Laws of reflection
- The incident ray, the reflected ray, and the normal at the point of incidences lie in the same plane.
- The angle of incidence is equal to the angle of reflection.
Deviation of light by a plane mirror
Let 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
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Real image |
Virtual image |
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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. |
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It can be obtained on screen |
It cannot be obtained on screen |
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It is formed by concave mirror only |
It is formed by both concave and convex mirror |
Terms used in Spherical mirrors
- Pole: The center of the spherical reflecting surface of the mirror is called pole, P.
- Center of Curvature: The center of the sphere which is a part of the mirror is called center of curvature, C.
- 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.
- Principal axis: It is the line joining the pole and the center of curvature of the mirror.
- 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.
- Focal length: The distance between the pole and principle focus of a mirror is called the focal length.
- Aperture: The diameter of the boundary of the mirror is called aperture.
- 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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