Objects that transmit light are either transparent or translucent.

The idea

Objects that let light through are of two kinds. A transparent material, such as clear glass or still water, transmits light with almost no scattering, so rays keep their directions and you can see a sharp image through it. A translucent material, such as frosted glass, tracing paper or tissue, also transmits light but scatters it as it passes, so light gets through while directions are jumbled and only a blur is seen. An opaque object transmits none at all. The same material can move between the two: polish frosted glass and it becomes transparent, because the scattering comes from the rough surface rather than the glass itself. A coloured transparent object also absorbs some wavelengths, which is what makes it a filter.

What earns the marks

• Defining transparent as transmitting light so that a clear image can be seen through it.
• Defining translucent as transmitting light while scattering it, so no clear image forms.
• Contrasting both with opaque, which transmits no light at all.
• Classifying named materials correctly, such as frosted glass as translucent.

Common mistakes and how to put them right

• Using translucent to mean partly coloured rather than partly scattering.

• Calling anything you cannot see through opaque, including frosted glass and tissue paper.

• Saying transparent objects reflect no light at all.

• Muddling transmitted light with reflected light when describing what a material does.

Exam tips

• Decide by asking whether a clear image can be seen through the material, not whether light gets through.
• Use the three words transparent, translucent and opaque precisely; examiners test the middle one.
• Mention scattering when you define translucent, because that is the distinguishing idea.