If all wavelengths are reflected equally the object appears white.

The idea

White is not a wavelength of its own. A surface looks white because it reflects all the visible wavelengths about equally, so the mixture leaving the surface is the same mixture that arrived. Very little is absorbed, which is why white surfaces warm up slowly: in the AQA cube experiment the shiny white cube rose only 2.0 °C while the matt black one rose 6.5 °C in the same time. Because a white object simply returns whatever it is given, its appearance follows the illumination: shine red light on white paper and it looks red, not white. Reflecting every wavelength is not the same as letting every wavelength through, which is what a clear, transparent object does.

What earns the marks

• Stating that a white surface reflects all the visible wavelengths roughly equally.
• Stating that very little of the incident light is absorbed.
• Stating that a white object takes on the colour of the light shining on it.
• Linking a white surface to poor absorption and so to a smaller temperature rise.

Common mistakes and how to put them right

• Treating white as a single colour of light with its own wavelength.

• Saying a white object looks white whatever colour of light falls on it.

• Confusing reflecting all colours with transmitting all colours, which describes clear glass.

• Assuming a matt white surface cannot reflect all colours because it looks dull.

Exam tips

• The mark usually needs three ideas: all wavelengths, reflected, and equally.
• Name the light source before naming the colour the object appears.
• Quote the temperature figures from a results table when you use them as evidence.