Ch 5 · 3.2.4 Dispersion of Light · O Level Physics 5054
Topic 5 of 7 in Waves — O Level Physics 5054 — 30 lessons.
Introduction
After rain, sunlight shining through water droplets paints a rainbow across the sky. Isaac Newton did the same thing indoors with a glass prism and a beam of sunlight, and showed that white light is really a mixture of colours. Splitting it up is called dispersion. (Syllabus section 3.2.4.)
Key ideas — Dispersion by a prism
When a narrow beam of white light enters a triangular glass prism, it is refracted, and different colours are refracted by different amounts. They leave the prism spread out into a band of colours called a spectrum.
🧑🏫 Teacher's explanation
Imagine a group of runners all sprinting through a patch of thick, muddy terrain. If they are all running at the same speed on the track, they stay together. But once they hit the mud, the faster runners might struggle less, while the slower runners get bogged down significantly more. Because they react to the mud differently, the group spreads out.
📖 Definitions
🔍 Worked example — Easy
List the seven colours of the visible spectrum in order of increasing wavelength.
🔍 Worked example — Standard
A beam of white light enters a glass prism. Which colour will be deviated (bent) the most from its original path?
🔍 Worked example — Exam-style
A student observes a spectrum from a prism. If the red light is refracted by an angle θ_red and the violet light is refracted by an angle θ_violet, state the relationship between θ_red and θ_violet.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe how a spectrum of colours is formed. [2] (b) State the colour that is refracted the most. [1] [3 marks]
Key ideas — The seven colours
The traditional seven colours of the visible spectrum, starting from the colour bent least:
🧑🏫 Teacher's explanation
Imagine a group of runners all starting a race at the same time. In a vacuum, they all run at exactly the same speed. However, if the track suddenly turns into a muddy field (like glass), some runners will slow down more than others. The 'heavy' runners (low frequency) barely slow down, while the 'light' runners (high frequency) struggle and change direction sharply.
📖 Definitions
🔍 Worked example — Easy
Arrange the following colours in order of increasing frequency: blue, red, violet, yellow.
🔍 Worked example — Standard
A beam of white light passes through a glass prism. Which colour will emerge from the prism at the largest angle to the incident beam?
🔍 Worked example — Exam-style
The wavelength of red light is λ_red = 7.0 × 10^-7 m and the wavelength of violet light is λ_violet = 4.0 × 10^-7 m. If the speed of light in a vacuum is c = 3.0 × 10^8 m/s, calculate the ratio of the frequency of violet light to the frequency of red light.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Name the phenomenon that causes the white light to split into a spectrum of colours. [1] (b) State the order of the seven colours in the spectrum, starting from the colour that is refracted the least. [1] (c) Explain, in terms of wavelength and frequency, why violet light is refracted more than red light. [2] [4 marks]
Practical notes
Worked example
A prism produces a spectrum on a screen. Which colour is (a) deviated most, (b) has the longest wavelength, (c) has the highest frequency?
🔍 Worked example — Easy
A student observes a spectrum produced by a prism. Identify which colour in the visible spectrum has the longest wavelength.
🔍 Worked example — Standard
A beam of white light is passed through a glass prism. Which colour is deviated (bent) the most from its original path?
🔍 Worked example — Exam-style
A spectrum is observed on a screen. If the frequency of light is f, and the speed of light in a vacuum is c, and the wavelength is λ, use the relationship c = fλ to explain why violet light bends more than red light.
⚠️ Common errors
Common mistakes
Practice
Answers: (1) The splitting of white light into its separate colours by refraction. (2) Red, orange, yellow, green, blue, indigo, violet. (3) Violet. (4) Each colour travels at a slightly different speed in glass, so each is refracted by a different amount; violet is refracted most and red least.