Introduction to Reflection | Physics Grade XI Reference Note

Optics

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Introduction to Reflection | Physics Grade XI Reference Note

Introduction to Reflection

The branch of science which deals with the phenomenon of light is called optics.
A ray of light is the path in which the light travels.
A beam is a collection of rays of lights.
Reflection is the phenomenon of returning the light after striking at a surface.

Laws of reflection

  1. The incident ray, the reflected ray, and the normal at the point of incidences lie in the same plane.
  2. The angle of incidence is equal to the angle of reflection.
Laws of ReflectionDeviation of light by a plane mirror
Glancing angle is made by an incident ray with the reflecting surface. Its value in a mirror varies from 0 degree at grazing incident to 90 degree at the normal incidence.

Deviation of light by plane mirrorLet us consider a plane mirror XY. When a ray of light AO is incident at point O on the surface of the mirror with glancing angle θ, the ray is reflected along OB as shown in the figure. ON is a normal drawn at point O.

AOC is the path of the incident ray before reflection and OB is the path of reflection. The angle δ between the paths of the incident ray and reflected ray is called the angle of deviation. We can write, angle of deviation, δ =

or, δ = 90 – r + θ = 90 – r + θ = θ + θ = 2θ
Thus, the angle of deviation of a ray by a plane mirror is equal to the twice of glancing angle.

Real and virtual images

Real image

Virtual Image

Real image

Virtual image

An image is said to be real when it is formed by the actual intersection of light rays as shown in the figure;

An image is said to be virtual when it is formed by the virtual intersection of light rays and do not actually pass through the position of an image but only appear to come from that point as shown in the figure.

It can be obtained on screen

It cannot be obtained on screen

It is formed by concave mirror only

It is formed by both concave and convex mirror

Terms used in Spherical mirrors

  1. Pole: The center of the spherical reflecting surface of the mirror is called pole, P.
  2. Center of Curvature: The center of the sphere which is a part of the mirror is called center of curvature, C.
  3. Radius of Curvature: The radius of the sphere which is a part of the mirror is called radius of curvature. It is denoted by R. PC is the radius of curvature.
  4. Principal axis: It is the line joining the pole and the center of curvature of the mirror.
  5. Principal focus: It is the point where all the parallel beam of light after reflection is believed to converge or appear to diverge from that point.
  6. Focal length: The distance between the pole and principle focus of a mirror is called the focal length.
  7. Aperture: The diameter of the boundary of the mirror is called aperture.
  8. Focal plane: A plane passing through the principle focus of a mirror at right angles to its principle axis is called the focal plane of the mirror.

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