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We see things because light from a light source reflects off their surface. Most surfaces are rough, or irregular, which scatters the light in all directions. However, if the surface is smooth, we see a regular reflection, like a mirror.

Light is reflected on plane mirrors according to the Law of Reflection: the angle of reflection is equal to the angle of incidence.

These angles are measured to the normal - an imaginary line perpendicular to the surface of the mirror.

Law of reflection
A plane mirror reflects incident light at the same angle to the normal.

Ray Diagrams

In order to identify where an image is being formed, we use 'ray diagrams'. These show the incidence and reflection rays of light, and also an imaginary line that extends behind the mirror. This is the ray that the eye perceives, and locates the image observed on the other side of the mirror the same distance as the physical object on the observer's side of the mirror.

Plane mirror reflection ray diagram
A plane mirror creates a virtual image behind the mirror equidistant to the object, as this ray diagram illustrates.

Virtual images are formed by tracing two light rays and obeying the law of reflection. First draw the normals for the rays from the top and bottom of the object. Light appears to go in a straight line, so extend the ray behind the mirror to a point at the same distance behind the mirror as the object is from the mirror. The image is the same size as the object

Plane mirror reflection ray diagram
A plane mirror at an angle does not change the rule that the angle of incidence equals the angle of reflection. In this diagram, how a virtual image is formed is shown.

Scattered light

Why do some materials, like metal and water, reflect light evenly, forming an image, but most materials, like wood and plaster, do not?

If the surface of a material is very even, the light can create a virtual image for the observer because the light rays maintain their relative angles after reflection. Rough surfaces, with bumps and cracks, reflect the light in many different directions, so the observer just sees a lot of rays in a jumbled pattern, and no image.

Surfaces that are flat and even appear shiny, and surfaces that are uneven appear matt


Shinyness is due to two things: the flatness of the surface, and the amount and frequency range retransmitted from the incident ray.

If some frequencies are absorbed by the surface, the light that is reflected has a colour, since white light requires all frequencies of visible light to be present. The silver on mirrors does not absorb any frequencies, and has a high reflectivity.

Content © Andrew Bone. All rights reserved. Created : January 13, 2014

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