Object colour depends on which wavelengths reach the eye
Definition
Absorption
The transfer of energy from electromagnetic radiation to a material when the radiation is taken in rather than reflected or transmitted.
- White light contains the visible wavelengths that the eye interprets as different colours.
- An opaque surface appears a particular colour because it reflects that colour or range of colours and absorbs most of the others.
- A red object in white light reflects red light and absorbs most other visible wavelengths.
- A white surface reflects most visible wavelengths, while a black surface absorbs most visible wavelengths.
- The observed colour depends on both the wavelengths in the incident light and the wavelengths the surface reflects.
- A red object illuminated only with blue light appears very dark because there is no red light to reflect and the blue light is mostly absorbed.
Filters transmit selected colours
Definition
Colour filter
A transparent material that transmits a selected wavelength or range of visible wavelengths and absorbs most other visible wavelengths.
- A red filter transmits red light and absorbs most other visible colours.
- A filter cannot create a missing colour; it can only transmit wavelengths already present in the incident light.
- White light passing through a blue filter appears blue because blue wavelengths are transmitted while most other wavelengths are absorbed.
- Red light incident on a blue filter produces little or no transmitted light because the blue filter absorbs the red light.
- Two filters in sequence transmit only wavelengths that both filters allow through.
Example
Predicting the observed colour
- A green leaf reflects green light and absorbs most other visible wavelengths.
- Under red light, the leaf receives no green wavelength to reflect.
- It absorbs most of the red light, so it appears very dark.
- If viewed through a green filter under white light, green light from the leaf is transmitted and the leaf remains visible.
Use a wavelength pathway to explain colour
- First identify the wavelengths present in the light source.
- Then state which wavelengths the surface reflects and which it absorbs.
- If a filter is present, state which remaining wavelengths it transmits and which it absorbs.
- The colour reaching the eye is the wavelength that survives every stage.
Common Mistake
- Do not say that an object contains or produces its colour; an opaque object appears coloured because of selective reflection and absorption.
- Do not say that a filter adds colour to light; a filter removes wavelengths by absorption and transmits selected wavelengths.
- Do not assume an object's colour is unchanged under coloured illumination.
Exam technique
Writing colour explanations
- Name the incident colour before describing the surface or filter.
- Use the precise verbs reflects, absorbs and transmits.
- Follow the light from source to surface, through any filter, and then to the eye.
Self review
- Explain why a red object appears red in white light.
- Explain why a white surface appears white.
- Define a colour filter.
- Predict the appearance of a blue object under red light and explain your answer.
- State what happens when red light reaches a blue filter.
