Human Eye and Colorful World Notes PDF

L-10 HUMAN EYE AND THE COLOURFUL WORLD The human eye is one of the most valuable and sensitive sense organs which helps us to see the colorful world around us. The human eye is like a camera. The eyeball is approximately spherical in shape with a diameter of about 2.3cm. Human eye has the following...

L-10 HUMAN EYE AND THE COLOURFUL WORLD The human eye is one of the most valuable and sensitive sense organs which helps us to see the colorful world around us. The human eye is like a camera. The eyeball is approximately spherical in shape with a diameter of about 2.3cm. Human eye has the following parts and functions. Refer fig 10.1 1.CORNEA: Cornea is a transparent membrane in front of the eye. Most of the refraction of the light rays entering the eye occurs at the outer surface of the cornea. 2. IRIS: Behind the cornea is a dark muscular diaphragm called iris which controls the size of the PUPIL. The pupil is a small opening in the iris through which the light enters the eye and it regulates and controls the amount of light entering the eye. In bright sunlight , the iris contracts the pupil to allow less light to enter the eye and in dim light , the iris expands the pupil to admit more light to see the objects clearly. Therefore it takes sometime to see clearly in dim light when we enter a room from bright sunlight outside and vice versa. 3.EYE LENS: It is a convex lens made up of transparent jelly like material. It is held in position by ciliary muscles. 4.CILIARY MUSCLES : helps to change the curvature of the lens and hence changes its focal length. These muscles are attached to the eye lens and can modify the shape of the eyelens which leads to the variation in focal length. When the muscles are relaxed, the lens becomes thin.Thus the curvature of the eyelens decreases and the focal length increases. This enable us to see distant object clearly. When the muscles are contracted, the lens becomes thick. Thus the curvature of the lens increases and the focal length decreases.This enable us to see nearby objects clearly. 5.RETINA: The lens helps to focus the image of objects on the RETINA The eye lens forms a real inverted image of the object on the retina by adjusting its focal length with the help of ciliary muscles.. The light sensitive cells in the retina then produce electrical signals which are carried by the optic nerves to the brain. The brain processes the information and then we see the object. POWER OF ACCOMMODATION OF THE EYE: The ability of the of the eye lens to adjust its focal length and thereby enable us to see object from a distance of 25cm to infinity. The ciliary muscles are capable of modifying the curvature of the lens and thereby affecting the focal length of the eye lens. However, the focal length of the eye cannot be reduced below a certain minimum limit. If you read a book very close to your eye, it will appear blurred. The minimum distance, at which objects can be seen most distinctly without strain is called the 'Least Distance of Distinct Vision' or the 'Near point of the eye'. For a young adult with normal vision, the near point is about 25cm. The farthest point up to which the eye can see objects clearly is called the 'Far point of the eye'. For a normal eye, it is infinity. Q. What happens to the image distance in the eye when we increase the distance of object from the eye? In human eye the image distance (distance between lens and retina) is fixed. Therefore no change happens when we increase the distance of the object from the lens. DEFECTS OF VISION AND THEIR CORRECTIONS. When the eye loses it’s power of accommodation ,the person cannot see the objects clearly. There are three major refractive defects of vision. 1.MYOPIA Refer fig.10.2 This is also known as nearsightedness. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. The far point is nearer than infinity. Image of a distant object is formed in front of the retina Causes:- 1.Excessive curvature of the eye lens Or 2.Elongation of the eye ball. Correction A concave lens of suitable power is used for correction. 2. HYPERMETROPIA OR HYPEROPIA Refer fig 10.3 Hypermetropia is also known as far- sightedness. A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. Image of nearby objects is formed behind the retina. Causes:- 1.Focal length of the eye lens is too long. OR 2. The eyeball has become too small Correction A convex lens of suitable power is used for correction. 3.PRESBYOPIA progressive form of farsightedness that affect most people by old age. The power of accommodation of the eye decreases with ageing. Most people find their near point receding gradually. Causes: Gradual weakening of ciliary muscles and diminishing flexibility of the eye lens. Correction:- It can be corrected using a convex lens. Some people who suffer from myopia develop hypermetropia too as they age. This can be corrected using BIFOCAL LENS. The common bifocal lenses consist of both concave and convex lenses. The upper portion consists of concave lens to facilitate the vision of faraway objects. The lower part is a convex lens to facilitate the vision of nearby objects. CATARACT: The medical condition in which the eye lens of a person progressively becomes cloudy ( or even opaque), resulting in blurred vision is called cataract. Correction:- The opaque lens is removed surgically and a new artificial lens is inserted in its place. NUMERICALS 1. A person cannot see objects nearer than 75 cms from his eyes, while a person with normal vision can see objects upto 25 cms from his eyes. Find the nature, the focal length and the power of the correcting lens used for the defective vision. Name the defect. Here the defect is hypermetropia, u = −𝟐𝟓cm v = −𝟕𝟓𝒄𝒎 f =? P=? 𝟏 𝟏 𝟏 For lens, 𝒇 = 𝒗 − 𝒖 𝟏 𝟏 𝟏 = −𝟕𝟓 + 𝟐𝟓 𝒇 𝟏 +𝟐 = 𝒇 𝟕𝟓 𝟕𝟓 𝒇= = +37.5cm ( the corrective lens is convex lens) 𝟐 𝟏 𝟏𝟎𝟎 𝟖 P = 𝒇 = 𝟕𝟓/𝟐 = 𝟑 = +𝟐. 𝟔𝟕𝑫 2. A person with myopic eye is not able to see objects beyond 2 m from him. Determine the nature, focal length and power of the lens used to correct the defect. Here, u = -∞ v = -2m f =? P=? 𝟏 𝟏 ⬚ 𝟏 𝟏 𝟏 For lens,𝒇 = 𝒗 − = −𝟐 + ∞ 𝒖 𝒇 𝟏 −𝟏 −𝟏 = + 0= 𝒇 𝟐 𝟐  f = -2 m (the corrective lens is concave lens) 𝟏 −𝟏 P = 𝒇 = = −𝟎. 𝟓𝑫 𝟐 REFRACTION OF LIGHT THROUGH A TRIANGULAR PRISM Refer fig 10.4 It has two triangular bases and three rectangular lateral surfaces. These surfaces are inclined to each other. When a ray of light passes through a glass prism it refracts twice at air-glass interface as well as at the glass air interface. At the first face PQ the ray of light bends towards the normal, whereas at the other face RS, ray of light bends away from the normal. Since the two refracting surfaces of the prism are not parallel, the emergent ray and the incident ray are not parallel to one another. In this case, the ray of light passing through the prism is found to bend towards the base of the prism. The angle between the incident ray and emergent ray is called ANGLE OF DEVIATION. The angle between its two lateral faces is called ANGLE OF PRISM. DISPERSION OF LIGHT REFER FIG 10.5 The phenomenon of splitting of white light into seven constituent colors on passing through a glass prism is known as dispersion of light.  Different colors deviate through different angles with respect to incident ray as they pass through the prism. The red light deviates the least and violet light maximum. REASON FOR DISPERSION 1.The refractive index of the glass is different for different colours & hence they refract at different angles. Violet has the shortest wavelength so it is most deviated and red has the longest wavelength so the least deviated. 2.The two refractive surfaces are not parallel. RECOMBINATION OF SPECTRUM OF LIGHT REFER FIG 10.6 The components of white light can be recombined to give back white light. The first prism P1 splits the white light into seven colors and the second prism P2 is in inverted position w.r.t to P1, again recombines all the colors to produce white light. It happens because the dispersion in the first prism is reversed by the second prism. RAINBOW FORMATION: Refer fig 10.8 It is a natural spectrum appearing in the sky after a rain shower. A rainbow is always formed in the direction opposite to that of Sun. Tiny water droplets present in the atmosphere act as prisms and disperse sunlight. They refract and disperse the incident sunlight and reflect it internally and finally refract it again when it comes out of the droplet. Due to dispersion of light and internal reflection different colors reach our eyes. Two conditions necessary to observe a rainbow: 1.It is seen in the sky only after a rain shower. 2.The position of sun should be always behind the observer. ATMOSPHERIC REFRACTION When light passes through the different layers of the atmosphere refraction takes place. This is called atmospheric refraction. The air in the atmosphere is not at the same temperature.  The refractive index of the atmosphere gradually increases as we come down towards the earth. The refractive index depends on various factors such as temperature, composition of air, dust particles etc. All these factors keep fluctuating which causes the fluctuation in the refractive index of the atmosphere. Q. Why do stars twinkle? The twinkling of a star is due to atmospheric refraction of starlight. Stars appear as point size in the sky, because they are very far away from us. The star light on entering the earth atmosphere undergoes refraction due to the difference in the refractive index of the various layers of the atmosphere. So the star light entering the eye fluctuates with time. Due to this, at times the star appear brighter and at some other time fainter. Thus the fluctuating brightness of the star gives rise to the twinkling effect of the star to the eye. Q. Why planets do not twinkle? This is because they are of larger size and much closer to the Earth so it can be taken as a collection of large number of point sized(extended) source of light. The dimming effect of one part of the planet is nullified by the brighter effect in its other parts. On the whole planets remains brighter and does not appear to twinkle. Q. Apparent position of star is slightly higher than the actual position. Give reason. Ref fig 10.9 Due to atmospheric refraction the stars seem higher than they actually are. A star light is refracted as soon as it enters the earth’s atmosphere. Air higher up in the sky is rarer but near the earth’s surface is denser. So the star light bends more. Though the actual position of star seems higher in the sky. ADVANCE SUNRISE AND DELAYED SUNSET  Refer fig 10.10 Light from the sun on entering the earth’s atmosphere passes from rarer to denser layers bending towards the normal every time. Therefore the sun appears to be raised while it is actually below the horizon. We can see the apparent rising sun about 2 min before it is actually above the horizon. Light from the sun on entering earth’s atmosphere passes from rarer to denser layers bends towards the normal every time. Therefore the sun appears raised while it is actually below the horizon. So we can see still the sun 2 minutes even after the sun has set below the horizon So time from sunrise to sunset is lengthened by about by 2 + 2= 4min because of atmospheric refraction. Q. Sun appears bigger during sunrise and sunset. Give reason. The apparent flattening of the Sun’s disc at sunset and sunrise is also due to the atmospheric refraction. SCATTERING OF LIGHT Light is scattered when it falls on various types of suspended particles in its path. The spreading of light by particles in a medium is termed as scattering of light. TYNDALL EFFECT The scattering of light by colloidal particles in the medium and the path of light becomes visible. This is known as Tyndall effect. SOME NATURAL APPLICATIONS OF TYNDALL EFFECT 1. When sunlight enters a dusty room through a small hole, the scattering of light by the dust particles in the air make the particles visible. 2. When sunlight passes through the canopy of dense forest the tiny water droplets in the mist scatter light and the path of light becomes visible. FACTORS ON WHICH THE COLOUR OF SCATTERED LIGHTDEPENDS:  1. The colour of scattered light depends on the size of the suspended particles in the medium such as dust, water droplets etc. Very fine particles scatter mainly blue light of shorter wavelength. Large size particles scatter red light of longer wavelength. If the size of particle is large enough, then the scattered light may appear even white. 2. Wave length: The rate of scattering depends the wavelength of the light. Blue light has shorter wavelength , therefore it scatters much more than the red light which has longer wave length. Q. Why does the sky appear blue? The particles in the air have size smaller than the wavelength of visible light. They scatter blue light of shorter wavelength more strongly than red light. Therefore the scattered blue light enters our eye and the sky appears blue. Q. Why does the sky appears dark instead of blue to an astronaut? Since there is no atmosphere in the outer space, scattering of light does not take place.Therefore the sky appears dark to an astronaut. Q. Why are danger signals lights red in colour? Danger signals are red in colour as red colour has greater wavelength and it is least scattered by fog or smoke. It can be seen in the same colour for a long distance.