Light and the electromagnetic spectrum — final check key

1. UK1 · Final check

Model answer. (a) White light is a mixture of all the colours. Glass refracts each colour by a slightly different amount, so the prism spreads them apart into a spectrum: dispersion. (b) Red has the longest wavelength; violet has the highest frequency. (c) Infrared lies beyond the red end and ultraviolet beyond the violet end; of the seven main regions, radio waves have the lowest frequency. (d) It is monochromatic light: light of a single frequency, which a prism bends but cannot spread.

A full answer includes

• (a) White light is a mixture of colours, and the glass refracts each by a different amount; dispersion.
• (b) Longest wavelength: red; highest frequency: violet.
• (c) Beyond the red: infrared; beyond the violet: ultraviolet; lowest frequency: radio waves.
• (d) Supplement: monochromatic light; it has a single frequency.

Also accept

• (a) mixture; each colour refracted differently; dispersion (b) red; violet (c) infrared; ultraviolet; radio waves (d) monochromatic: one frequency

Watch for

• “The prism makes the colours.” — They were already in the white light; the prism only spreads them apart.

2. UK2 · Final check

Model answer. (a) Infrared and ultraviolet are both electromagnetic waves, so they travel at the same speed in a vacuum. (b) Time = distance ÷ speed = 6.0 × 10⁸ m ÷ 3.0 × 10⁸ m/s = 2.0 s. (c) Medical scanning uses X-rays, which pass through soft tissue but are absorbed by bone, so the bone shows on the picture; an electric grill uses infrared, which the surface of the food absorbs and warms up; a room is lit with visible light, which the eyes detect. (d) Too much infrared causes skin burns; too many microwaves cause internal heating of body cells.

Checked automatically

• (b) Supplement: Time: 2.0 s (within ±0.05 s)
  • Common wrong answer 0.5 : You divided the speed by the distance. Time = distance ÷ speed.

A full answer includes

• (a) The same: all electromagnetic waves travel at the same speed in a vacuum.
• (b) Supplement: 6.0 × 10⁸ m ÷ 3.0 × 10⁸ m/s = 2.0 s.
• (c) X-rays; infrared; visible light; with a reason, for example: X-rays pass through soft tissue but bone absorbs them.
• (d) Infrared: skin burns; microwaves: internal heating of body cells.

Also accept

• (a) the same (b) 2.0 s (c) X-rays; infrared; visible light, with a reason (d) infrared skin burns; microwaves internal heating

Watch for

• “Ultraviolet is faster because its frequency is higher.” — A higher frequency goes with a shorter wavelength, not a higher speed.