LIGHT REFLECTION AND REFRACTION

Transcription

1 A ray of light is the straight line path along which the light travels in a homogenous medium. A beam of light is a group of rays of light emitted by a source of light. A beam of light is of three types: o Parallel beam o Divergent beam o Convergent beam 1. Reflection of light When a light ray travelling in a medium encounters a boundary leading into a second medium, part of the incident ray is thrown back into the original medium. This phenomenon is called as the reflection of light. Reflection is of two types: i. Diffuse reflection / Irregular reflection It is the reflection of light from any rough surface. ii. Specular reflection / Regular reflection It is the reflection of light from any smooth surface. Laws of Reflection i. The angle of incidence (i) is equal to the angle of reflection (r). ii. The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane. 2. Image When the light rays meet or appear to meet after reflection from a mirror, it is called as an image. a) Real image This image is formed by actual intersection of light rays after reflection. b) Virtual image This image is formed by producing the reflected rays backward after reflection. 3. Mirrors A plane mirror is a piece of glass whose one side is polished by using silver paint which is covered by a coating of red paint to protect the silver layer. It gives a virtual image. A curved mirror is a mirror with a curved reflecting surface, which may be either concave or convex. The curved mirrors converge or diverge the rays of light incident on them. E.g. spherical mirror.

2 A spherical mirror is a part of hollow glass sphere whose one surface is polished. There are two types of spherical mirrors: a) Concave mirror (Converging mirror) It has a reflective surface that bulges inward (away from the incident light) and whose outer surface is polished. It gives real images. b) Convex mirror (Diverging mirror) - Its inner surface is polished and the outer side is the reflecting surface. The straight line passing through the pole and the centre of curvature of the mirror is called the principal axis. Principal focus is a point on the principal axis where the rays of light parallel to the principal axis meet or appear to meet after reflection from the mirror. The distance between the pole (P) and the focus (F) is called as the focal length. It is represented as f. The focal length is half the curvature. Uses of concave mirror 1) It is used as a shaving mirror since it forms a large image when it is placed close to the face. 2) It is also used in solar devices like solar cooker, as it converges sun rays over a small area to produce high temperature. 3) It is used as a reflecting mirror for projector lamps. 4) It is used in torches and head lights. 5) The dentists use concave mirrors to see large images of the teeth of patients. Uses of convex mirrors 1) It is used in street lights. 2) It is used as a rear view mirror in automobiles as it gives erect as well as diminished image due to which it has wider field of view. According to the new Cartesian sign convention, the pole (P) of the mirror is taken as the origin. The principal axis of the mirror is taken as the x-axis (X X) of the coordinate system. The conventions are as follows i. The object is always placed to the left of the mirror. This implies that the light from the object falls on the mirror from the left-hand side. ii. All distances parallel to the principal axis are measured from the pole of the mirror.

3 iii. All the distances measured to the right of the origin (along + x-axis) are taken as positive while those measured to the left of the origin (along x-axis) are taken as negative. iv. Distances measured perpendicular to and above the principal axis (along + y-axis) are taken as positive. v. Distances measured perpendicular to and below the principal axis (along y-axis) are taken as negative. Mirror formula It is a relation between the object distance (u), the image distance (v) and the focal length (f) of a mirror. According to this formula, the three are related as: The linear magnification is defined as the ratio of the height of the image (h) to the height of the object ( ). It is denoted by (m). Mathematically we have, m =

4 4. Refraction of light When a ray of light enters from one transparent medium to another in an oblique angle i.e. except in normal angle, bending takes place. This bending of light ray is called as refraction. Laws of refraction : i. The incident ray, the refracted ray and the normal to the surface of separation of the two media at the point of incidence all lie in the same plane. ii. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to the inverse ratio of the two indices of refraction of the two media. The refractive index of medium 2 with medium 1 is given by the ratio of the speed of light in medium 1 and the speed of light in medium 2. This is represented as. This can be expressed as: Since the refractive index is a ratio of two similar quantities, it does not have a unit. 5. Refraction by spherical lenses Lens is a transparent medium which is formed by joining two pieces of spherical glass. There are two types of lenses: i. Converging or convex lens This type of lens are thicker at the centre and thinner at the edges.

5 ii. Diverging or concave lens This type of lens are thicker at the edges and thinner at the centre. The converging and diverging lens are further classified as follows: Convex lenses : 1) Meniscus 2) Planoconvex 3) Bi-convex or double convex Concave lenses : 1) Meniscus 2) Planoconcave 3) Bi-concave or double concave 6. Lens formula This formula gives the relationship between object distance (u), image distance (v) and the focal length (f). The lens formula can be expressed as: 7. Linear magnification of lenses Linear magnification (m) is defined as the ratio of the size of the image ( ) to the size of the object (h). It can be expressed as: m =

6 8. Power of a lens Different lenses bend the rays of light falling on it by different amounts. This ability of a lens is called as its power. The power of lens (P) is defined as its ability to converge or diverge a beam of light falling on it. It is measured as the reciprocal of focal length of a lens in metre. Thus, P = S.I unit of power of lens is dioptre. One dioptre is the power of a lens whose focal length is 1m. 9. Power of combination of two or more lenses If two or more lenses are placed together to form a combined lens, then the power of this combined lens is equal to the sum of the powers of individual lenses. P = P 1 + P 2 + P Terms related to a lens a) Optical centre of the lens It is defined as the centre of the lens through which light can pass without any deviation. b) Principal axis It is defined as the line passing through the optical centre and is perpendicular to the line joining its edges. c) Principal focus It is defined as a point on the principal axis where all the light rays which are parallel to principal axis either converge or appear to diverge from, after refraction. d) Aperture The effective width of a lens through which refraction takes place is called as the aperture. 11. Applications of spherical lenses a) A convex lens is used as a magnifying glass e.g. by a watch maker, a student in the laboratory etc. b) In the microscope and a telescope for viewing near and distant objects and in other optical instruments. c) Lenses are used in a camera to form the image of an object on the screen or photographic film. d) A concave lens of suitable focal length is used to cure myopia. e) A convex lens is used to get rid of hypermetropia. f) Lenses are also used in photocopies, projector etc.