CBSE Class 10 Science The Human Eye and the Colourful World Notes Set 03

These CBSE Class 10 Science notes for Chapter 10, The Human Eye and the Colourful World, cover the structure of the human eye, the defects of vision (myopia, hypermetropia and presbyopia) with their correction, cataract, refraction of light through a glass prism, dispersion of white light, the rainbow, atmospheric refraction and the scattering of light.

Important Notes and Formulas

Structure of the Human Eye

A human eye is roughly 2.3 cm in diameter and is almost a spherical ball filled with some fluid. It consists of the following parts:

1. Sclera: It is the outer covering, a protective tough white layer called the sclera (white part of the eye).
2. Cornea: The front transparent part of the sclera is called the cornea. Light enters the eye through the cornea.
3. Iris: A dark muscular tissue and ring-like structure behind the cornea is known as the iris. The colour of the iris indicates the colour of the eye. The iris also helps regulate the amount of light entering the eye by adjusting the size of the pupil.
4. Pupil: A small opening in the iris is known as a pupil. Its size is controlled by the iris. It controls the amount of light that enters the eye.
5. Lens: Behind the pupil, there is a transparent structure called a lens. By the action of ciliary muscles, it changes its shape to focus light on the retina. It becomes thinner to focus on distant objects and becomes thicker to focus on nearby objects.
6. Retina: It is a light-sensitive layer that consists of numerous nerve cells. It converts images formed by the lens into electrical impulses. These electrical impulses are then transmitted to the brain through the optic nerve. The light-sensitive cells of the retina are of two types, cones and rods.
   (a) Cones: Cones are the cells that are more sensitive to bright light. They help in detailed central and colour vision.
   (b) Rods: Rods are the cells that are more sensitive to dim light. They help in peripheral vision.
7. Optic nerve: It carries the electrical signals from the retina to the brain.

At the junction of the optic nerve and retina, there are no sensory nerve cells. So, no vision is possible at that point and it is known as a blind spot.

An eye also consists of six muscles. These include the medial rectus, lateral rectus, superior rectus, inferior rectus, inferior oblique, and superior oblique. The basic function of these muscles is to control the movement of the eye.

Power of accommodation: The ability of the eye lens to adjust its focal length is called the power of accommodation. The distance between the eye lens and the retina remains fixed, so the ciliary muscles change the curvature of the lens. A normal eye has its near point at about 25 cm and its far point at infinity.

Defects of Human Eyes

Myopia

Near-sightedness, or myopia, is the defect of vision due to which a person can see nearby objects clearly, but finds it difficult to see far-off objects distinctly.

In a myopic eye, the eyeball is longer (from front to back) than normal, or the cornea is bulged, so the focal length of the eye is decreased. The eye becomes unable to focus the image of distant objects on the retina. This causes the image to be focused in front of the retina instead of on the retina. As a result of this defect of vision, the distant objects look blurred. The maximum distance at which a myopic eye can see an object clearly is known as its far point; for a myopic eye the far point is nearer than infinity.

Myopia: image formed in front of the retina

Causes of Myopia
It is caused due to:

1. High converging power of the eye lens (because of its short focal length). Due to this, the image is formed in front of the retina, and a person cannot see distant objects.
2. Eyeball being too long. If the eyeball is too long, then the retina is at a larger distance from the eye lens. In this case, the image is also formed in front of the retina.

Correction
Myopia can be corrected by wearing spectacles containing a concave lens. When a concave lens of suitable power is used for the myopic eye, the concave lens first diverges the parallel rays of light from a distant object. Therefore, a virtual image is formed at the far point of the myopic eye. Then, since the rays of light appear to be coming from the eye's far point, they are easily focused by the eye lens, and the image is formed on the retina. Thus, a concave lens is used for a myopic eye to decrease the converging power of the eye lens.

Correction of myopia using a concave lens

Hypermetropia

Far-sightedness, or hypermetropia, is the defect of vision due to which a person can see distant objects clearly, but finds it difficult to see nearby objects distinctly. The near point of a hypermetropic eye is more than 25 cm away, and the image of a nearby object is formed behind the retina.

Hypermetropia: image formed behind the retina

Causes of Hypermetropia
It is caused due to:

1. Low converging or focusing power of the crystalline eye lens (because of its large focal length).
2. Eyeball being too short.
3. Hypermetropia can be present in babies at the time of their birth, but as they grow older, the eyeball lengthens to normal, and the defect is cured naturally.

Correction
The near point of an eye having hypermetropia is more than 25 cm. Therefore, this defect can be corrected by putting a convex lens in front of the eye. When a convex lens of suitable power is placed in front of the hypermetropic eye, the convex lens first converges the diverging rays of light coming from a nearby object, so that a virtual image of the nearby object is formed at the near point of the eye. Since the light rays now appear to be coming from the eye's near point, the eye lens easily focuses and forms the image on the retina. Thus, a convex lens is used for hypermetropia to increase the converging power of the eye lens.

The hypermetropic eye needs a lens of positive power, which indicates that the corrective lens required is convex.

Correction of hypermetropia using a convex lens

Presbyopia

The eyes lose their power of accommodation with ageing. As people grow old, the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens make it more difficult for the eye to focus on close objects. This causes the near point to recede away in older people gradually. As a result, these people may find it difficult to see nearby objects distinctly without corrective eyeglasses. This defect, caused by the loss of flexibility of the eye lens, is called presbyopia. Sometimes, a person may have both myopia and hypermetropia.

Causes of Presbyopia
It is caused due to:

1. Gradual weakening of the ciliary muscles.
2. Decreasing flexibility of the eye lens.

Both these occur with the eye's natural ageing between 40 and 60 years of age, although the person may have had normal vision throughout their life.

Correction
Presbyopia is corrected by using bi-focal lenses, which consist of both concave and convex lenses. The upper portion consists of a concave lens. It facilitates distant vision. The lower part is a convex lens. It facilitates near vision. Such a difference is not visible in progressive lenses, which also function similarly but do not have a line distinguishing the two lenses.

Cataract

Yet another defect of the eye that usually comes in old age is the cataract. It is the medical condition in which the lens of a person's eye becomes progressively cloudy, resulting in blurred vision. It decreases the eye's vision gradually and can lead to a total loss of eye vision. It can be restored by surgery: the opaque lens is removed, and an artificial lens is inserted in its place. Spectacle lenses cannot correct this defect.

Refraction of Light Through a Prism

When a ray of light is incident on a rectangular glass slab, after refracting through the slab, it gets displaced laterally. As a result, the emergent ray comes out parallel to the incident ray. Unlike a rectangular slab, the sides of a glass prism are inclined at an angle to each other. This angle is called the angle of the prism.

Prism: A prism is a transparent refracting medium bounded by two plane surfaces, inclined to each other at a certain angle. It has two triangular bases and three rectangular lateral surfaces.

The angle of prism: The angle between the two lateral (refracting) faces is called the angle of the prism.

The angle of deviation: The angle between the incident ray and the emergent ray is called the angle of deviation.

Refraction of light through a triangular glass prism

Dispersion of White Light by a Glass Prism

The phenomenon of splitting of white light into its seven constituent colours when it passes through a glass prism is called dispersion of white light. The various colours seen are Violet, Indigo, Blue, Green, Yellow, Orange and Red. The sequence of colours is remembered as VIBGYOR. The band of seven colours is called the spectrum. The different component colours of light bend through different angles with respect to the incident ray. The red light bends the least while the violet bends the most.

Dispersion of white light by a prism

For violet colour, the wavelength is minimum and for red colour the wavelength is maximum, i.e. the frequency for violet colour is maximum and for red colour the frequency is minimum.

Composition of white light: White light consists of seven colours, i.e., violet, indigo, blue, green, yellow, orange and red.

Monochromatic light: Light consisting of a single colour or wavelength is called monochromatic light, for example, sodium light.

Polychromatic light: Light consisting of more than one colour or wavelength is called polychromatic light, for example, white light.

Recombination of white light: Newton found that when an inverted identical prism is placed in the path of the dispersed light, then after passing through the second prism, the colours recombine to form white light.

Rainbow

A rainbow is a natural spectrum of sunlight. It is formed due to the dispersion of sunlight by the tiny water droplets present in the atmosphere.

Formation of the rainbow: The water droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop. Due to the dispersion of light and internal reflection, different colours reach the observer's eye.

Rainbow formation

Conditions for the formation of a rainbow are:

1. The formation of a rainbow involves a series of physical phenomena: refraction, dispersion and internal reflection.
2. A rainbow is always formed in a direction opposite to that of the Sun, i.e. the Sun is always behind the observer.
3. The red colour appears on top and violet at the bottom of the rainbow.

In the figure, at A refraction and dispersion take place, at B internal reflection takes place, and at C refraction and dispersion take place again.

Atmospheric Refraction

The refraction of light caused by the Earth's atmosphere (having air layers of varying optical densities) is called atmospheric refraction.

Apparent position of a star: It is due to atmospheric refraction of starlight. The temperature and density of the different layers of the atmosphere keep varying. Hence, we have different media. A distant star acts as a point source of light. When the starlight enters the Earth's atmosphere, it undergoes refraction continuously, due to the changing refractive index, i.e., from rarer to denser. It bends towards the normal. Due to this, the apparent position of the star is different from its actual position. The star appears higher than its actual position.

Apparent position of a star

Twinkling of a star: It is also due to atmospheric refraction. A distant star acts like a point source of light. As the beam of starlight keeps deviating from its path, the apparent position of the star keeps on changing because the physical condition of the Earth's atmosphere is not stationary. Hence, the amount of light entering our eyes fluctuates: sometimes the star appears bright and sometimes dim. This is the twinkling effect of the star. Planets do not twinkle because they are much closer and behave as extended sources made up of a large number of point sources, so the total variation in the amount of light entering the eye averages out.

Advance sunrise and delayed sunset: Because of atmospheric refraction, the Sun is visible about 2 minutes before the actual sunrise and about 2 minutes after the actual sunset.

Scattering of Light

According to Rayleigh's law of scattering, the amount of scattered light is proportional to \( \dfrac{1}{\lambda^4} \) (\( \lambda \) = wavelength). The scattering of light decreases with increase in wavelength.

Tyndall effect: The scattering of light by the particles of a colloid (fine particles of dust, smoke or water droplets) that makes the path of the light beam visible is called the Tyndall effect. Examples: shining a flashlight beam into a glass of milk, and the visible beam of headlights in fog. The water droplets scatter the light, making the headlight beams visible.

Colour of the sky: The sunlight that reaches the Earth's atmosphere is scattered in all directions by the gases and dust particles present in the atmosphere. The sky appears blue because the size of the particles in the atmosphere is smaller than the wavelength of visible light, so they scatter the light of shorter wavelength (the blue end of the spectrum). The blue colour is scattered more and hence the sky appears blue.

Some applications of scattering of light in daily life are:

1. The Sun appears reddish at sunrise and sunset. Near the horizon, sunlight travels a longer path through the atmosphere, so most of the blue light is scattered away and the red light reaches us.
2. The Sun appears white at noon, when light travels the shortest path through the atmosphere and little of the blue is scattered away.
3. The sky's blue colour forms due to the scattering of light by the molecules of nitrogen and oxygen and other fine particles in the atmosphere.
4. The absence of an atmosphere is what causes the sky to look dark at very high altitudes.
5. Red light is used as a danger signal because, due to its longer wavelength, it is the least scattered by fog or smoke and can be seen from a long distance.
6. Clouds look white because the water droplets in them are large compared with the wavelength of light, so they scatter all colours almost equally.