Dispersion of Light by a Prism
Dispersion is the separation of visible light into its constituent colours when it passes through a prism. This occurs because each colour (wavelength) of light is refracted by a different amount when entering and leaving the glass:contentReference[oaicite:1]{index=1}.
Key Terms
- Spectrum — a band of colours produced when light splits according to wavelength (e.g. rainbow).
- Monochromatic light — light of a single frequency or wavelength.
- White light — a mixture of many colours of visible light, each with its own wavelength.
Explanation of Dispersion
- White light is a combination of several colours, each having different frequency and wavelength.
- When white light enters a prism, each colour is refracted by a different amount.
- Red light (longest wavelength) bends the least.
- Violet light (shortest wavelength) bends the most.
- Thus, a spectrum of colours appears on the other side of the prism.
Order of Colours (Visible Spectrum)
Red → Orange → Yellow → Green → Blue → Indigo → Violet
(Mnemonic: ROYGBIV)
Why Dispersion Occurs
- Each colour travels at a different speed in glass.
- Violet light slows more than red light.
- Hence, violet deviates most, and red deviates least:contentReference[oaicite:2]{index=2}.
Exam Tips
- Label angle of incidence (i), angle of refraction (r), and angle of emergence (e) when drawing prism diagrams.
- State that dispersion occurs due to different speeds of colours in glass — not because frequencies change.
- Remember: Red has the longest wavelength, Violet has the shortest.
- Infra-red and ultra-violet are just beyond red and violet in the spectrum.
Applications
- Explains formation of rainbows — sunlight dispersed by raindrops.
- Used in spectrometers to study the wavelengths of light from different sources.
Summary Table
| Colour | Wavelength (nm) | Deviation in Prism | Speed in Glass |
|---|---|---|---|
| Red | ~700 | Least | Fastest |
| Orange | ~620 | Low | High |
| Yellow | ~580 | Moderate | Medium |
| Green | ~530 | More | Slower |
| Blue | ~470 | High | Slower |
| Violet | ~400 | Most | Slowest |
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