We see variety of objects in the world around us. However we are unable to see anything in a dark room On lighting up the room things become visible. What makes things visible? During the day sunlight helps us to see the objects. An object reflects light that falls on it. This reflected light received by our eyes enables us to see things. We are able to see through a transparent medium as light is transmitted through it. There are a number of wonderfull phenomenon associated with light such as image formation by mirrors, the twinkling of stars, the beautiful colours of rainbow bending of light by a medium, and so on. we shall study in this course the reflection of light by spherical mirrors and refraction of light and their application in real life situations. Light rays change direction when they reflect off a surface, move from one transparent medium into another, or travel through a medium whose composition is continuously changing. The law of reflection states that, on reflection from a smooth surface, the angle of the reflected ray is equal to the angle of the incident ray. (By convention, all angles in geometrical optics are measured with respect to the normal to the surfacethat is, to a line perpendicular to the surface.) The reflected ray is always in the plane defined by the incident ray and the normal to the surface. The law of reflection can be used to understand the images produced by plane and curved mirrors. Unlike mirrors, most natural surfaces are rough on the scale of the wavelength of light, and, as a consequence, parallel incident light rays are reflected in many different directions, or diffusely. Diffuse reflection is responsible for the ability to see most illuminated surfaces from any positionrays reach the eyes after reflecting off every portion of the surface. When light traveling in one transparent medium encounters a boundary with a second transparent medium (e.g, air and glass), a portion of the light is reflected and a portion is transmitted into the second medium. As the transmitted light moves into the second medium, it changes its direction of travel; that is, it is refracted. The law of refraction, also known as Snells law, describes the relationship between the angle of incidence (1) and the angle of refraction (2), measured with respect to the normal (perpendicular line) to the surface, in mathematical terms: n1 sin 1 = n2 sin 2, where n1 and n2 are the index of refraction of the first and second media, respectively. The index of refraction for any medium is a dimensionless constant equal to the ratio of the speed of light in a vacuum to its speed in that medium.
