HUMAN EYE AND THE COLOURFUL WORLD Class: 10 (Boys) Sub: PHYSICS NOTES The Human Eye: The human eye is a sensitive sense organ and acts like a camera, which enable us to capture the colourful picture of the surroundings. It works on the refraction of light through a natural convex lens made of transparent living material and enables us to see things around us. The parts of the human eye and their functions are : Sclera It is the outer part of the eye which is quite tough and white in colour. Cornea It is the front part of the eye. Made of transparent substance and it is bulging outwards. It outer surface is convex in shape. The light coming from the objects enters the eye through cornea. Iris- It s behind the cornea and is dark muscular diaphragm which controls the size of the pupil. A hole in the centre of the iris is called the pupil of the eye. Pupil It is the hole in the middle of the iris which appears black as no light is reflected from it but regulates and controls the amount of light entering the eye. The size of the pupil depends on the brightness of the light. If the amount of light received by the eye is large then the iris contracts the pupil and reduces the amount of light entering the eye. But, if the amount of light received by the eye is small, the iris expands the pupil so that more light may enter the eyes. Eye-lens It is a convex lens made of transparent, soft and flexible material like a jelly made of proteins. Its flexible, so it can change its shape to focus light on to the retina. Aqueous humour It is the space between cornea and eye-lens and is filled with fluid mostly water to maintain correct pressure balance. Vitreous humour It is the space between eye-lens and retina is filled with a transparent jelly like substance which supports the back of the eye.
Ciliary muscles Ciliary muscles change the thickness of eye-lens while focusing ie. The focal length of eye-lens can be changed by changing its shape by the action of the ciliary muscles. Retina It is the screen on which the image is formed in the eye and is a delicate membrane having enormous number of light-sensitive cells called rods and cones which respond to the intensity of light and colour of objects respectively by generating electrical signals, which are sent to the brain via the optic nerves. The brain interprets these signals after which we perceive object as they are. Blind spot It is a small area of the retina insensitive to light where optic nerve leaves the eye. Working of the Eye: The light rays enters through the cornea of the eye, pass through the pupil and fall on the eye-lens which is a convex lens and converges the light rays and produces a real and inverted image of the object on the retina which is conveyed to the brain by the optic nerve and gives rise to the sensation of vision. Persistence of Vision: It is the time for which the sensation of an object continue in the eye which is about 1/16 th of a second. Power of Accommodation: The ability of eye lens to adjust its focal length with the help of ciliary muscles is called accommodation. Ciliary Muscles RELAXED CONTRACT 1. Eye lens become thin 1. Eye lens become thick 2. Increases the focal length 2.Decreases the focal length 3. Enable us to see distant object clearly. 3.Enable us to see nearby object clearly Range of Vision of a Normal Human Eye: Least distant vision is the minimum distance at which objects can be seen most distinctly without strain. Also called near point of the eye. For normal vision its 25 cm. The farthest point upto which the eye can see objects clearly is called the far point of the eye. Its infinity for a normal eye. So, normal eye can see objects clearly between 25 cm and infinity.
Defects of Vision and Their Correction: Cataract: The crystalline lens of the people at old age becomes milky and cloudy which causes partial or complete loss of vision. Restored by cataract surgery by removing extra growth. Myopia: ( Short-sightedness or Near-sightedness) It is that defect of vision due to which a person can see objects nearby clearly but cannot see distant objects clearly. A myopic eye has its far point nearer than infinity. Image of distant objects is formed in front of to the retina and not at the retina. The reason for this defect - Excessive curvature of the eye lens - The elongation of the eye ball. It is corrected by - Concave lens of appropriate power Hypermetropia: ( Far-Sightedness) It is that defect of vision due to which a person can see distant objects clearly but cannot see nearby objects clearly. The near point is farther away from the normal near point. A hypermetropic eye has its least distance of distinct vision greater than 25cm. The light rays from a close by object are focused at a point behind the retina. The reason for this defect - the focal length of the eye lens is too long - the eyeball has become too small.
Corrected by- - convex lens of appropriate power Presbyopia: It is that defect of vision due to which the power of accommodation of the eye usually decreases with age. The near point gradually recedes and finds it difficult to see nearby objects comfortably and distinctly without corrective eye glasses. In such conditions, a person may suffer from both myopia and hypermeteropia. Reason of the defect - by the gradual weakening of the ciliary muscles - diminishing flexibility of the eye lens corrected by- - bi-focal lenses of both concave and convex lenses-upper position consists of concave lens and lower position consist of convex lens. -It is also corrected by contact lenses and surgical procedures. Refraction of light through a Prism: Prism It has two triangular bases and three rectangular lateral surfaces. These surfaces are inclined to each other. The angle between its two lateral faces is called Angle of Prism. Ref Act 11.1
Here: PE Incident ray EF Refracted ray FS Emergent ray <A- Angle of the prism <i Angle of incidence <r Angle of refraction <e Angle of emergence <D Angle of deviation- It is the angle between the incident ray and emergent ray. Dispersion of white light: Glass prism Spectrum of white light : It is the band of seven colours formed on a white screen, when a beam of white light is passed through a glass prism. ie. VIBGYOR Dispersion of light : The splitting up of white light into seven colours on passing through a transparent medium like a glass prism is called dispersion of light. It occurs because colours of white light travel at different speeds through the glass prism. The red colour has the maximum speed in the glass prism, so the red colour is deviated the least and violet colour has the minimum speed in the glass prism, so the violet colour is deviated the maximum. So, the colours are arranged in the order of increasing frequency but decreasing wavelength ie:from red on the top to violet at the bottom. Re-combination of Spectrum colours to give white light: Newton by his experiments showed that the seven coloured lights of the spectrum can be recombined to give back white light.
Here, the recombination of seven colours, produced by first prism, happens as the second prism has been placed in reverse position due to which the refraction produced by the second prism is equal and opposite to that produced by the first prism. Formation of Rainbow: The rainbow is an arch of seven colours visible in the sky which is produced by the dispersion of sun s light by raindrops in the atmosphere. The rainbow is a natural spectrum of sunlight in the sky formed in a direction opposite to that of the sun, when the sun is shining and it is raining at the same time. Each raindrop acts as a tiny glass prism splitting the sunlight into a spectrum. As white sunlight enters and leaves these raindrops, the various coloured rays present in white light are refracted by different amounts due to which an arch of seven colours called rainbow is formed in the sky. A ray of white sunlight AB enters the raindrop at point B and undergoes refraction and dispersion to form a spectrum. This spectrum undergoes total internal reflection at point C within the raindrop and finally refracted out of the raindrop at point D. This spectrum produced by the raindrops in the atmosphere is seen to the observer on the earth.
Atmospheric Refraction - The refraction of light caused by the earth s atmosphere(having air layers of varying optical densities) is called atmospheric refraction. Twinkling of Stars: The twinkling of a star is due to the atmospheric refraction of star s light. When the light coming from a star enters the earth s atmosphere, it undergoes refraction due to the varying optical densities of air at various altitudes. The atmosphere keeps on changing and this continuously changing atmosphere refracts the light from the stars by different amounts from one moment to the other. When the atmosphere refracts more star-light towards us, the star appears to be bright and when the atmosphere refracts less light, then the star appears to be dim and thus the starlight reaching our eyes increases and decreases continuously due to atmospheric refraction and appears that the stars are twinkling. The stars are point sources of light and the continuously changing atmosphere is able to cause variations in the light coming from a point-sized star(due to refraction) because of which the star appears to be twinkling. The planets on the other hand do not seem to twinkle as planets are closer to earth and are seen as extended source of light i.e. the collection of large number of point sized sources of light. Therefore the total amount of light entering our eyes from all individual point source will average out to zero and nullify the twinkling effect. Apparent Star Position: Due to atmospheric refraction, the stars seem to be higher in the sky.
The temperature and density of different layer of atmosphere keeps varying due to which we have different medium. Distant star act as point source of light. When the starlight enters the earth s atmosphere it undergoes refraction continuously, due to changing refractive index i.e. When it enters from rarer to denser, it bends towards the normal. The refractive index of air medium gradually increases with a decrease in altitude. The continuous bending of starlight towards the normal results in a slight rise of the apparent position of the star. Thus, the star appear higher than its actual position. Advance Sunrise and delayed sunset: We can see the sun about 2 minutes before the actual sunrise and 2 minutes after the actual sunset because of atmospheric refraction. When the sun is slightly below the horizon, then the sun s light coming from less dense air to more dense air is refracted downwards as it passes through the atmosphere. Because of this atmospheric refraction, the sun appears to be raised above the horizon when actually it is slightly below the horizon. So, the time from sunrise to sunset is lengthened by about 2 + 2 = 4 minutes and if the earth had no atmosphere then the day would have been shorter by about 4 minutes. Apparent flattening of the sun s disc at sunrise and sunset is also due to atmospheric refraction. Scattering of Light: To throw light in various random directions or it s the spreading of light as a result of collision with particles of medium.
Tyndall Effect: It is the scattering of light by particles in its path. When a beam of light strikes the minute particle of earth s atmosphere having the suspended particles of dust and molecules of air, the path of beam become visible. This phenomenon of scattering of light by the colloidal particle gives rise to Tyndall Effect. To observe Tyndall effect, the particles diameter should be less than 1/20 th the wavelength of the light used. It can be observed when sun light passes through a canopy of a dense forest, light through a hole in a dark room, light through a cloud, headlight beams on a foggy day. The colour of the scattered light depends on the size of the scattering particles Very Fine particle(like air molecules-nitrogen & oxygen) Large size particle Very large enough(like dust particles and water droplets) (scatter mainly light of shorter wavelength i.e. Blue and is mostly easily scattered) (Scatter light of longer wavelength i.e. Red) (The sky appear white) The cloud appears White: The size of water droplet(scattering particle) is very large, hence it scatters all wavelength of light almost equally.
The colour of sky is blue: When sunlight passes through the atmosphere, most of the longer wavelength lights( such as red, orange, yellow)present in it do not get scattered much by the air molecules and hence pass straight through. The shorter wavelength blue light is, scattered all around the sky by air molecules in the atmosphere since the air and other fine particles in the atmosphere are smaller in size and scatters light of shorter wavelength and whichever direction we look, some of this scattered blue light enters our eyes and sky looks blue. The outer space, the sky looks dark and black: It s because there is no atmosphere containing air in the outer space to scatter sunlight. Since there is no scattered light to reach our eyes in outer space, therefore, the sky looks dark and black there which is why the astronauts who reach outer space find the sky to be dark and black. Danger signal lights are red: It s because the red coloured light having longer wavelength is the least scattered by fog or smoke particles and so red light can be seen as it is even from a distance.
Red Sun at Sunrise and Sunset: Sun appears 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. During Sunrise and sunset the sun is near the horizon, and has to travel the greatest distance through the atmosphere to reach us. Due to which the blue light (having shorter wavelength) are scattered away from our line of sight. So, the light reaching us directly from the rising sun or setting sun is light of longer wavelength(red). Thus, sun and its surroundings appear red. Sun appears white at noon: When the sun is overhead at noon, then the sunlight has to travel a relatively shorter distance through the atmosphere. Due to this, the scattering of blue as well as red is very less at noon and hence the sun in the sky appears white to us. Experiment to study scattering of light: Ref activity 11.3 pg 196 *******