The plane mirror and image formation

How an image is formed in a plane mirror

A plane mirror is a flat, smooth surface that reflects almost all the light it receives in an orderly manner. When you stand in front of a mirror, every point on your body sends out light rays which are reflected according to the law i = r, and your brain reconstructs an image from these reflected rays.

Properties of the image

The image formed by a plane mirror has specific characteristics:

• It is virtual: it cannot be projected onto a screen; it appears to be behind the mirror.
• It is symmetrical about the plane of the mirror.
• It is the same size as the object.
• The distance between the object and the mirror is equal to the distance between the mirror and the image.

Practical example

If you stand 1.5 metres from a mirror, your virtual image appears to be 1.5 metres behind the surface of the mirror, i.e. 3 metres away from you in total.

Left-right reversal

A well-known effect: in a mirror, your right hand appears to become your reflection’s left hand. In reality, this is not a left-right reversal but a front-back reversal (depth is reversed), which creates this impression.

A classic pitfall

Never say that an image in a plane mirror is ‘projected’ onto the mirror: it is virtual; it exists only in the optical extension of the rays, not physically on the surface.