The electromagnetic spectrum: recall that light is an electromagnetic wave

The idea

Light is one member of the electromagnetic spectrum, so it is an electromagnetic wave rather than a sound wave or a water wave. An electromagnetic wave is produced by oscillating electric and magnetic fields that travel together through space, and it needs no material medium, which is why light reaches us from the Sun across empty space. Visible light occupies only a narrow band of wavelengths, roughly 400 nm to 700 nm, sitting between infrared and ultraviolet. In a multiple-choice question you may be asked to pick the correct description of light, to identify which statement about light is true, or to select the region of the spectrum that visible light belongs to. Read every option and reject any that calls light a mechanical or longitudinal wave, or that says light needs a medium.

What earns the marks

• Identifies light as an electromagnetic wave that belongs to the electromagnetic spectrum.
• States that light travels through a vacuum without needing a material medium.
• Recognises that visible light is a narrow band of the spectrum between infrared and ultraviolet.
• Rejects options that describe light as a sound wave, a mechanical wave or a longitudinal wave.

Common mistakes and how to put them right

• Describes light as a sound wave or a mechanical wave.

  Put it right by stating that light is an electromagnetic wave.

• Claims that light needs a medium such as air to travel.

  Put it right by noting that light crosses empty space from the Sun.

• Places visible light outside the electromagnetic spectrum.

  Put it right by locating it between infrared and ultraviolet.

Exam tips

• Read all four options before choosing, because distractors often swap wave types.
• Underline the key word in the question, such as electromagnetic or vacuum, before deciding.
• Eliminate any option that mentions a medium or a longitudinal wave for light.