red laser beam passing through a diffraction grating. image: en.academic.ru
Diffraction is a test for wave behaviour. When a ray of light passes through a diffraction grating, the energy of the incident beam is split into a series of interference fringes. Constructive interference is occurring at each location where a fringe (or spot) is observed because the rays are in phase when they arrive at these points.
diffraction spots projected on to a wall. image: microscopy uk
We can measure the relative positions of the fringes in a diffraction pattern to determine the wavelength of the light used. The diffraction grating equation is
- m is the diffracted order – some resources may use n instead of m
- λ is the wavelength
- d is the line spacing.
Here is an infrared diffraction experiment you can try at home to calculate the wavelength of the infrared LED in a remote control.
I’ve attached a set of pdf notes and questions on diffraction. These notes use n rather than m for the diffracted order.