Geometrical vs wave optics
Advanced
Ray optics is valid when objects are much larger than λ.
Ray optics is valid when objects and apertures are much larger than the wavelength. When sizes approach λ, diffraction and interference appear.
Lens design uses rays; resolution limits use waves.
The two pictures are consistent in their domains.
In more detail
Geometrical optics treats light as rays and explains imaging with lenses and mirrors. Wave optics explains interference, diffraction and polarisation, which the ray model cannot describe.
Real-world uses
- Designers use ray optics for most lens and camera systems.
- Diffraction and interference are essential for gratings and coatings.
- Lens designers use ray optics for large lenses and wave optics when the aperture is comparable to the wavelength.
Open the interactive lesson, with simulation and quiz →
Frequently asked questions
When is the ray model valid?
When the features of the optical system are much larger than the wavelength of light.
Do the two models contradict each other?
No. Geometrical optics is the short-wavelength limit of wave optics.
When can I ignore the wave nature of light?
When the objects and apertures are much larger than the wavelength, so diffraction and interference are small.
Related topics in Optics
Colour and the eye · Additive and subtractive colour mixing · Photometry