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 main parts of the Human Eye: Cornea, Iris pupil,ciliary muscles,Eye lens,Retina and optic Nerve. Cornea The front part of the eye. Made of a transparent substance and is bulging outwards. The outer surface is convex in shape. Light from objects enters the eye through it. Iris. Behind the cornea. A flat,coloured,ring-shaped membrane. Controls the amount of light entering the eyes. It automatically adjusts the size of the pupil according to the intensity of light. If amount of light large-Iris contracts the pupil and reduces the amoun t of light. If light is less- Iris expands the puil so that more light enters the eyes. Pupil. A Hole in the middle of the Iris. It appears black because no light is reflected from it. Eyelens. A convex lens made of a transparent,soft and flexible material like a jelly made of proteins. Eyelens can change its shape (thick or thin) to focus light on the retina. It is held in position by suspensory ligaments –one end is connected to the eye lens and the other to the ciliary muscles. Ciliary muscles. Change the thickness of the eyelens while focussing. Main diff between a camera and an eye. Retina. The screen on which the image is formed in the eye. It is behind the eyelens and at the back part of the eye. It is like a film in a camera. A delicate membrane having a large number of light-sensitive cells called Rods and cones which respond to the intensity of light and colour of objects respectively. Blind spot. A junction of optic nerve and retina in the eye, where there are no rods and cones. No vision is possible at this spot. A small area of the retina insensitive to light where the optic nerve leaves the eye. When the image of an object falls on the blindspot, we cant see the object. Aqueous Humour A watery liquid filled between cornea and Eyelens. Vitreous Humour A Transparent jelly-like substance between eyelens and the retina. Rods Rod-shaped cells in the retina. Sensitive to dim light. We are seeing things in dimlight due to the rods. Nocturnal animals (owl) have a large number of rods. Cones Cone-shaped cells in the retina. Sensitive to bright light. Respond to colours, cause the sensation of colour of objects in our eyes. To see colours and distinguish between the colours. Do not function in dim light. The light from the object enters through the cornea,pass through the pupil and fall on the eyelens. Eyelens converges thelight rays and forms a real and inverted image on the retina. The image formed in the retina is conveyed to the brain as electric signals through the optic nerve and gives rise to the sensation of vision.The electric signals are generated due to the activation of rods and cones in the retina. Physics concept: Eye- A convex lens and a screen. A normal eye can see the far-away objects and the near-by objects due to the power of Accommodation. Distant object view: The rays of light will be diverging in the beginning and then becomes parallel. The convex eyelens should have a low converging power to focus the image on the retina. For low converging power,f should be large and is quite thin. Near-object view: The rays of light will be diverging when they reach the eye. Convex eyelens -high converging power-low focal length-lens should be thick. The power of Accommodation is possible by changing the focal length of the eyelens by the action of ciliary muscles. Distant -object view: The ciliary muscles of the eye are fully relaxed. Pull the suspensory ligaments . The suspensory ligaments will be tight. The eyelens become stretched and become thin. Focal length becomes large and converging power becomes low. In this condition,the eye is said to be unaccommodated because it is the relaxed state of the eye. Near-by object view: Ciliary muscles contract. Suspensory ligaments loose. The Eye-lens bulges under its own elasticity and becomes thick. Focal length less, Converging power large In this condition,the eye is said to be accommodated The ability of an eye to focus the distant objects as well as the nearby objects on the retina by changing the focal length of its lens is called accomodation. A normal eye has a power of accommodation which enables objects as far as infinity and as close as 25 cm to be focussed on the retina. The power of accommodation of normal vision is 4 dioptres. Far point The farthest point from the eye at which an object can be seen clearly. It is at infinity. Near point, The nearest point up to which the eye can see the object clearly. It is 25 cm. Therefore, The range of vision of a normal Human Eye is from infinity to 25 cm. They are also known as the defects of the eye.some common defects: Myopia, Hypermetropia,presbyopia. These are the refractive defects of vision because they are caused by the incorrect refraction of light rays by the eye-lens. Can be corrected by using suitable spherical lenses. Short-sightedness-short sight is normal,but long sight is defective. Cannot see the distant objects clearly. The far point is less than infinity. The causes: High converging power of eyelens. Or The Eye ball being too long. In an eye suffering from Myopia, the ciliary muscles do not relax sufficiently,to make the lens low converging power. And the lens thin. In another case, when the eyeball is too long,the retina is at a larger distance from the eyelens. So -the image formed in front of the retina for both the cases. The defect can be corrected by using concave lens. The distant object can be made to appear as if it were at the farpoint of the eye by using a concave lens. The concave lens should be of such a focal length that it produces a virtual image of the distant object at the farpoint of the myopic eye. The purpose of cocave lens is to reduce the converging power of the eyelens. Cannot see the near-by objects. Can see the far-away object. The nearpoint is more than 25 cm . Opposite of Myopia. The causes: Low converging power of Eye lens (because of large focal length.) Or Due to eyeball being too short. The ciliary muscles being weak, cannot make the eyelens thicker to increase its converging power. The image is formed behind the retina. The eye ball short-the retina is at a shorter distance from the eyelens. The image is formed behind the retina. Can be corrected by using convex lens. Its Focal length should be such that it forms a virtual image of the object at the nearpoint of the hypermetropic eye. The purpose of convex lens is to increase the converging power of the eyelens. The defect of vision due to which an older person cannot see the nearby objects clearly. In old age, ciliary muscles become weak and the eyelens become inflexible. The nearpoint will be much more than 25 cm. It is old age Hypermetropia. Caused by the loss of power of accommodation of the eye due to old age. It is corrected by convex lens. Both Myopia and Hypermetropia-upper: concave, lower: convex The medical condition in which the lens of the eye of a person becomes progressively cloudy resulting in blurred vision. The eyelens becomes cloudy due to the formation of a membrane over it. It decreases the vision of the eye gradually. Surgery-the eyelens can be removed and a new artificial lens is inserted. No spectacle lenses for correction. Glass prism A transparent object made of glass having two triangular ends and three rectangular sides. The opposite faces of the prism are not parallel to each other.They are inclined at an angle to one another. The angle between its opposite faces is called the angle of the prism. For a rectangular slab, the incident ray and emergent ray are parallel to one another. For a prism,the incident ray and emergent ray are not parallel to one another. In refraction through a prism,the emergent ray is deviated from its original direction by a certain angle. The angle between the incident ray produced and the emergent ray produced is known as the angle of deviation. Refraction of light through a glass prism Dispersion of Light Newton found that if a beam of white light is passed through a prism,the white light splits to form a band of seven colourson a white screen. The band of seven colours formed on a white screen,when a beam of white light is passed through a prism is called spectrum of white light. This splitting of white light into seven colours is known as the Dispersion of Light. Dispersion occurs because the colours of white light travels at different speeds through a prism.Due to this,each colour is refracted at a different angle,with the result that seven colours are spread out to form a spectrum. Red colour: Maximum speed, Deviation the least. Forms the upper part of the spectrum. Violet:Minimum speed,Deviation the max, Forms the lower part of the spectrum. Re-combination of spectrum colours to give white light Formation of Rainbow An arch of seven colours visible in the sky which is produced by the dispersion of sun’s light by raindrops in the atmosphere. It is actually a natural spectrum of sunlight in the sky. It is formed when the sun is shining and it is raining at the same time. It is formed in a direction opposite to the sun. It is produced by the dispersion of white sunlight by the raindrops in the atmosphere. Each tiny raindrop act as a prism splitting the sunlight into a spectrum. A ray of sunlight enters the raindrop and undergoes refraction and dispersion to form a spectrum. This spectrum undergoes total internal reflection within the raindrop and finally refracted out of the raindrop. This spectrum produced by raindrops in the atmosphere is seen from the earth as Rainbow. Red at the top,violet at the bottom Some of the air layers of the atmosphere are cold and some are hot. Hot:optically rarer. Cold:optically denser. In the same atmosphere, we have air layers of different optical densities.. When light rays pass through the air layers,refraction of light takes place. The Refraction of light caused by the earth’s atmosphere having air layers of different optical densities is known as Atmospheric Refraction. Look at objects through the hot air over a fire,the objects appear to be moving slightly. Air just above the fire: hot: optically rarer. Above that: cold:optically denserDue to the Atmospheric refracton,we feel the objects are moving slightly.small scle Twinkling of stars. Stars seem Higher than they actually are. Advance sunrise and delayed sunset. Star-A point-sized object. Light coming from the star enters the earth’s atmosphere. Due to the varying optical densities of air at various altitudes,it undergoes Atmospheric Refraction. The atmosphere is continuously changing. It refracts the light from the stars by different amounts from one moment to the next. When the atmosphere refracts more sunlight towards us: the star appears to be bright. When the atmosphere refracts less sunlight towards us: the star appears to be dim. The star-light reaching our eyes increases and decreases continuously due to Atmospheric Refraction. So the star appears to twinkle. A planet-collection of point sources of light. The dimming effect produced by some of the point sources of light is nullified by the brighter effect produced by the point sources of light in the other part. Thus,the brightness of the planet always remain the sameand hence it does not appear to twinkle.