HUMAN EYE.pdf

HUMAN EYE BY LOVLEEN ARYA #8882172075 Human Eye : It acts like a camera, enables us to capture the colourful picture of the surroundings. It forms an inverted, real image on light sensitive surface called Retina. The Various parts of eye and their func...

HUMAN EYE BY LOVLEEN ARYA #8882172075 Human Eye : It acts like a camera, enables us to capture the colourful picture of the surroundings. It forms an inverted, real image on light sensitive surface called Retina. The Various parts of eye and their functions 1. Cornea : It is a thin membrane through which light enters. It forms the transparent bulge on the front of eyeball. Most of the refraction occurs at the outer surface of the cornea. 2. Eyeball : it is approximately spherical in shape, with a diameter of about 2.3cm. 3. Iris : It is a dark muscular diaphragm that controls the size of pupil. It is behind the cornea. 4. Pupil : It regulates and control the amount of light entering the eye. It is the black opening between aqueous humour & lens. 5. Crystalline eye lens : It provides the focussed real & inverted image of the object on the retina. It is composed of a fibrous, jelly like material. This is a convex lens that converges light at retina. 6. Ciliary muscles : It helps to change the curvature of eyelens and hence changes it’s focal length so that we can see the object clearly placed at different positon. 7. Retina : Thin membrane with large no. of sensitive cells. 8. When image is formed at retina, light sensitive cells get activated and generate electrical signals. These signals are sent to brain via optic nerve. Brain analyses these signals after which we perceive object as they are. How pupil works ? The pupil of an eye provide a variable aperture, whose size is controlled by iris a) When the light is bright : Iris contracts the pupil, so that less light enters the eye. b) When the light is din : Iris expand the pupil, so that more light enters the eye. Pupil opens completely, when iris is relaxed. Persistence of Vision : It is the time for which the sensation of an object continue in the eye. It is about 1/16th of a second. Power of Accommodation : The ability of eye lens to adjust it focal length is called power of accommodation with the help of ciliary muscles. 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 Near point of the Eye For point of the Eye It is 25cm for normal eye. The minimum It is infinity for normal eye. It is the distance at which an object can be seen farthest point upto which the eye can see most distinctly without strain. object clearly. DEFECTS OF VISION AND THEIR CORRECTION 1. CATARACT : The image cannot be seen distinctly because eye lens become milky and cloudy. This condition is known as cataract. It can cause complete or partial loss of vision. This can be corrected by surgical removal of extra growth (cataract surgery) 2. Myopia : (Near Sightedness) A defect in which a person can see nearby object clearly, but cannot see distant object distinctly. Here , Image formed in front of the retina. Normal O Image Eye Object formed at Retina 1 Myopic O O Eye The Reason of defect 1. Excessive curvature of eye lens (thick, decrease focal length) 2. Elongation of the eye ball. CORRECTION Corrected by using a Concave Lens of appropriate power.  1 O O Correction of Myopia (3) Hypermetropia (Far - Sightedness) – A defect in which a person cannot see nearby object clearly, but can see distant object distinctly. Image is formed at a point behind the retina NORMAL Image formed EYE N at Retina Hypermetropic NORMAL eye EYE N N 1 The Reason of defect 1. Increase in focal length of the eye lens (Thin eye lens) 2. Eye ball has become too small. CORRECTION Corrected by using a Convex Lens of appropriate power. N 1 N Correction of Hypermetropic eye 4. Presbyopia As we become old, the power of accommodation of the eye usually decreases, the near point gradually recedes away. This defect is called Presbyopia. Person may suffer from both myopia and hypermetropia. Reasons of defect- Gradual weakening of ciliary muscles and decreasing the flexibility of the eye lens.  Correction- Using of Bifocal lens with appropriate power. Bifocal lenses consist of both concave and convex lens, upper position consist of concave lens and lower portion consist of convex lens. ACCORDNG TO SYLLABUS , TOPICS INCLUDED ARE FROM HERE : 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. At the first interface( air to glass ) , the light ray has bent towards the normal. At the second interface ( glass to air ) , the light ray has bent away from the normal. Angle of Deviation (D) : The angle between the incident ray and emergent ray. Dispersion for white light by a Glass Prism Inclined refracting surfaces of glass prism show exciting phenomenon. Splitting of White light into band of colours The splitting of light into its component colours is called Dispersion. The band of the coloured components of light beam as called Spectrum i.e. VIBGYOR. Why do we get the different colors ? The different component colours of light bends at different angles with respect to incident angle. The red light bends the least while the violet bends the most.Thus , the rays of each colour emerge along different paths and thus become distinct. ISSAC NEWTON : He was the first, who obtained spectrum of sunlight by using glass prism. He tried to split the spectrum of white light more by using another similar prism, but he could not get any more colours. He repeated the experiment using second prism in on inverted position with respect to the first prism. He a llowed all the colours of spectrum to pass through second prism. He found white light emerging on the other side of second prism. R R V V He concluded that sun is made up of seven visible colours i.e ‘VIBGYOR’ RAINBOW It is the spectrum of sunlight in nature. It is formed due to the dispersion of sunlight by the tiny water droplets, present in atmosphere. A rainbow is always formed in a direction opposite to that of sun. Water droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally (internal reflection) and finally refract it again, when it emerges out of the water droplet. Due to dispersion and internal reflection of light, different colours reach to observer’s eye. Red colour appears on top & violet at the bottom of rainbow. Rain drop A At ‘A ‘Refraction & dispersion B takes place At ‘B’Internal refraction takes place C At ‘C’ Refraction & dispersion takes place R V Atmospheric Refraction The refraction by the earth’s atmosphere is called ATMOSPHERIC REFRACTION. The hotter air is lighter ( less dense ) than the cooler air above it and has a refractive index slightly less than that of the cooler air. The temperature and density of different layers of atmosphere keep varying. Hence ,we have different mediums. Since the physical conditions are not stationary , the apparent position of the object as seen through the hot air fluctuates. This wavering is an effect of Atmospheric refraction. Apparent Star Position– It is due to atmospheric refraction of star light. Distant star act as point source of light. When the starlight enter the earth’s atmosphere it undergoes refraction continuously, due to 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 actual position. The star appears slightly higher than its actual position when viewed near the horizon. Twinkling of Star– It is also due to atmospheric refraction. The apparent position of star keeps on changing because physical conditions of earth’s atmosphere are not stationary. Since the stars are very distant , they are point sized sources of light. As the path of light coming from the star goes on varying rapidly , the amount of light entering our eyes fluctuate - sometimes bright and sometimes faint which is the TWINKLING EFFECT. Why Planets do not twinkle?  Planets are closer to earth and are seen as extended source of light i.e. the collection of large no. of point sized sources of light. Therefore, the total amount of light entering our eyes from all individual point sources will nullify the twinkling effect. Advance Sunrise and delayed sunset This is also due to atmospheric refraction. Because of this sun is visible about 2 minutes earlier than actual sunrise and about 2 minutes after the actual sunset. Apparent position Atmosphere of sun Horizon EARTH Actual Sun Scattering of Light When a beam of light interacts with the particles of matter, it is redirected in any different directions. This phenomenon is called SCATTERING OF LIGHT. Tyndall Effect– When a beam of light strikes the minute particles of earth’s atmosphere - suspended particles of dust and molecule of air the path of beam become visible. The phenomenon of scattering of light by the colloidal particles gives rise to Tyndall Effect. It can be observed when sunlight passes through a canopy of a dense forest.Here, tiny water droplets in the mist scatter the light. The colour of the scattered light depends on the size of the scattering particles Very fine particles Large size particles Very large (scatter mainly (Scatter light of enough blue colour longer wave (The sky having short length i.e. red) appear wavelength) white) (1) Why do clouds Appear white? The size of water droplets (scattering particle) is very large, hence scatters all wavelengths of light almost equally. Thus , the clouds appear white. (2) Why is the colour of the sky blue? The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of visible light. Since blue has shorter wavelength than red, hence it will be scattered the most and this scattered blue light enters our eyes. Q. If there would have been no earth’s atmosphere , what would happen ? Ans. There will be no scattering and sky will appear dark. Q. Why does sky appear dark to passengers flying at very high altitudes ? Ans. This is because scattering is not prominent at such great heights. (3) Colour of the Sun of Sunrise and Sunset While sunset and sunrise, the colour of the sun and its surrounding appear red. During sunset and sunrise, the sun is near horizon, and therefore the sunlight has to travel larger distance in atmosphere. Due to this ,most of the blue light (shorter wavelength) is scattered away by the particles. The light of longer wavelength (red colour) will reach our eye. This is why sun appears red in colour. (4) Why are the danger signals or signs made of red colour? Red colour scatters the least when strikes the small particle of fog and smoke because it has the maximum wavelength (visible spectrum). Hence at large distance also, we can see the red colour clearly. (4) At noon sun appear white– At noon the, sun is overhead and sunlight would travel shorter distance relatively through the atmosphere. Hence, at noon, the Sun appear while as only little of the blue and violet colours are scattered.