NCERT Solutions for Class 10th Science
Chapter 10. Light-Reflection and Refraction
- Light seems to travel in straight lines.
- The laws of reflection of light (i) The angle of incidence is equal to the angle of reflection. (ii) The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.
- Image formed by a plane mirror is always virtual and erect. The size of the image is equal to that of the object.
- The reflecting surface of such mirrors can be considered to form
a part of the surface of a sphere. Such mirrors, whose reflecting surfaces are spherical, are called spherical mirrors.
- A spherical mirror, whose reflecting surface is curved inwards,
that is, faces towards the centre of the sphere, is called a concave mirror.
- A spherical mirror whose reflecting surface is curved outwards, is called a convex mirror.
- The centre of the reflecting surface of a spherical mirror is a point called the pole.
- The reflecting surface of a spherical mirror forms a part of a sphere.
This sphere has a centre. This point is called the centre of curvature.
- The radius of the sphere of which the reflecting surface of a spherical mirror forms a part, is called the radius of curvature.
- A straight line passing through the pole and the centre of curvature of a spherical mirror. This line is called the principal axis.
- The distance between the pole and the principal focus of a spherical mirror is called the focal length.
- The diameter of the reflecting surface of spherical mirror is called its aperture.
- The nature, position and size of the image formed by a concave mirror depends on the position of the object.
- Concave mirrors are commonly used in torches, search-lights and
vehicles headlights to get powerful parallel beams of light.
- Convex mirrors are commonly used as rear-view (wing) mirrors in
- The distance of the principal focus from the pole is called the focal length.
- Light travels in vacuum with an enormous speed of 3×108 m s-1. The speed of light is different in different media.
- A ray of light travelling from a rarer medium to a denser medium slows down and bends towards the normal. When it travels from a denser medium to a rarer medium, it speeds up and bends away from the normal.
- A lens have two spherical surfaces, bulging outwards. Such a lens is called a convex lens. It is thicker at the middle as compared to the edges. Convex lens converges light rays. Hence convex lenses are called converging lenses.
- A lens have two spherical surfaces curved inwards. It is thicker at the edges than at the middle. Such lenses diverge light rays.Such lenses are called concave lens or diverging lenses.
- The SI unit of power of a lens is ‘dioptre’.
- The power of a convex lens is positive and that of a concave lens is negative.