flexible convex lens whose focal length can be changed.
5. Ciliary muscles: hold the lens and change its curvature.
6. Retina: the screen that holds rod cells (dim light) and cone cells (bright light and colour).

Power of accommodation

The ability of the eye lens to change its focal length so that both near and far objects are focused sharply on the retina is called the power of accommodation. When the ciliary muscles relax, the lens becomes thin and its focal length increases, which lets us see distant objects. When the ciliary muscles contract, the lens becomes thicker and its focal length decreases, which lets us see nearby objects.

The near point is the nearest point at which the eye can see an object clearly. For a normal adult eye it is about 25 cm, and this is called the least distance of distinct vision. The far point is the farthest point the eye can see clearly, and for a normal eye it is at infinity.

Defects of vision and their correction

1. Myopia (short-sightedness): the person sees nearby objects clearly but not distant objects. The image of a distant object forms in front of the retina, because the eyeball is too long or the eye lens is too curved. It is corrected by a concave lens of suitable power.
2. Hypermetropia (long-sightedness): the person sees distant objects clearly but not nearby objects. The image of a nearby object forms behind the retina, because the eyeball is too short or the focal length of the eye lens is too long. It is corrected by a convex lens of suitable power.
3. Presbyopia: the power of accommodation decreases with age as the ciliary muscles weaken and the lens becomes less flexible. The person may have both myopia and hypermetropia, and bifocal lenses are used.
4. Cataract: the eye lens becomes cloudy, usually with age, which causes blurred vision. It is treated by surgery.

Corrective lens power is found from P = 1/f with f in metres. For myopia, the lens must form a virtual image of a distant object at the far point of the person, so f is negative and equal to the far point distance. For hypermetropia, the lens must form a virtual image of an object at 25 cm at the person's near point.

Refraction of light through a prism and dispersion

When white light passes through a glass prism it splits into its component colours, called a spectrum. This is dispersion. The sequence is violet, indigo, blue, green, yellow, orange and red (VIBGYOR). Each colour bends by a different amount, with red bending the least and violet the most, because they travel at different speeds in glass. A second inverted prism placed correctly recombines the colours into white light. Newton used this to show that the prism does not create colours but only separates them.

A rainbow is a natural spectrum seen in the sky after rain, when the sun is behind the observer. Tiny water drops act as prisms. Sunlight is refracted and dispersed when it enters a drop, reflected internally from the back of the drop, and refracted again when it leaves. A rainbow is therefore due to dispersion, refraction and internal reflection.

Atmospheric refraction

The refraction of light by the earth's atmosphere, whose density changes with height, is called atmospheric refraction. It causes the twinkling of stars, because the star's light bends by slightly different amounts as the air moves, so its apparent position and brightness change. Planets do not twinkle because they are much closer, so they appear as extended sources made of many point sources, and the variations average out. Atmospheric refraction also makes the sun visible about 2 minutes before actual sunrise and about 2 minutes after actual sunset, and flattens the disc of the sun near the horizon.

Scattering of light

When light passes through fine particles in air, they scatter it and make its path visible. This is the Tyndall effect. Very fine particles scatter mainly blue light, which has a short wavelength, so the sky looks blue. At sunrise and sunset sunlight travels a longer path through the air, most of the blue light is scattered away and the red light reaches us, so the sun looks reddish. Red light is scattered least, which is why danger signals are red. An astronaut sees a dark sky because there is no atmosphere to scatter light.

Worked examples

Example 1. A myopic person has a far point at 80 cm. Which lens should be prescribed, and what is its power?
The lens must form a virtual image of a distant object at 80 cm, so v = −80 cm and u = −∞, which gives f = −80 cm = −0.8 m. P = 1/f = −1.25 D. A concave lens of power −1.25 D is needed.

Example 2. A person has a near point at 50 cm. What lens lets her read a book held at 25 cm?
The lens must form a virtual image at 50 cm of an object at 25 cm, so u = −25 cm and v = −50 cm. \( \dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u} = -\dfrac{1}{50} + \dfrac{1}{25} = \dfrac{1}{50} \), so f = +50 cm and P = +2 D. A convex lens of power +2 D is needed.

Example 3. A person needs a lens of power +4.5 D for correcting her vision. Name the defect and find the focal length.
The positive power means a convex lens, so she has hypermetropia. f = 1/4.5 m ≈ 0.22 m = 22 cm.

Quick Check Questions