White light and colour

1. White light, such as sunlight or the light from a white LED bulb, is a mixture of all the colours of visible light.
2. The colours of the visible spectrum, in order of decreasing wavelength, are red, orange, yellow, green, blue, indigo and violet.
3. Red light has the longest wavelength, at about $700nm$, and violet light has the shortest, at about $400nm$.
4. Each colour is a range of wavelengths, and your eye and brain interpret each range as a particular colour.
5. The colour of an object depends on which wavelengths from the light falling on it end up reaching your eye.
6. Light reaches your eye from an object only if the object reflects it or transmits it, because absorbed light never leaves the object.

The colour of opaque objects

1. An opaque object transmits no visible light, so every wavelength falling on it is either reflected or absorbed.
2. Most objects show differential absorption, which means they absorb some wavelengths strongly and reflect others.
3. The colour you see is the colour of the wavelengths that are reflected into your eye.
4. A red post box looks red in daylight because its paint reflects red wavelengths and absorbs the other colours in white light.
5. A green leaf looks green because it reflects green wavelengths and absorbs most of the red and blue light.
6. A white object, such as a sheet of paper, reflects all visible wavelengths roughly equally, so the full mixture reaches your eye and it looks white.
7. A black object, such as a car tyre, absorbs all visible wavelengths, so very little light is reflected and it looks black.
8. The absorbed light transfers energy to the object, which is why a black car seat becomes much hotter in summer sunshine than a white one.
9. An object can only reflect colours that are present in the light shining on it.
10. A red post box seen under pure blue light looks black, because it absorbs the blue light and there is no red light for it to reflect.
11. A white object takes on the colour of the light shining on it, so a white shirt looks green under green light.

Transparent objects and colour filters

1. A transparent coloured object, such as a bottle of green glass, transmits some wavelengths and absorbs the rest.
2. Its colour is the colour of the light it transmits, because that is the light that passes through to your eye.
3. A colour filter is a thin sheet of coloured plastic or glass that transmits one band of colours and absorbs all the others.
4. A red filter placed in front of a white lamp transmits red light and absorbs every other colour, so the light that emerges is red.
5. The filter does not add colour to the light or change one colour into another, because it can only remove the wavelengths it absorbs.
6. Red light shone at a red filter passes through almost unchanged, because red is the colour that the filter transmits.
7. Blue light shone at a red filter is absorbed, so almost no light emerges and the screen behind it looks dark.
8. White light sent through a red filter and then a green filter is almost completely blocked, because the red filter removes everything except red and the green filter then absorbs the red.
9. Every filter absorbs a little of the colour it transmits as well, so the light that emerges is always slightly dimmer than the light that went in.
10. Stage lighting and traffic lights both use this idea, with a white lamp behind a coloured filter giving the coloured beam.

Specular reflection and scattering

1. Every ray that reflects from a surface obeys the law of reflection at the exact point where it strikes, with the angle of reflection equal to the angle of incidence.
2. Specular reflection happens at a very smooth surface, such as a mirror, a still pond or polished metal.
3. On a smooth surface the normals at neighbouring points all point the same way, so a set of parallel incident rays leaves as a set of parallel reflected rays.
4. The reflected rays keep their arrangement, so specular reflection produces a clear image, as you see in a bathroom mirror.
5. Scattering, also called diffuse reflection, happens at a rough surface, such as paper, cloth or a painted wall.
6. On a rough surface the normals at neighbouring points point in many different directions, so parallel incident rays are reflected in many different directions.
7. Scattered light does not form an image, but it reaches your eye from every viewing angle, which is why you can read a poster from anywhere in a classroom.
8. Most objects around you are seen by scattered light, and their colour comes from the wavelengths that they scatter rather than absorb.
9. A gloss-painted door shows a bright shiny patch where light is reflected specularly, while a matt-painted wall of the same colour looks evenly lit from every angle.
10. A wet road at night looks like a mirror because a film of water makes its surface smooth, while the same road when dry scatters the light from headlights.
11. A translucent material, such as frosted glass or tracing paper, transmits light but scatters it as it passes through, so objects seen through it look blurred.