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Q & A: How mirrors work

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Most recent answer: 10/22/2007
Q:
how dose mirror work?
- muhannad (age 13)
impington village colege, uk
A:
Hi Muhannad,

Mirrors are some of the simplest devices to make and also some of the most useful. To make a mirror, all you need is a polished piece of shiny material. Most metals are shiny when polished, and there is an important reason for this. Metals are very good conductors of electricity. For this reason, the electric field inside a metal has to be zero. If the electric field were not zero inside a metal, then electrons will move very quickly to cancel any such field. If a field is applied from outside, then electrons will move around to cancel it inside.

Light consists of traveling electric and magnetic waves. When a light wave strikes a shiny surface, that is, a flat one which conducts electricity well, then the electric field must vanish everywhere on the surface. The only way to satisfy this condition and to have a light wave coming in is if there is also a light wave coming out, with exactly the opposite electric field as the incoming wave in all places on the surface of the mirror at all times. This process is very similar to the reflection of a mechanical wave on a rope or chain that is attached to a post at one end. If you give the free end of the rope a good shake (just once), you can watch the wave travel to the fixed end at the post, and then an upside-down wave will come back at you. Here are some of this process and others. Reflection of an electromagnetic wave from a mirror corresponds to the "hard boundary" where the endpoint is fixed at zero for the electric field.

If the light wave hits a mirror not quite head on but at an angle, then the reflected wave comes off at the same angle w.r.t. to a line drawn perpendicular to the plane of the mirror. This process (plus another animation!) is described on our .

Mirrors can be curved to focus light as in telescopes, or curved the other way to allow a wider range of view, as in the right-hand-side rear-view mirrors in many cars these days.

Tom

(published on 10/22/2007)

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