Dispersion

Overview

Keep the prism diagram and the colour pattern tightly connected in this lesson. Students should see dispersion as refraction applied to white light, not as a separate effect where the prism somehow creates colours.

Key knowledge and explanations

• Define dispersion as the separation of white light into its component colours by refraction in a prism.
• Explain that white light contains a range of visible frequencies, and different colours slow down and refract by different amounts in glass. Red deviates least and violet deviates most.
• Teach the seven visible colours in order as red, orange, yellow, green, blue, indigo, and violet, then make the reverse relationship clear: red has the longest wavelength and lowest frequency, while violet has the shortest wavelength and highest frequency.
• Define monochromatic light as light of a single frequency and contrast it with white light.
• Use one large labelled prism diagram so students can connect entry refraction, exit refraction, and the final spectrum in a single representation.

Lesson flow

1. Start with a retrieval prompt on refraction and ask whether all colours of light would bend by the same amount in glass.
2. Demonstrate dispersion with a prism or a simulation, then have students sketch the incident white ray, the prism, and the emerging colour spectrum in the correct order.
3. Practise explaining the spread of colours and rehearse the visible-spectrum order in both wavelength and frequency terms.
4. Finish with a short compare-and-contrast task where students explain the difference between white light and monochromatic light.

Checks for understanding

• Use a hinge question with four prism diagrams and ask which one shows the colours in the correct order.
• Ask students to place the seven colours in order and then identify which end of the spectrum has the highest frequency.
• Give one quick prompt asking whether a laser beam is monochromatic and require students to justify the answer.

Common mistakes or misconceptions

• Students often think the prism adds colour to the light. Keep returning to the idea that the colours were already present in white light before the prism.
• Some mix up the order of wavelength and frequency. Use one table or diagram that shows the two orders are reversed.
• Students may use monochromatic to mean bright or pure. Reinforce that it means light of a single frequency.

Follow-up

• Set a short prism-diagram and spectrum-labelling task so students rehearse the colour order and the explanation for different amounts of refraction.
• Carry forward accurate ray-diagram conventions into the lenses lesson, where students will again need to track how light changes direction through transparent materials.