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The Human Eye and the Colourful World

THE HUMAN EYE

The eye consists of:

i) Eye Ball- It is nearly spherical in shape. It preserves the shape of the eye and protects it from injury.

ii) Cornea- It is a thin layer of transparent material covering the iris. The main function of cornea is to protect the eyes. It also helps focusing light.

iii) Iris- Iris is the coloured part of the eyes. It controls the amount of light that can pass through the eye lens to the retina.

iv) Pupil- The dark spot in the centre of the iris is called pupil. Pupil is the opening of the eye.

v) Eye Lens- The eye lens is in the front of the eye ball. It makes a real and inverted image of the object on the retina. It is held in its place by ciliary muscles attached to the eye ball.

vi) Ciliary Muscles- The Ciliary muscles surrounding the eye lens can increase or decrease the thickness of the lens, thus making its focal length shorter or longer.

vii) Retina- The retina is a thin tissue that has many layers of cells. It consists of nerve cells that are sensitive to the brightness as well as to the colours of light.

• Retina contains light sensitive cells- RODS and CONES.

• Rods- are sensitive to dim light.

• Cones- are sensitive to bright light. Cones sense colour too.

viii) Optical nerve- The nerve cells carry message about the image from the retina to the brain in the form of special signals. The brain then interprets these signals and enables us to see the objects.

ix) Blind Spot- At the junction of the optic nerve and the retina, there are no sensory cells, so no vision is possible at that spot. This is called the blind spot.

Power of Accommodation

The ability of the eye lens to adjust its focal length is called accommodation.

The least distance of distinct vision is 25cm.

The far point of the eye is infinity.

DEFECTS OF VISION AND THEIR CORRECTION

1). Cataract - Eye lens looses its transparency due to deposition of opaque material on it. It can be corrected by simple surgery.

2). Myopia or shortsightedness caused due to excessive curvature of the eye-lens or elongation of the eyeball.

It can be corrected by using concave lens

3). Hypermetropia or long-sightedness caused due to either the eyeball becomes too short or the focal length of the eye-lens becomes too large.

It can be corrected by using convex lens.

4). Presbyopia -at old age person cannot read correctly due to the stiffening of the ciliary muscles and the decrease in flexibility of the eye lens.

This defect can be corrected by using convex lens or bifocal lens of suitable focal length.

5). Asigmatism- A person cannot simultaneously see both the horizontal and vertical view of an object with the same clarity. It is due to the irregular curvature of the cornea.

It is corrected by using a cylindrical lens.

DISPERSION OF WHITE LIGHT BY A GLASS PRISM

Glass Prism – A triangular glass prism is a transparent object made of glass having two triangular ends and three rectangular sides (or rectangular faces).

The rectangular sides of a glass prism are not parallel to each other but the triangular ends are parallel.

a) .The refraction of light through the rectangular sides of the prism (which are not parallel to each other) is different in behavior than the refraction of the ray through a rectangular slab (where faces are parallel).

b). The refraction of a glass prism is always studied through its rectangular sides (the inclined sides). The emergent ray of light in a glass prism is not parallel to the incident ray of light because the opposite faces of the glass prism (where the refraction takes place) are not parallel to one another.

c). When a ray of light passes through a prism, it bends towards the base (the thicker part) of prism. Angle of Deviation – It is the angle between incident ray and emergent ray. It is the peculiar (triangular) shape of the glass prism which makes the emergent ray bend with respect to the

I) . Dispersion of Light –

a). Newton discovered by his experiments with glass prisms that white light consists of a mix of seven colours. Those seven colours are denoted by the word VIBGYOR (V=violet, I=indigo, B=blue, G=green, Y=yellow, O=orange, R=red). Dispersion of Light – The splitting up of white light into seven colours on passing through a medium like glass prism is called as dispersion of light. Spectrum of White Light – The band of seven colours formed on a white screen, when a beam of white light is passes through a glass prism, is called as spectrum of white light.

b). Cause of Spectrum – The dispersion of white light occurs because the angle of refraction (angle of bending) of lights of different colours is different when passing through the glass prism. The red colour is deviated the least, so it forms the upper part of the spectrum. The violet colour is deviated the maximum, so violet colour appears at the bottom of the spectrum. Recombination of seven colours may be produced by joining a second prism to placed in reversed position to the first where the light emerges from the first prism. The rainbow is an arch of seven colours visible in the sky which is produced by the dispersion of sunlight by raindrops in the atmosphere.

ATMOSPHERIC REFRACTION

The atmosphere contains layers of air having different optical densities.

a). These different layers of air hence produce refraction of light. The refraction of light caused by the earth’s atmosphere is called atmospheric refraction.

b). The twinkling of a star is due to the atmospheric refraction of star’s light. The continuous changing atmosphere refracts the light from the stars by different amounts from one moment to the next. Stars appear to twinkle because the stars are very far and hence point sized, hence the changing can cause them to twinkle. Planets are close and they appear bigger, so the changes in atmospheric layers can not cause variation in the light coming from them, so they do not appear to twinkle. Optical density of the atmosphere increases as we move down – air is rarer upward in the atmosphere and dense as we move closer to earth. The progressive change in the optical density of the atmosphere causes the light rays emanating from the star to bend, so stars seem to be higher in the sky than they actually are. We can see the sun about 2 minutes before the actual sunrise and about 2 minutes after the actual sunset because of atmospheric refraction.

SCATTERING OF LIGHT

c). Scattering of Light – Scattering of light means to throw the light in various random directions. Light is scattered when it falls on various types of suspended particles in its path.

e). The colour of the scattered light depends on the size of scattering particles –

o The larger particles of dust and water droplets present in the atmosphere scatter the light as it is, so the scattered light also appears white. o The extremely minute particles such as air molecules present in the atmosphere scatter mainly the blue light present in the sunlight.

Tyndall Effect

d). Tyndall Effect – The scattering of light by particles in its path is called Tyndall Effect. Tyndall discovered that when white light consisting of seven colours is passes through a clear liquid having small suspended particles in it, then the blue colour of white light having shorter wavelength is scattered much more than the red colour having longer wavelength. The blue light present in sunlight is scattered 10 times more than the red light.

Why is the colour of the clear sky Blue?

a). The scattering of blue component of the white sunlight by air molecules present in the atmosphere causes the blue colour of the sky.

b). If the earth had no atmosphere then? – There would have been no scattering of sunlight at all. No light of the sky would have entered our eyes and the sky would have looked dark and black. The sky looks dark and black instead of blue in the outer space because there is no atmosphere.

Colour of the Sun at Sunrise and Sunset

The sun and the surrounding sky appear red at sunrise and sunset because at that time most of the blue colour present in the sunlight has been scattered out and away from our line of sight, leaving behind mainly red colour in the direct sunlight beam that reaches our eyes.

Assignment

Q1. discuss how the light rays from an object are focussed on the retina.

Q2. What is the function of iris and pupil of the eye ?

Q3. How does the focal length of the eye lens change as per distance of the object in front of eye?

Q4. What is power of accommodation of eye?

Q5. Draw a neat labelled diagram of a human eye.

Model Answers

Ans 1. Our eye work like a camera. Light enters the eye through cornea,which is a transparent bulge on the front surface of eyeball. Most of the refraction of light rays entering the eye takes place at the outer surface of the cornea. The crystalline lens provides the fine adjustment of focal length required to focus objects situated at different distances on the retina.

Ans 2. The iris controls the size of the pupil. It adjusts in size and therefore, helps in regulating the amount of light entering the eye through pupil. When the light is very bright, the pupil becomes very small.When light is dim, it opens up completely through the relaxation of the iris.

Ans 3. Ciliary muscles present in eyes help to change in the curvature of eye lens which can change its focal length. When the muscles are relaxed,the lens is thin and its focal lengh is more. When the ciliary muscles contract and the eye lens becomes thicker. Hence the focal length of eye lens decreases.

Ans 4. The ability of the eye lens to adjust its focal length is called accommodation.

Ans 5. Refer to the Figure 11.1 in NCERT.