Explain how colour is related to differential absorption, transmission and…

The idea

An object looks coloured because of what happens to the different wavelengths of white light when they reach it. A surface absorbs some wavelengths and reflects others; the wavelengths that reach the eye determine the perceived colour. A red apple in white light absorbs most wavelengths and reflects mainly red, so it appears red. A filter transmits selected wavelengths and absorbs the rest, so a green filter lets green through and blocks red and blue. Specular reflection occurs at a smooth surface, where parallel rays reflect in one direction and a clear image can form. Scattering occurs at a rough surface, where light is reflected in many directions, so no clear image forms and the surface looks matt. A coloured surface under light of a different colour may appear black if that light contains no wavelength the surface can reflect.

What earns the marks

• Explains that an object appears a given colour because it reflects or transmits that wavelength and absorbs the others.
• Distinguishes absorption from reflection and transmission when accounting for the colour seen.
• Describes specular reflection at a smooth surface as reflection in a single direction that can form a clear image.
• Describes scattering at a rough surface as reflection in many directions that prevents a clear image forming.
• Applies the idea to a case such as a red object in blue light appearing black because no red light is available to reflect.

Common mistakes and how to put them right

• States that an object gives out the colour it appears.

  Put it right: Explain instead that it reflects or transmits that wavelength and absorbs the rest.

• Treats specular reflection and scattering as the same process.

  Put it right: Distinguish reflection in one direction at a smooth surface from reflection in many directions at a rough surface.

• Assumes a red object always looks red.

  Put it right: Explain that in light with no red component it absorbs what arrives and appears black.

Exam tips

• Name the wavelengths absorbed and the wavelengths reflected or transmitted, rather than saying only that the object is that colour.
• Use the words specular and scattering precisely, linking each to smooth or rough surfaces and to whether a clear image forms.
• Check the colour of the incident light before predicting the colour seen, because the available wavelengths limit what can be reflected.