1. Why wave optics is needed
Ray optics explains many things like reflection, refraction and image formation. But some phenomena cannot be understood by treating light as straight-line rays. Effects like interference, diffraction and polarisation only make sense when I think of light as a wave.
This wave behaviour becomes important when the size of obstacles or openings is comparable to the wavelength of light.
2. Light as a wave
In wave optics, light is treated as an electromagnetic wave that has oscillating electric and magnetic fields. These fields vibrate perpendicular to each other and to the direction of travel.
This wave nature explains many fine details that ray optics cannot.
3. Wavefronts: a useful way to picture light
A wavefront is a surface where all points vibrate in phase. It helps me visualise how light propagates as a wave. Depending on the source, different shapes of wavefronts can appear.
3.1. Types of wavefronts
- Plane wavefront: From a distant light source.
- Spherical wavefront: From a point source.
- Cylindrical wavefront: From a line source.
Wavefronts move forward just like ripples on water.
4. Huygens' principle: how waves move
To understand how a wavefront travels, I use Huygens' principle. It states that every point on a wavefront acts as a source of tiny secondary wavelets. The new wavefront is the surface tangent to these wavelets.
This principle neatly explains reflection and refraction using the wave model.
5. Why ray optics is not enough
There are many observations that ray optics cannot explain:
- Patterns of bright and dark fringes
- Light bending around edges
- Certain orientations of light being blocked
These effects arise only because light behaves like a wave.
6. Three key phenomena in wave optics
Most of wave optics revolves around three main ideas:
6.1. Interference
When two light waves overlap, they add up. Sometimes they make the light brighter, and sometimes they cancel out. This creates a pattern of alternating bright and dark fringes.
The famous Young’s double slit experiment shows interference clearly.
6.2. Diffraction
Diffraction is the bending and spreading of waves around edges or through small openings. Even though ray optics predicts sharp shadows, diffraction shows that the edges blur slightly.
The smaller the opening (close to wavelength size), the more spreading occurs.
6.3. Polarisation
Light waves vibrate in many directions. When vibration is restricted to one direction, the light becomes polarised. This is a purely wave property and cannot be explained using rays.
7. Where wave optics shows up in daily life
Once I understood wave optics, many everyday effects became clearer:
- The rainbow-like colours in soap bubbles (interference)
- The spreading of light through a narrow crack (diffraction)
- Glare reduction in polarised sunglasses (polarisation)
- Colour patterns on thin oil films (interference)
8. How wave optics connects to modern physics
Wave optics is not only beautiful but also essential in many technologies:
- Optical fibres
- Lasers
- Holography
- Anti-reflective coatings
- Precision instruments like interferometers
All of these depend on understanding how light waves add, bend and vibrate.