Wavelengths that are not reflected are absorbed.

The idea

For an opaque object the incident light has only two possible fates: it is reflected or it is absorbed, because none is transmitted through. The two shares add up to all of the incident light, so once you know which wavelengths are reflected you also know that everything else has been absorbed. A green leaf reflects the middle of the spectrum and absorbs the red and blue. Absorbed light is not destroyed; its energy is transferred to the internal energy store of the surface, which is why a black car roof in sunlight becomes far hotter than a white one. That same idea carries straight over into the infrared work: a matt black cube absorbs the most and shows the largest temperature rise.

What earns the marks

• Stating that any wavelength not reflected by an opaque surface is absorbed.
• Naming the colours absorbed by a given object, such as a green leaf absorbing red and blue.
• Stating that no light is transmitted through an opaque object.
• Linking absorption to an increase in the internal energy, and so the temperature, of the surface.

Common mistakes and how to put them right

• Saying the light that is not reflected simply disappears or is destroyed.

• Saying the unreflected light passes through an object described as opaque.

• Describing absorption as reflection that happens inside the object.

• Forgetting that absorbed light warms the surface, so missing the energy mark.

Exam tips

• Answer in two parts: what is reflected, then what is absorbed.
• Bring in the temperature rise when a question mentions sunlight or heating.
• Use 'absorbed' rather than 'taken in' or 'soaked up' to match the mark scheme.