Lesson overview
Light and the electromagnetic spectrum: What lies beyond the red end of a rainbow?
Let sunlight through a slit in a blind and a glass prism, and name the seven colours it spreads into on a white card. Lay thermometers with blackened bulbs in the blue, in the red, just beyond the red and in the shade, and find where it warms most; slide a strip of security ink beyond the violet. Supplement: shine a red laser through the same prism. Then put seven devices on the rungs of the electromagnetic spectrum and find what all its waves share.
Suggested sequence
- Lesson 1 · Light split into colours. Dispersion of white light by a glass prism and the seven colours of the spectrum in order; infrared beyond the red and ultraviolet beyond the violet, found with thermometers and security ink; the regions of the electromagnetic spectrum in order of wavelength and of frequency; Supplement: monochromatic light. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · The electromagnetic spectrum at work. All electromagnetic waves travel at the same speed in a vacuum; the uses of each region, from radio waves to gamma rays, and the harm too much of some regions does; Supplement: 3.0 × 10⁸ m/s in a vacuum, approximately the same in air. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 2 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 2 questions, 15 min, lesson closed.
Learning goals
Part 1: Light split into colours
- Describe the dispersion of light as illustrated by the refraction of white light by a glass prism; know the traditional seven colours of the visible spectrum in order of frequency and in order of wavelength (3.2.4.1, 3.2.4.2)
- Supplement: Recall that visible light of a single frequency is described as monochromatic (3.2.4.3)
- Know the main regions of the electromagnetic spectrum in order of frequency and in order of wavelength, with infrared beyond the red end of the visible spectrum and ultraviolet beyond the violet end (3.3.1)
Part 2: The electromagnetic spectrum at work
- Know that all electromagnetic waves travel at the same high speed in a vacuum (3.3.2)
- Supplement: Know that the speed of electromagnetic waves in a vacuum is 3.0 × 108 m/s and is approximately the same in air (3.3.6)
- Describe typical uses of the different regions of the electromagnetic spectrum: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays (3.3.3)
- Describe the harmful effects on people of excessive exposure to electromagnetic radiation: microwaves, infrared, ultraviolet, X-rays and gamma rays (3.3.4)
Misconceptions to listen for
- “A is violet, because it is at the top, like the top of the spectrum chart.” — Look at how much each ray is bent, not where it is on the page. The ray bent least is red.
- “All three rays are bent equally; the glass only colours them.” — The colours are already in the white light. The prism refracts each one by a different amount, which spreads them apart.
- “The prism adds the colours to the light, so white light is not a mixture.” — The colours were in the white light all along: a second prism turned the other way gathers them back into white.
- “The colour refracted most is red.” — Red is refracted least. Violet, at the other end of the spectrum, is refracted most.
- “Indigo comes after violet.” — Violet is the last colour, at the end refracted most; indigo is between blue and violet.
- “Yellow comes before orange.” — Orange lies between red and yellow, as in a rainbow.
- “Red has the longer wavelength and the higher frequency.” — At the same speed, a longer wavelength goes with a lower frequency: fewer crests pass each second.
- “Red and blue light have the same wavelength but travel at different speeds in air.” — In air all the colours travel at practically the same speed. They differ in wavelength and frequency.
- “Violet, blue, orange, red: violet is bent most, so it must have the longest wavelength.” — The colour bent most has the shortest wavelength. Red, bent least, has the longest.
- “The prism splits the green light into more colours.” — Green from the spectrum is already one narrow band of colour; the second prism bends it but it stays green.
Scope
The dispersion of white light by a glass prism (3.2.4.1); the seven colours of the visible spectrum in order of frequency and of wavelength (3.2.4.2); Supplement: monochromatic light, of a single frequency (3.2.4.3); the main regions of the electromagnetic spectrum in order of frequency and of wavelength (3.3.1); all electromagnetic waves travelling at the same high speed in a vacuum (3.3.2); typical uses of each region (3.3.3) and the harmful effects of too much of some of them (3.3.4); Supplement: the speed of electromagnetic waves, 3.0 × 10⁸ m/s in a vacuum and approximately the same in air (3.3.6). Communication with electromagnetic waves (3.3.5 and 3.3.7–3.3.10) is in its own module; v = fλ is in Waves in a ripple tank; the wavelength of each region, the photon and how the eye sees colour are not covered here.