1. Dispersion of white light
- Dispersion: the splitting of white light into its constituent colours by a prism. The band of colours is the spectrum: violet, indigo, blue, green, yellow, orange, red (VIBGYOR).
- Cause: in a material medium, light of different colours travels at different speeds, so the refractive index depends on wavelength: . Hence violet is deviated most and red least.
- In vacuum all colours travel at the same speed, m s⁻¹.
- A prism only separates colours already present; a second, identical prism placed inverted recombines them into white light.
- Monochromatic light (one wavelength, for example a sodium lamp) is deviated but not dispersed.
| Colour | Speed in glass | μ in glass | Deviation |
|---|---|---|---|
| Red | Highest | Least | Least |
| Violet | Lowest | Greatest | Greatest |
Formulas
- Refractive index depends on colour
Key terms
- Dispersion
The splitting of white light into its constituent colours on passing through a prism.
- Spectrum
The band of colours obtained when white light is dispersed.
- Monochromatic light
Light of a single wavelength (single colour).
2. The visible spectrum
- Wavelength range about 400 nm (violet) to 700–800 nm (red); m.
- Order of increasing wavelength: violet, indigo, blue, green, yellow, orange, red; frequency increases the other way ().
- Colour is decided by frequency, which does not change on refraction.
| Colour | Wavelength (about) | Frequency (about) |
|---|---|---|
| Violet | 400 nm | 7.5 × 10¹⁴ Hz |
| Green | 530 nm | 5.7 × 10¹⁴ Hz |
| Red | 700 nm | 4.3 × 10¹⁴ Hz |
Formulas
- Wave equation for light
Key terms
- Wavelength
The distance between two successive crests (or compressions) of a wave.
3. The electromagnetic spectrum
| Region | Wavelength (about) | Source or use |
|---|---|---|
| Gamma rays | below 10⁻¹¹ m | Radioactive nuclei; cancer treatment |
| X-rays | 10⁻¹¹ to 10⁻⁸ m | Fast electrons hitting a metal; bone images |
| Ultraviolet | 10⁻⁸ to 4 × 10⁻⁷ m | Sun, arcs; sterilising |
| Visible | 4 × 10⁻⁷ to 8 × 10⁻⁷ m | Sun, lamps; seeing |
| Infrared | 8 × 10⁻⁷ to 10⁻³ m | Hot bodies; remote controls |
| Microwaves | 10⁻³ to 0.3 m | Ovens, radar, mobile phones |
| Radio waves | above 0.3 m | Broadcasting |
Common properties: produced by accelerated charges; transverse electric and magnetic fields; travel in vacuum at m s⁻¹; obey ; not deflected by electric or magnetic fields; can be reflected and refracted; carry energy.
| Order | Increasing wavelength | Increasing frequency |
|---|---|---|
| First | Gamma rays | Radio waves |
| Last | Radio waves | Gamma rays |
Key terms
- Electromagnetic wave
A transverse wave of changing electric and magnetic fields that can travel through vacuum at the speed of light.
4. Infrared and ultraviolet radiation
Infrared (IR): beyond red. Produced by hot bodies (Sun, heater, human body). Detected by a thermopile or blackened-bulb thermometer. Properties: strong heating effect; less scattered by haze and fog; absorbed by water vapour and CO₂; reflected by polished surfaces. Uses: remote controls, night vision and thermal cameras, haze photography, physiotherapy, drying paint, weather satellites, greenhouses.
Ultraviolet (UV): beyond violet. Produced by the Sun, electric arcs, mercury vapour lamps (quartz tubes). Detected by photographic film and fluorescence (zinc sulphide). Properties: causes fluorescence; kills germs; causes chemical changes (tanning); absorbed by ordinary glass, transmitted by quartz; absorbed by the ozone layer. Uses: sterilising water and instruments, checking forged notes, vitamin D formation. Harmful in excess: sunburn, eye damage, skin cancer.
| Feature | Infrared | Ultraviolet |
|---|---|---|
| Position | Beyond red | Beyond violet |
| Detector | Thermopile | Fluorescent screen, film |
| Glass | Partly transmitted | Absorbed (quartz transmits) |
| Main effect | Heating | Chemical, germicidal |
Key terms
- Thermopile
A detector of infrared radiation that produces a current when warmed by the radiation.
- Fluorescence
The emission of visible light by a substance while ultraviolet radiation falls on it.
5. Scattering of light
- Scattering: absorption and re-emission of light in all directions by small particles.
- For particles much smaller than the wavelength (air molecules): scattered intensity . Blue is scattered about 5 times as much as red.
- Blue sky: air molecules scatter blue most; skylight is mainly blue.
- Red Sun at sunrise and sunset: long path through air removes blue by scattering; red reaches the eye.
- Red danger signals: red is scattered least and penetrates fog farthest.
- White clouds: large droplets scatter all colours equally.
- Black sky in space or on the Moon: no atmosphere, no scattering.
| Observation | Reason |
|---|---|
| Blue sky | Blue scattered most |
| Red sunrise | Blue scattered out on a long path |
| Red signals | Red scattered least |
| White clouds | Large drops scatter all colours |
Formulas
- Scattering by very small particles
Key terms
- Scattering of light
The process by which small particles absorb light and send it out again in all directions.
Exam tips
Common mistakes
- Saying red is deviated (or scattered) most.
- Saying the prism “produces” colours.
- Mixing up the order of the electromagnetic spectrum, especially X-rays and gamma rays, or infrared and microwaves.
- Forgetting that ordinary glass absorbs ultraviolet.
- Explaining the red sunrise without mentioning the longer path through air.
How marks are usually awarded
- Dispersion: definition (1), cause — μ depends on wavelength (1), order of deviation (1).
- IR and UV: each property or use 1 mark; a named detector or source earns credit.
- Scattering: name the particles, the wavelength dependence and the conclusion.