Railway Science (Physics-Light and Optics) (Part-III)

Principal focal plane. Any ray of light that passes through the mirror is always parallel to the principal axis. A ray of light that passes through the focus of the mirror becomes parallel to the principal axis of the mirror after reflection. A ray of light passing through the centre of curvature of any mirror is reflected back along the same path. Any incident ray which isn’t parallel to the principal axis is also reflected diagonally and the incident ray and the reflected ray always follow the laws of reflection i.e. the angles formed by these rays are equal to each other.

Un-Deviated. The ray passing  through the optical centre does not deviate because the curvature of the lens at the incident and exit points are exactly opposite so the deviation at the first point is cancelled at the other point. For a thin lens the ray appears to be passing straight but for a thick Lens there will be a lateral shift i.e incident ray will be parallel to emergent ray but displaced.

Only (a) and (b) are correct. Pole: The geometric center of the spherical surface of the mirror is called the pole of the mirror. Principal axis: It is the straight line joining the pole of the mirror to its centre of curvature. It is straight for all kinds of mirrors. Centre of curvature of a mirror is the centre of the sphere of which the mirror is a part.

Rules for sign convention: The distances which are measured along the direction of the incident ray are taken as positive. The distances which are measured opposite to the direction of the incident ray are taken as negative. The region above the principal axis is considered positive. The region below the principal axis is considered negative. All measurements should be taken from the Pole of the mirror.

(S - I) and (S - III) only. Any ray of light that passes through the mirror is always parallel to the principal axis. Any ray of light that passes through the mirror always passes through the principal focus (f) of the mirror after reflection. A ray of light passing through the centre of curvature of any mirror is reflected back along the same path. Any incident ray which isn’t parallel to the principal axis is also reflected diagonally and the incident ray and the reflected ray always follow the laws of reflection i.e. the angles formed by these rays are equal to each other.

μ = c/v . Refractive index refers to the measure of the bending of a ray of light when it passes from one medium to another medium. It is defined as the ratio of the speed of light in a vacuum to the speed of the light in the medium. Some refractive indices - air (1.0003); water (1.333); and diamond (2.417).

Convex lens. Water acts as a convex lens and magnifying glass. Whenever light travels from one medium to another, the rays bend towards or away from the normal. This is called the refraction of light. When light moves from rarer to denser mediums, the rays are bent towards the normal.

Plane, convex and concave. If in the magic mirror the child looks at her head of the same size then in the top part the plane mirror is used. His middle portion of the body is looking smaller, then in the middle part a convex mirror is used. The convex mirror produced the erect and diminished image. The bottom part of the mirror produces a bigger image of the legs. The image is bigger since it is produced by a concave mirror. A concave mirror produces an enlarged and erect image.

Principal axis. Paraxial rays are nothing but a set of incident rays on the mirrors which lie very close to the principal axis. The focal length (f) is defined as the distance between the focus and the pole of the mirror. The radius of curvature (R) is defined as the radius of the mirror that forms a complete sphere.

0°. As the ray of light passes through the centre of curvature of a concave mirror it strikes the mirror along the normal (i.e. it incidences on to the mirror at 90 degree and 0 degree with normal). Hence the incident ray coincides with the normal. Therefore the angle of incidence is 0 degree.