5.3P Explain how colour of light is related to a differential absorption…

The idea

White light contains a range of colours. When light meets a surface, some colours are absorbed more than others; this differential absorption decides which colours are reflected or scattered to the eye, so the surface looks coloured. A red surface absorbs most colours strongly but reflects red strongly, so it appears red. A white surface absorbs little and reflects most colours; a black surface absorbs most colours and reflects little. A filter works by transmission: its pigment or dye absorbs some colours and transmits others. A red filter transmits red light and absorbs the rest, so white light seen through it looks red. A green object viewed through a red filter looks dark because little red light reaches the eye from it.

What earns the marks

• States that a surface appears a given colour because it reflects that colour strongly and absorbs other colours more.
• Explains that a filter transmits certain colours and absorbs the others, so the transmitted light has the colour of the filter.
• Predicts that a red object seen through a green filter looks dark because the object reflects little green light.
• Distinguishes absorption at a surface from transmission through a filter as two different processes.

Common mistakes and how to put them right

• Claims a filter adds colour to the light.

  Put it right by stating the filter absorbs some colours and transmits the rest.

• Assumes a coloured object emits its own colour.

  Put it right by explaining that the object reflects that colour from the light falling on it.

• Expects a red object to look red through any filter.

  Put it right by noting that the filter must transmit red light for the object to appear red.

Exam tips

• Link each colour to which wavelengths are absorbed and which are reflected or transmitted.
• Use the words absorb, reflect and transmit precisely, matching each to surface or filter.
• Check predictions for coloured objects seen through a differently coloured filter by asking which colours reach the eye.