Visible light (physics only) — AQA GCSE Physics

Both kinds of reflection obey the same law: the angle of incidence equals the angle of reflection, both measured from the normal, the line drawn at 90° to the surface where the ray strikes. On a smooth surface such as a mirror or still water, the normals at neighbouring points are parallel, so a parallel beam stays parallel after reflecting. That is specular reflection, and it is why you see a clear image. On a rough surface the normals point in many different directions, so parallel rays leave at many different angles and the light is scattered. That is diffuse reflection, and no image forms. The light is not absorbed any more strongly, just spread out, which is why a sheet of white paper looks bright but shows no reflection of your face. Each individual ray still obeys the law of reflection.

A colour filter is a piece of coloured transparent material. It transmits the wavelengths of its own colour and absorbs the rest, so the light that comes out is both coloured and dimmer than the light that went in. A blue filter transmits blue and absorbs the green and red parts of white light; a red filter transmits red and absorbs the rest. Colourless glass absorbs almost nothing, so white light passes through and stays white. The missing colours are not sent back or bent away, they are absorbed, and their energy raises the internal energy of the filter. Put a blue filter and a red filter in series and almost no light gets through at all, because whatever the first one transmits the second one absorbs.

Opaque means no light passes through, so everything you see from the object has bounced off its surface. The surface reflects some wavelengths much more strongly than others, and the mixture that is reflected into your eye is the colour you perceive. A tomato looks red in white light because the long wavelength part of the spectrum is strongly reflected while the rest is absorbed. Because the colour depends on the reflected light, it depends on the light shining on the object as well as on the surface itself: put that tomato under pure green light and it looks black, because there is no red in the incident light for it to reflect. Real surfaces reflect a spread of wavelengths, which is why the specification says more strongly reflected rather than only reflected.

For an opaque object the incident light has only two possible fates: it is reflected or it is absorbed, because none is transmitted through. The two shares add up to all of the incident light, so once you know which wavelengths are reflected you also know that everything else has been absorbed. A green leaf reflects the middle of the spectrum and absorbs the red and blue. Absorbed light is not destroyed; its energy is transferred to the internal energy store of the surface, which is why a black car roof in sunlight becomes far hotter than a white one. That same idea carries straight over into the infrared work: a matt black cube absorbs the most and shows the largest temperature rise.

White is not a wavelength of its own. A surface looks white because it reflects all the visible wavelengths about equally, so the mixture leaving the surface is the same mixture that arrived. Very little is absorbed, which is why white surfaces warm up slowly: in the AQA cube experiment the shiny white cube rose only 2.0 °C while the matt black one rose 6.5 °C in the same time. Because a white object simply returns whatever it is given, its appearance follows the illumination: shine red light on white paper and it looks red, not white. Reflecting every wavelength is not the same as letting every wavelength through, which is what a clear, transparent object does.

A black surface absorbs all, or nearly all, of the visible wavelengths that land on it, so almost no light is reflected back to your eye and you see black. Black is the absence of reflected light, not a colour being sent out. Because a black object has nothing to reflect whatever colour you shine on it, it stays black under red, green or blue light, unlike a white or coloured object. The absorbed energy is transferred to the internal energy store of the surface, so black objects warm quickly: in the AQA cube experiment matt black gave the largest temperature rise, 6.5 °C, against 2.0 °C for shiny white. Surface texture matters as well as colour, so matt black absorbs more than shiny black.

Objects that let light through are of two kinds. A transparent material, such as clear glass or still water, transmits light with almost no scattering, so rays keep their directions and you can see a sharp image through it. A translucent material, such as frosted glass, tracing paper or tissue, also transmits light but scatters it as it passes, so light gets through while directions are jumbled and only a blur is seen. An opaque object transmits none at all. The same material can move between the two: polish frosted glass and it becomes transparent, because the scattering comes from the rough surface rather than the glass itself. A coloured transparent object also absorbs some wavelengths, which is what makes it a filter.

Questions on this content chain several steps together, so work through them in order: what light leaves the source, what the filter transmits and absorbs, what the surface reflects and absorbs, and what finally reaches the eye. Colourless glass transmits all of the white light, so nothing changes. Blue glass transmits only the blue and absorbs the other colours, so anything seen through it is lit by blue light alone. A red book viewed through a blue filter then looks black, because the only light reaching it is blue and the red surface absorbs blue. Each transmitted colour and each absorbed colour is credited separately, so name both rather than describing the result as the light changing colour.