1.To consolidate some terms and definitions before moving on 2.To describe a standing wave practical related to microwaves which is the basis of common exam questions Book Reference : Pages

A monochromatic light source provides light of a single wavelength i.e. A single colour. The definition can be extended with the words “highly monochromatic” and “mostly monochromatic”

For Young’s double slit experiment we use two slits to provide two coherent light sources. The practical does not work with light from two nearby lamps since the light waves are emitted at random therefore any interference between the waves would also be random and we would not see the characteristic interference pattern Waves are said to be coherent if there is a constant phase difference, (can be 0) between them and they are the same wavelength

A collimated light source produces light where the rays exhibit a high degree of parallelism. They diverge/converge slowly as the wave propagates

Many wave/optics experiments require light sources with specific properties in terms of coherency and colour composition. Here are some of the options: White light Either natural sunlight or light from a from a lamp. This contains the full spectrum of colours, ( nm). Can be passed through a coloured filter to provide “mostly monochromatic” light, i.e. Light dominated by a single colour

Vapour Lamps & Discharge Tubes Typically produce light with a dominate colour “mostly monochromatic”.

Lasers Lasers are a convenient source of “highly monochromatic” light (within 1nm). The colour produced will depend upon the composition of the laser (e.g. Helium-neon red light 635nm) The light is very parallel, (collimated) and can be readily focussed to a concentrated spot with a very small area. (hence the capacity to burn and the H&S issues) Laser light is coherent since the emission of photons is not random unlike other light sources

Coherent Light 1.same wavelength or frequency  2.same phase or constant phase difference  Laser Light 1.monochromatic waves of single frequency/wavelength  2.collimated produces an approximately parallel beam  3.coherent waves produced are in constant phase  4.polarised vibrations in 1 plane only 

Recall the standing wave on a string diagrams...

The same properties apply to microwaves... Note Microwaves are reflected by metal See HW questions and virtual practical