Reflection of Light: Definition, Laws, Types and Diagrams

When a light ray falls on a polished surface, it bounces back. This phenomenon is called reflection of light. We are able to see various objects around us due to reflection as when the light ray after reflection reaches our eyes, it helps us see things. In reflection of light, the angle of incidence of the light is equal to the angle of reflection. The incident ray, the normal, and the reflected ray all lie on the same plane.  As far as plane mirrors are concerned, the images formed are virtual, erect, laterally inverted, and of the same size as the object. They are as far behind the mirror as the objects are in front of them. However, in this article we will focus on reflection through spherical mirror. 

Whether the mirror is curved inwards or outwards, makes the distinction between concave and convex mirrors, respectively. While convex mirrors are diverging in nature, concave ones are converging. 

Keyterms: Light ray, reflection, spherical mirror, concave mirror, convex mirror, plane mirrors, nature

Read Also: Light-Reflection and Refraction Revision Notes

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Some important definitions

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• The centre of curvature(C) is defined as the centre of the hollow glass sphere from which spherical mirrors are formed. 
• The pole(P) is the midpoint of the spherical mirror. 
• The principal axis is defined as a straight line which passes through the pole and the centre of curvature of the mirror. 
• The radius of curvature(R) is the line joining the centre of curvature and the pole of the spherical mirror.
• The focal length is defined as the distance that exists between the principal focus and the pole of the mirror. 
• The aperture is the maximum width of the spherical mirror in which reflection of light takes place. 

The principal focus varies slightly for the two kinds of mirrors. 

• For concave mirrors (c), it is simply the point of intersection of a number of rays parallel to the principal axis. 
• For convex mirrors (d), it is the point at which reflected rays appear to arise from on the principal axis.

(c)

Concave Mirror

(d) 

Convex Mirror

For spherical mirrors with small apertures, the radius of curvature is twice the focal length. 

R = 2f

Hence, the principal focus lies midway between P and C.

Read More About Image Formation by Lenses

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Ray Diagrams

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While an infinite number of rays originate from a source of light, only two of these are used while drawing a ray diagram to enhance clarity,and the intersection of these two determines the image’s position of the point object. 

• A ray of light incident parallel to the principal axis passes through the principal focus after reflection in the case of a concave mirror. 
• In the case of convex mirrors, it appears to diverge from the principal focus.
• When a ray of light passes through the principal focus of a concave mirror, upon reflection, it emerges parallel to the principal axis. 
• In the case of convex mirrors, the only difference is that the ray is directed towards the principal focus of the mirror. 

A ray of light 

• For concave mirrors, passing through the centre of curvature.
• or directed in the direction of the centre of curvature for convex mirrors is reflected backwards along the same path. The cause is that the incident rays fall on the mirror along the normal to the reflecting surface.
• A ray incident obliquely to the principal axis, towards the pole of the mirror for concave and convex mirrors respectively gets reflected back obliquely. Moreover, the incident and reflected rays obey laws of reflection at the point of incidence, leading to the formation of equal angles with the principal axis. 

We will now take a close look at the images formed by concave mirrors for different positions of the object. 

• It is real, inverted, highly diminished, and point-sized. 
• It is real, inverted, and diminished. 
• The image formed is real, inverted, and is of similar size of the object. 
• It is real, inverted, and enlarged.
• It is real, inverted, and highly enlarged. 
• It is virtual, erect, and enlarged. 

Read More About Uses of Convex Lens

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Concave Mirrors

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Some common applications of concave mirrors include torches and headlights for vehicles in order to procure powerful parallel beams of light. They are also used by dentists and by haircut personnel to get magnified images. Further, they have widespread use in sunlight concentrators in solar heat furnaces. 

• When the object is at infinity, the image is formed at the focus F behind the mirror. It is virtual, erect, highly diminished, and point-sized.
• When the object is between infinity and the pole P of the mirror, the image is formed between P and focus F, behind the mirror. It is virtual, erect and diminished.

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Convex Mirrors

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Convex mirrors are particularly useful because they provide albeit diminished but erect images. They provide a wider field of view owing to their outward curvature. They’re used in rear-view mirrors. This typically includes a larger field of vision than what would be obtained by using a plane mirror. 

Mirror Formula

• The object distance (u) is the distance of the object from its pole in a spherical mirror. 
• The image distance (v) is the distance of the image from the pole of the mirror. 
• The focal length(f) is the distance of principal focus from the mirror’s pole. 

The Mirror Formula can thus be written as 

1/v + 1/u = 1/f

Magnification

Magnification(m) from a spherical mirror provides an understanding of the relative extent to which the image of an object is magnified with respect to the object’s size. Alternatively, it is the ratio of the height of the image(h’) to the height of the object(h). 

m = h’/h = – (v/u)

Also Read:

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New Cartesian Sign Convention

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According to this sign convention, the mirror’s pole i.e. P is taken as the origin and the principal axis is taken as the x axis. 

The five standard rules followed are: 

(1) The object is always placed to the left of the mirror, implying that the light from the object falls directly on the mirror from the left-hand side. 

(2) All the distances which are parallel to the principal axis of the mirror are measured from its pole. 

(3) All the distances which are 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. 

(4) Distances that are measured perpendicular to and above the principal axis(along + y-axis) are taken to be as positive. 

(5) Distances that are measured perpendicular to and below the line of principal axis (along - y axis) are taken to be as negative. 

This is as depicted in (u)

New Cartesian Sign Convention