Lightnotes-1

LIGHT Light is a form of energy. It is an electromagnetic wave as it does not require any medium for its propagation. The straight line path assumed by light in its propagation is called a ray of light. Rectilinear Propagation of light: - Light always travels in straight lines when propagated through a medium between two points; this is called rectilinear propagation of light. All EM waves including light move with a constant speed in vacuum which is given by Laws of reflection The angle of incidence is equal to the angle of reflection The incident ray, the normal at the point of incidence and reflected ray lie in the same plane. Image The rays from a point object incident on a mirror when reflected either intersect or appear to intersect at a point which is called the image of that object. Real Image Virtual Image It is formed by actual intersection of the reflected or refracted rays. It is formed when the reflected or refracted rays appear to intersect. It can be obtained on a screen. It cannot be obtained on a screen. It is always inverted. It is always erect. An image formed by plane mirror is Virtual Erect Laterally inverted Same size as the object At the same distance from the mirror as the distance of the object from the mirror. Spherical Mirrors Concave Mirror A part of a hollow spherical glass which is silvered from the outer side so that the inner side becomes reflecting, is called a concave mirror. It is also called a converging mirror. Convex Mirror A part of a hollow sphere, silvered from the inner side, so that the outer side becomes reflecting is called convex mirror. It is also called a diverging mirror. Centre of Curvature The centre of that sphere of which the mirror is a part is called the centre of curvature of the mirror. Radius of curvature The radius of the sphere of which the mirror is a part is called radius of curvature of the mirror. Aperture The diameter of the reflecting surface of a mirror is called its aperture. Pole The centre of the reflecting surface of spherical mirror is called its pole. Principle Axis The line joining the centre of curvature of a spherical mirror to its pole is called its principle axis. Focus The rays incident parallel to the principle axis of a spherical mirror when reflected either meet or appear to meet at a point on the principle axis. This point is called as the focus of the spherical mirror. Focal Length The distance between the pole and the focus of a spherical mirror is called its focal length. Incident ray Reflected ray Through F or towards F Parallel to the principle axis. Parallel to the principle axis. Through F or appear to diverge from F. Through C or towards C Retrace back. Uses of concave Mirror Concave mirror is used in solar concentrators. The sun rays can be concentrated at one point to obtain a high temperature using a concave mirror as it converges the rays coming from infinity at its focus. Concave mirrors reflect the light rays incident on them from the focus, parallel to the principal axis. Thus, to obtain a parallel beam of light as in vehicle lamps or search lights a source of light can be kept at the focus of a concave mirror. When the object is placed between the focus & the pole of a concave mirror a magnified virtual & erect image is formed. That is why they are used in making shaving mirrors and make-up mirrors. Uses of convex mirror Converse mirror are used in making rear view mirrors used in automobiles for the following reason. It always makes a virtual, erect and diminished image of an object. It has a wide range of vision.