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Mathsacumen.com Light Incident ray :The light ray, which strikes any surface, is called the incident ray Reflected ray: The ray that comes back from the surface after reflection is known as the reflected ray. Point of incidence: The point where the light ray strikes is called point of incidence Normal: The line which is perpendicular to the surface at point of incidence is called the normal to the surface at the point of incidence. Angle of incidence(∟i) : The angle between the normal and the incidence ray is known as the angle of reflection. Angle of reflection(∟r): The angle between the normal and the reflected ray is known as the angle of reflection. Laws of reflection of light: The incident ray, the reflected ray, and the normal {at the point of incidence}, all lie in the same plane. The angle of reflection is always equal to the angle of incidence. Lateral inversion: An image formed by a mirror the left of the object appears on the right and the right appears on the left. This is known as lateral inversion Regular Reflection and Diffuse Reflection of Light Regular Reflection: In regular reflection, a parallel beam of incident light is reflected as a parallel beam in one direction. Regular reflection of light occurs from smooth surfaces like that of a plain mirror. Images are formed by regular reflection of light. Diffuse reflection: In Diffuse reflection, a parallel beam of incident light is reflected in different directions. The diffuse reflection of light takes place from rough surfaces A sheet of paper produces diffuse reflection of light. No image is formed in diffuse reflection of light. Vinod kumar.k Page 1 Mathsacumen.com Characteristics of image formed by a plane mirror It always form virtual, erect image. Size of image is equal to size of object Image and object are equidistant from the mirror. Image is laterally inverted. Uses of plane mirrors We commonly use plane mirrors in our homes to look at our reflection. In beauty parlour, plane mirrors kept at an angle are used to view the side of the head. Plane mirrors parallel to each other ae used to view the back of the head. Plane mirrors are used to reflect light on an object. For example, during outdoor shooting of film metal sheets are used as plane mirrors to reflect sunlight on the actors. They are also used in solar cookers to reflect light on the food being cooked. They are used in periscopes. From a submarine under the sea, a sailor can see objects and enemy ships on the surface of the sea by using a periscope. Periscope : A Periscope is a device through which a person can se the objects that are out of the direct line of sight. For example, by using a periscope, we can see the objects on the other side of a high wall which cannot be seen by us directly. Periscope has two plane mirrors arranged parallel to one another. Each plane mirror, however, makes an angle of 450 with the side of the tube. Some of the uses of periscopes are given below: Periscope is used to see over the heads of a crowd A Periscope is used by soldiers sitting in bunker to observe the enemy activities outside {over the ground}. A Periscope is used by a navy officer sitting in a submarine to see ships over the surface of water in the sea. Vinod kumar.k Page 2 Mathsacumen.com Multiple Images: When two plane mirrors are kept inclined at an angle, they can form multiple images of an object. If two plane mirrors are inclined at an angle , then the number of images formed in them is given by the formula: No. of images formed = (3600/ ) - 1 Kaleidoscope : The kaleidoscope is an instrument which produce multiple reflections of coloured glass pieces {or coloured plastic pieces} and create beautiful patterns. Construction: The Kaleidoscope consists of three long and narrow strips of plane mirrors inclined at 600 to one another forming a hollow prism, and fitted into a cardboard tube. One end of the cardboard tube is closed by an opaque disc {cardboard disc} having a small hole at its centre. The other end of cardboard tube is closed with circular discs of glass. The inner disc being of transparent glass {clear glass } and the outer disc of ground glass {translucent glass}. A number of small pieces of different coloured glass {or plastic } and having different shapes are kept between the two glass discs {which can move around freely in the space between the two glass discs}. When we hold the kaleidoscope tube towards light and look inside it through the small hole, we see beautiful patterns of coloured glass. 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. The seven colours of the spectrum of white light are: violet, indigo, blue, green, yellow, orange and Red. A human eye: The human eye is like a camera. Its lens system forms an image on alight sensitive screen called the retina. The eye ball is approx. spherical in shape with a diameter of 2.3cm. The human eye has the following parts:(a) Cornea:-The transparent spherical membrane covering the front of the eye. (b) Iris:-The coloured diaphragm between the cornea and lens. (c) Pupil:-The small hole in the iris. (d)Eye lens:-Its is a transparent lens made of jelly like material. (e) Ciliary muscles:-These muscles hold the lens in position. (f) Retina:-The back surface of the eye. Vinod kumar.k Page 3 Mathsacumen.com (g)Blind spot:-The point at which the optic nerve leaves the eye. An image formed at this point is not sent to the brain. (h) Aqueous humour:-A clear liquid region between the cornea and the lens. (i) Vitreous humour:-The space between eye lens and retina is is filled with another liquid called Vitreous humour. Persistence of vision The image of an object seen by our eyes persists {or remains}on the retina for about 1/16th of a second even after the object has disappeared from our view. The ability of an eye to continue to see the image of an object for a very shot duration even after the object has disappeared from view, is called persistence of vision. Defects of the Eye: a) Myopia (short-sightedness) : In this defect the person is able to see the near by objects but not able to see the far-off objects. This defect can be corrected by using spectacles containing Concave lens. b) Hypermetropia (Far-sightedness) : In this defect the person is able to see the far-off objects but not able to see the near by objects. This defect can be corrected by using spectacles containing Convex lens.c) Cataract : Cataract develops when the eye lens of person becomes cloudy (or even opaque) due to the formation of membrane over it. Cataract decreases the vision of the eye gradually. It can even lead to total loss of vision of the eye. The opaque lens is removed from the eye of the person by surgical operation & a new artificial lens is inserted in its place. The visually- impaired: There are an estimate 180 million visually impaired people in the world while the number in india is around 7,00,000. Some of them are born with defects and others lose their eyesight because of diseases such as cataract. Since visually impaired people depend a lot on the senses of felling and hearing, they often develop a very keen sense of tough and hearing. Use of Braille: The Braille system is a method of writing for the blind. It employs groups of raised dots to represent printed letters and numbers. It was invented by Louis Braille of France in the early 1800s. It is not a separate language, but just another way to read and write English or any other language. Vinod kumar.k Page 4