Red-shift (physics only): This effect is called red-shift.

The idea

The name is literal. Visible light runs from violet at the short wavelength end to red at the long wavelength end, so when the lines in a galaxy's spectrum shift towards longer wavelengths they shift towards the red end, and the effect is called red-shift. The name says nothing about the colour the galaxy appears to be; it describes the direction the lines have moved. The matching effect for an object approaching us is blue-shift, a shift towards shorter wavelengths, and a few nearby galaxies show it. The idea is similar to the change in pitch you hear as a siren passes, although for the most distant galaxies the wavelength grows because space itself expands while the light is travelling.

What earns the marks

• Naming the effect as red-shift.
• Linking the name to the red end of the visible spectrum, the longer wavelengths.
• Naming blue-shift as the equivalent effect for an approaching source.
• Stating that the shift is measured from a spectrum rather than seen as a change of colour.

Common mistakes and how to put them right

• Answering red-shift to a question that asked what causes the effect rather than what it is called.

• Saying that distant galaxies look red to the eye.

• Confusing red-shift with the red appearance of a cool star.

• Calling a shift towards shorter wavelengths a red-shift.

Exam tips

• A one mark naming question wants the term only, so do not spend time explaining it.
• Keep red-shift and blue-shift paired in your notes so that you never swap them over.
• Check the direction of the shift on the figure before you name it.