Learning outcomes
- Describe dispersion of white light by a prism.
- Recall the seven visible colours in order.
- Relate colour order to wavelength and frequency.
- Explain why a spectrum forms.
13.1 White light and colour
White light contains a continuous range of visible wavelengths. A glass prism separates these wavelengths into a spectrum because refractive index depends slightly on wavelength. Each colour changes speed and direction by a different amount in glass.
The separation of colours is called dispersion. A narrow slit and collimated beam produce a clearer spectrum because overlapping rays are reduced. A second prism in the opposite orientation can recombine the colours approximately into white light.
13.2 Colour order
The traditional seven colours are red, orange, yellow, green, blue, indigo and violet. From red to violet, wavelength decreases and frequency increases. Red is refracted least by ordinary glass and violet most.
A mnemonic can help recall the order, but understanding the trends is more useful. Because all colours travel at approximately the same speed in a vacuum, the higher frequency colours have shorter wavelengths according to v = fλ.
13.3 Refraction through a prism
At the first prism face, light refracts toward the normal. At the second face it refracts away. The non-parallel faces make the angular separations add rather than cancel, so the emergent colours spread into a spectrum.
Violet generally has a larger refractive index in glass than red, so it bends more. Frequency does not change at the air–glass boundary; wavelength and speed change inside the glass.
13.4 Natural spectra
Rainbows form when sunlight is refracted and dispersed as it enters water droplets, internally reflected, and refracted again on leaving. Many droplets contribute light of different colours to the observer at different angles.
A rainbow explanation should not be reduced to “water acts like a prism”; mention refraction, dispersion and internal reflection. Detailed angle calculations are not required.
13.5 Colour and objects
An object appears coloured because it reflects or transmits certain visible wavelengths more strongly and absorbs others. A red object in white light reflects mainly red light. Under blue-only illumination it may appear dark because little blue is reflected.
This topic supports understanding but should not be confused with the syllabus requirement to know the visible spectrum order. The spectrum concerns components of light; object colour concerns selective reflection and absorption.
Worked examples
Frequency trend
Red and violet light travel at the same speed in vacuum. Violet has shorter wavelength, so it must have higher frequency.
Prism reasoning
A white beam enters glass. The frequencies remain fixed, but different colours have different speeds and refractive indices, so they emerge at different angles.
Practical focus
Investigation
Pass a narrow white-light beam through a triangular prism onto a white screen. Rotate the prism to obtain a sharp spectrum. Record the colour order and identify which end is deviated most.
Examination guidance
- Use the correct order from red to violet or reverse it clearly.
- Dispersion is separation because different wavelengths refract by different amounts.
- Do not state that colour frequency changes in the prism.
Check your understanding
- Which visible colour has the longest wavelength?
- Which is refracted most by a normal glass prism?
- Why does a prism produce a spectrum but a parallel-sided block usually does not leave a large angular separation?
Answers
- Red.
- Violet.
- The non-parallel prism faces make colour deviations accumulate, whereas a block’s second parallel face largely restores the original direction.
