Explain the effects of differences in the velocities of electromagnetic…

The idea

Electromagnetic waves travel at different speeds in different substances. In a vacuum all electromagnetic waves travel at the same speed, about 3.0 × 10⁸ m/s, but in glass, water or other transparent media the speed is lower, and the reduction depends on the substance and on wavelength. When a wave crosses a boundary at an angle and its speed changes, its direction changes: this is refraction. Entering a slower medium, the wave bends towards the normal; leaving into a faster medium, it bends away from the normal. A wave travelling along the normal does not change direction even though its speed changes. Because speed varies with wavelength in a given substance, different colours refract by different amounts, which separates white light into a spectrum in a prism. In a multiple-choice item, link the speed difference to the direction change and to the normal.

What earns the marks

• States that electromagnetic waves travel at different speeds in different substances, with the speed in a vacuum being the maximum.
• Explains that a change in speed at a boundary at an angle causes the wave to change direction, which is refraction.
• Identifies that a wave entering a slower medium bends towards the normal and one entering a faster medium bends away from the normal.
• Recognises that a wave travelling along the normal changes speed without changing direction.
• Links the wavelength dependence of speed in a substance to the separation of colours by a prism.

Common mistakes and how to put them right

• Claims a wave always bends at a boundary regardless of the angle of incidence.

  Put it right: A wave along the normal changes speed but not direction.

• Reverses the bending rule.

  Put it right: A wave slowing down bends towards the normal, not away from it.

• Assumes all colours travel at the same speed in glass.

  Put it right: Different wavelengths have different speeds, which is why a prism disperses light.

Exam tips

• Identify whether the wave is speeding up or slowing down before deciding which way it bends.
• Check the angle of incidence: if the wave travels along the normal, state that no change of direction occurs.
• Use the phrase towards or away from the normal explicitly to make the direction of bending unambiguous.