The Human Eye and the Colourful World · Lesson 5 of 8
Dispersion of White Light by a Glass Prism
“White light enters alone and leaves as a seven-colour team.”
• Explain dispersion of white light and identify the visible spectrum. • State the VIBGYOR sequence and compare the deviations of red and violet light. • Explain why different colours follow different paths through a prism. • Describe the recombination of a spectrum into white light using an inverted prism. • Explain rainbow formation as a sequence of refraction, dispersion and internal reflection. • Perform an activity that produces a spectrum from sunlight.
A narrow beam of sunlight looks colourless before it enters a prism. On a screen beyond the prism, however, it can spread into a brilliant band from red to violet. The prism has not painted the light. It has separated colour components that were already present together in the white beam.
The splitting of light into its component colours.
The band of coloured components produced when a beam of light is separated into its constituent colours.
The visible sequence is Violet, Indigo, Blue, Green, Yellow, Orange and Red. The acronym VIBGYOR records this order. In the usual prism arrangement, red appears at the end with the least deviation and violet at the end with the greatest deviation. The exact orientation on a page depends on how the prism is placed, but the relative order and relative bending remain the same.
A prism refracts each colour by a different amount because the refractive behaviour of glass depends on the colour, or wavelength, of light. Red light bends the least and violet light bends the most. Since each colour emerges along a slightly different direction, the components that overlapped in white light spread apart and become visible as a spectrum.
| Position in VIBGYOR | Colour | Relative deviation in a glass prism |
|---|---|---|
| First | Violet | Greatest |
| Second | Indigo | Very large |
| Third | Blue | Large |
| Fourth | Green | Intermediate |
| Fifth | Yellow | Moderate |
| Sixth | Orange | Small |
| Seventh | Red | Least |
Problem
A spectrum is labelled at only two ends. One end has the least deviation and the other has the greatest. Identify the colours.
- 1.Recall the relative bending of visible colours in glass.
- 2.Red light is deviated least, so the least-deviated end is red.
- 3.Violet light is deviated most, so the most-deviated end is violet.
Problem
Why does a glass slab usually not display a broad spectrum as clearly as a prism?
- 1.The first surface of either object can separate colours slightly because they refract by different amounts.
- 2.In a slab, the second surface is parallel to the first and tends to bring the separated directions back towards parallel emergence, leaving very small colour separation in the outgoing beam.
- 3.In a prism, the second face is inclined, so the colour-dependent deviations add to a visible angular spread.
Problem
A learner says that a prism creates seven new colours because sunlight contains only white. Evaluate the claim.
- 1.A second prism can recombine the separated spectrum into white light.
- 2.Individual colours sent through another prism do not split into additional colour components in the same way.
- 3.These observations show that the first prism separates components already present in white light; it does not manufacture them.
Activity
Purpose: To observe the spectrum produced when sunlight passes through a triangular glass prism. Materials: A thick sheet of cardboard, a narrow slit, a glass prism and a white screen or pale wall. Use sunlight indirectly through the slit; never look directly at the Sun.
• Make a small hole or narrow slit in the centre of the cardboard. • Allow sunlight to pass through the slit so that a narrow beam is produced. • Place the prism in the beam and allow light to fall on one refracting face. • Turn the prism slowly until the emerging light forms a clear band on a nearby screen. • Record the colour sequence and identify the colours at the two ends.
Observation: A band of colours appears rather than a single white patch. Violet lies at the more-deviated end and red at the less-deviated end. Explanation: Each colour is refracted through a different angle. Conclusion: White sunlight contains several colour components that a prism can separate.
Recombination Of The Spectrum Of White Light
Isaac Newton used a prism to obtain the spectrum of sunlight. When he tried to split the separated colours further with another similar prism, no new set of colours appeared. He then placed a second identical prism in an inverted position relative to the first. The first prism spread white light into a spectrum; the second produced opposite deviations that brought those colours together again.
A beam of white light emerged from the second prism. This is recombination. It provides strong evidence that sunlight is made of the seven visible colour components. Any light that produces a spectrum similar to sunlight is commonly called white light.
The joining of the separated colours of a spectrum to form white light again.
A coloured filter removes many components and transmits selected wavelengths. A prism, by contrast, separates components through unequal refraction. Recombination with an inverted prism distinguishes separation from the creation of coloured light.
Rainbow Formation
A rainbow is a natural spectrum that often appears after a rain shower. Countless tiny water droplets remain suspended in the atmosphere, and each suitable droplet acts like a small prism. Sunlight entering a droplet is refracted and dispersed. The separated light reflects internally from the rear surface and is refracted once more as it leaves the droplet.
Because the colours emerge along different directions, only certain droplets send a particular colour towards the observer. The combined light from a vast number of droplets produces coloured arcs. A rainbow is seen in a direction opposite to the Sun, so the Sun must be behind the observer. The same geometry can produce a rainbow-like spectrum near a waterfall or water fountain on a sunny day.
Sunlight enters and refracts → colours disperse → separated light reflects internally → light refracts again on leaving → different colours reach the observer from different droplets.
Quiz
What is the band of colours produced by dispersion called?
Which colour is deviated least by a glass prism?
What did the inverted second prism show in Newton’s experiment?
Which sequence correctly describes light in a raindrop?
Where should the Sun be when an observer sees a rainbow ahead?
Practice Problems
- Write the colour sequence of the visible spectrum. Answer: Violet, indigo, blue, green, yellow, orange and red, remembered as VIBGYOR.
- Why does violet separate farther from red after passing through a prism? Answer: Glass refracts the colour components by different amounts. Violet is deviated most and red least, so their emergent directions become separated.
- Describe evidence that a prism does not create the colours of sunlight. Answer: A second inverted prism brings the separated spectrum together to produce white light again. This shows that the colours were components of the original beam.
- A student sees a coloured band but labels red at the most-deviated end. Correct the interpretation. Answer: Red must be at the least-deviated end and violet at the most-deviated end. The remaining colours lie between them in VIBGYOR order.
- Explain rainbow formation in four optical stages and state the observer-Sun arrangement. Answer: Sunlight refracts on entering a droplet, disperses into colours, reflects internally and refracts again on leaving. The rainbow is seen opposite the Sun, so the Sun is behind the observer.
Key Takeaways
• Dispersion is the splitting of light into its component colours. • The visible spectrum follows the VIBGYOR sequence. • Red light is deviated least by glass, while violet light is deviated most. • Unequal refraction sends different colours along different emergent paths. • An inverted second prism can recombine the spectrum into white light. • The recombination experiment shows that sunlight contains the visible colour components. • A rainbow forms through refraction, dispersion, internal reflection and refraction on emergence. • A rainbow appears opposite the Sun, with the Sun behind the observer.