3.2.4 Dispersion of light
- Syllabus
- 0625–2026–2027
- Topic
- 3.2.4
- Level
- —
Learning objectives
3.2.4.1Dispersion of light as illustrated by• Describe the dispersion of light as illustrated by the refraction of white light by a glass prism3.2.4.2Traditional seven colours of the• Know the traditional seven colours of the visible spectrum in order of frequency and in order of wavelength3.2.4.3Recall that visible light of a single• Recall that visible light of a single frequency is described as monochromatic
Explain how a prism disperses white light
Dispersion is the separation of white light into its component colours because different frequencies are refracted by different amounts.
At the first air–glass face of a prism, each colour slows and bends towards its normal by a different amount. At the second glass–air face, each bends away from its normal. Because the faces are not parallel, the differences increase and the emerging rays spread into a spectrum.
| Colour | Speed in glass | Refraction and total deviation through the prism |
|---|---|---|
| red | greatest among visible colours | least |
| violet | smallest among visible colours | greatest |
A prism does not create colours or change one colour into another. White light already contains the visible frequencies; the prism separates them. A single-frequency ray refracts but does not split into a spectrum.
Order the seven visible colours
The traditional visible-spectrum sequence from lowest frequency to highest frequency is red, orange, yellow, green, blue, indigo, violet.
| Required order | Seven colours |
|---|---|
| increasing frequency | red → orange → yellow → green → blue → indigo → violet |
| increasing wavelength | violet → indigo → blue → green → yellow → orange → red |
In a vacuum, and approximately in air, every visible colour has the same speed. Since wave speed equals frequency multiplied by wavelength, higher frequency then means shorter wavelength, so the two colour orders run in opposite directions.
State the direction requested before writing the list. Red has the longest visible wavelength and lowest visible frequency; violet has the shortest visible wavelength and highest visible frequency.
Recognise monochromatic light
Monochromatic visible light has a single frequency.
In one specified medium, a single frequency also corresponds to a single wavelength. Frequency is the safest defining quantity because it remains unchanged when light crosses a boundary, while wavelength changes with speed.
| Light | Frequency content | Passage through a glass prism |
|---|---|---|
| monochromatic light | one frequency | refracts as one ray; no colour spectrum is formed |
| white light | a range of visible frequencies | refracts by different amounts and disperses into a spectrum |
Monochromatic does not mean one amplitude, one direction or no refraction. It specifies frequency content only; a monochromatic ray can still reflect and refract.