Reflection and refraction
Physics · Key Stage 3 · The School
Reflection has a law
The angle of incidence equals the angle of reflection, both measured from the NORMAL — the line at 90° to the surface, not from the surface itself. Measuring from the surface is the commonest error in this topic and turns every answer wrong by exactly 90°.
Refraction: bending at a boundary
Light travels slower in glass or water than in air. Entering at an angle, one side of the beam slows first, so the beam bends TOWARD the normal going in and AWAY from it coming out. That is why a straw in a glass looks broken, and why a pool is always deeper than it looks.
Colour
White light is a mixture, which a prism separates by refracting each colour by a different amount. An object looks red because it REFLECTS red and ABSORBS the rest — the colour is in the light, not the object. Under pure red light a blue jumper looks black: there is no blue to reflect.
Under white light a red jumper looks red, because white contains every colour and the jumper reflects the red and absorbs the rest. Under pure RED light it still looks red — there is red to reflect. Under pure BLUE light it looks black, because there is no red arriving and the blue is absorbed. Nothing about the jumper changed. Colour is a property of the light reaching your eye.
Coming OUT of glass into air, light bends away from the normal. Increase the angle and the bending increases, until at the CRITICAL ANGLE the beam skims along the surface. Past that it cannot leave at all and reflects back inside: TOTAL INTERNAL REFLECTION. It is not a special rule, just refraction taken to its limit. It is what keeps a signal inside an optical fibre for miles, and what makes a cut diamond flash.