1. Why we think of light as a wave
Light shows behaviours that cannot be explained if it were only a stream of particles or rays. Effects like bright and dark fringes, colours on soap bubbles, and light bending around obstacles all point to one idea — light behaves like a wave.
These effects appear only when I think of light spreading out and interfering like ripples on water.
2. Light as an electromagnetic wave
Light is an electromagnetic wave made up of oscillating electric and magnetic fields. These fields vibrate:
- perpendicular to each other,
- and perpendicular to the direction in which light travels.
This makes light a transverse wave.
3. Wavefronts help visualise light
Instead of imagining light as thin rays, wave optics uses wavefronts — surfaces on which every point vibrates in phase. This helps me picture how the wave spreads out.
3.1. Types of wavefronts
- Plane wavefront: From a faraway light source.
- Spherical wavefront: From a point source.
- Cylindrical wavefront: From a long, thin source like a slit.
4. Wavelength and frequency of light
Every wave is described by its wavelength and frequency. Light of different colours has different wavelengths:
- Violet → shorter wavelength
- Red → longer wavelength
The wavelength decides how much light bends around edges and how interference patterns look.
5. Huygens’ principle: how waves move forward
Huygens’ principle makes the wave picture easier: each point on a wavefront acts like a tiny source of new waves. The next wavefront is the smooth surface touching all these secondary waves.
This principle explains reflection and refraction without needing rays.
6. Evidence that light is a wave
Several experiments make the wave nature of light very clear:
6.1. Interference
When two light waves overlap, they add up. Sometimes they strengthen each other (bright fringes) and sometimes they cancel (dark fringes). This pattern can appear only if light is a wave.
6.2. Diffraction
Diffraction is the bending and spreading of light when it passes through narrow openings or around sharp edges. A sharp particle beam would not bend like this, but a wave does.
6.3. Polarisation
Polarisation is possible only for transverse waves. Since light can be polarised, it must be a transverse wave. This is one of the strongest evidences supporting the wave model.
7. How wavelength influences wave effects
The wave effects depend strongly on the wavelength:
- Shorter wavelengths (violet) bend less during refraction but diffract less.
- Longer wavelengths (red) bend more during dispersion and diffract more.
These differences explain why interference patterns have coloured fringes and why dispersion occurs in prisms.
8. Everyday examples of the wave nature of light
Wave effects appear in many familiar situations:
- Colours in oil films or soap bubbles → interference
- Blurring at the edges of shadows → diffraction
- Glare reduction using polarised sunglasses → polarisation
- Rainbow-like patterns on CDs → diffraction
9. Why both wave and ray models are useful
In many daily situations, obstacles and lenses are much larger than the wavelength of light. In those cases, the wavefront bends very little, so light travels almost in straight lines — the ray model is enough.
But whenever patterns, colours, or bending around small openings appear, the wave model becomes essential.