Reflection
Light Sound and Waves

Not just mirrors

Physics Narrative for 11-14 Supporting Physics Teaching

Reflection from any surface

The law of reflection is usually illustrated in science textbooks with flat or plane mirrors. You should be aware, however, that it applies to the reflection of light at any surface, not just the shiny ones that you might usually associate with reflection.

For example, each point on a stone wall reflects light such that the angle of incidence equals the angle of reflection. Here you need to picture a tiny piece of the wall's surface, which acts as a plane reflector. Having to hand a piece of rock containing mica flakes, which are shiny, might help to make the link to the reflection of light from a duller rock.

Once again, you can see why the angles of incidence and reflection are measured in relation to the normal line. In this case of reflection from an uneven stone wall, the reflecting surface does not provide a straight line to measure angles against. Reflection from such a rough surface is sometimes referred to as diffuse reflection. It's what enables us to see many of the objects in our environment.

Paint and reflections

Surfaces are made to be reflective in different ways to create different moods.

DIY shops sell different kinds of paint which are designed to provide contrasting effects not only in terms of colour but also in the ways in which light is reflected from the surface. The two extremes of finish are gloss and matt, with satin in between.

For gloss paint the particles at the surface of the paint are very small and when the paint dries they end up forming what is in effect a very flat, plane surface which acts just like a mirror. Any light shining onto this surface is reflected regularly and it may be possible to see the image of an object (your face!) as light is reflected.

At a microscopic level the surface of matt paint, when it dries, is very uneven. When magnified it resembles a pebbled beach. Here the light from a luminous source will hit each pebble and whilst each small part of each pebble will reflect light according to the laws of reflection, the overall effect is that the light is scattered in all directions. Our eyes gather light from parts of many different pebbles as diffuse reflection occurs, making the matt finish appear dull in comparison with the gloss surface.

Reflection
is formalised by Law of Reflection
can be exhibited by Progressive Wave
has the special case Total Internal Reflection
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