6.2.6a Colour and reflection: specular and diffuse (physics only)

Each colour is a narrow band of the visible spectrum

1. Each colour within visible light has its own narrow band of wavelength and frequency, so colours correspond to different ranges of wave properties, not just to names.
2. As with all electromagnetic waves, a shorter wavelength means a higher frequency, so red light and violet light differ in both wavelength and frequency.
3. Colour is set by wavelength and frequency, while brightness is about intensity, a separate idea.

Specular reflection: a smooth surface reflects in one direction

1. When light meets a very smooth surface, such as a mirror or still water, parallel rays reflect in parallel directions, so the reflected rays stay ordered.
2. Because the rays stay ordered, a smooth surface can form a clear image.
3. Examples of specular reflection are a mirror image, a reflection in calm water, and glare from polished metal.

Diffuse reflection: a rough surface scatters the light

1. A surface that looks flat can have tiny bumps on a small scale, and each part reflects light in a different direction, scattering the rays.
2. Because the reflected rays are scattered, a rough surface does not form a clear image, though the light is still reflected.
3. You can see white paper because it reflects light to your eyes, but you see no image in it because the reflection is diffuse.
4. Examples of diffuse reflection are light from paper, a painted wall, and rough stone or fabric.

6.2.6b Colour of objects and filters (physics only)

The colour we see depends on which wavelengths reach the eye

1. The colour of an opaque object is set by which wavelengths it reflects most strongly; the wavelengths it does not reflect are absorbed.
2. A red object looks red in white light because it reflects red wavelengths strongly and absorbs most others.
3. If all visible wavelengths are reflected equally the object looks white; if all are absorbed it looks black.
4. The colour you see therefore depends on both the wavelengths in the light shining on the object and the wavelengths the object reflects or absorbs.
5. Under blue light a red object can look nearly black, because there is little red for it to reflect and it absorbs the blue.

Transparent and translucent objects transmit light

1. Objects that transmit light are either transparent or translucent: clear glass is transparent, while frosted glass is translucent because it scatters the light.
2. A coloured transparent or translucent object acts like a filter, transmitting some wavelengths strongly and absorbing others, as blue glass transmits blue and absorbs most others.

Colour filters transmit some wavelengths and absorb the rest

1. A filter does not add colour: it removes wavelengths by absorbing them and lets the rest through.
2. A red filter transmits red strongly and absorbs most other wavelengths, so white light through it comes out mostly red.
3. If the light does not contain a wavelength the filter can transmit, very little gets through: blue light through a red filter comes out nearly black.