Recall the main groupings of the continuous electromagnetic spectrum…

The idea

The electromagnetic spectrum is a continuous family of transverse waves that all travel at the speed of light in a vacuum and differ in wavelength and frequency. Learn the groupings in order of increasing frequency (decreasing wavelength): radio waves, microwaves, infrared, visible, ultraviolet, x-rays and gamma rays. The visible region splits into the colours red, orange, yellow, green, blue, indigo and violet, with red having the longest wavelength and violet the shortest. A multiple-choice question may ask you to pick the correct order, identify a missing grouping, or place a colour correctly within the visible band, so memorise the sequence and the position of the colours.

What earns the marks

• Selects the groupings in the correct order: radio waves, microwaves, infrared, visible, ultraviolet, x-rays and gamma rays.
• Recognises that the spectrum is continuous, with neighbouring groupings differing in wavelength and frequency rather than in speed in a vacuum.
• Places the visible colours in order from red (longest wavelength) through to violet (shortest wavelength).
• Identifies that all groupings travel at the same speed in a vacuum and are transverse electromagnetic waves.
• Rejects distractors that reverse the order or insert a non-electromagnetic wave such as sound.

Common mistakes and how to put them right

• Reverses the order of the spectrum.

  Put it right: Radio waves have the longest wavelength and gamma rays the shortest.

• Places visible light in the wrong position.

  Put it right: Visible sits between infrared and ultraviolet.

• Lists the visible colours out of order.

  Put it right: Arrange them red through violet by decreasing wavelength.

• Treats sound as part of the spectrum.

  Put it right: Sound is a mechanical wave and is not electromagnetic.

Exam tips

• Use a memory phrase for the order, such as the initial letters of each grouping, and check it against the options.
• Remember the visible colours run red to violet as wavelength decreases.
• Eliminate options that put gamma rays first or radio waves last.
• Check whether the question asks for increasing frequency or increasing wavelength before choosing.