Explain, with examples, that all electromagnetic waves transfer energy…

The idea

Every electromagnetic wave carries energy away from a source and delivers it to an observer or detector, without needing a medium. The source emits the wave; the wave travels; the observer absorbs some of that energy. Examples across the spectrum show the same pattern. A lamp emits visible light that warms your skin or lights a room. A microwave oven's microwaves carry energy into food, heating it. Infrared from a heater warms a person nearby. Ultraviolet from the Sun carries energy to skin, causing tanning or burning. X-rays carry energy into a detector to form an image. Gamma rays from a radioactive source carry energy that a Geiger–Müller tube detects. In each case identify the source, the wave as the carrier, and the observer or detector receiving the energy.

What earns the marks

• States that electromagnetic waves transfer energy from a source to an observer or detector.
• Identifies a named source and a named observer or detector for a given EM wave example.
• Explains that the wave carries energy through space without needing a medium.
• Applies the source-to-observer energy transfer to more than one region of the spectrum, such as infrared or microwaves.

Common mistakes and how to put them right

• Describes EM waves as carrying only light rather than energy.

  Put it right by stating that every EM wave transfers energy.

• Names a wave without identifying a source and an observer.

  Put it right by giving both ends of the transfer.

• Claims EM waves need a medium to carry energy.

  Put it right by stating that they transfer energy through a vacuum.

Exam tips

• For each example, name the source, the wave and the observer or detector so the energy pathway is complete.
• Use examples from different parts of the spectrum to show the rule applies to all EM waves.
• Check that a chosen option shows energy moving from source to observer, not the reverse.