Waves: Catch-Up Learning Objectives Book Reference : Pages 199-201 To consolidate some terms and definitions before moving on To describe a standing wave practical related to microwaves which is the basis of common exam questions

Definitions : monochromatic 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”

Definitions : Coherence 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

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

Practical Light Sources 1 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, (350-650nm). Can be passed through a coloured filter to provide “mostly monochromatic” light, i.e. Light dominated by a single colour

Practical Light Sources 2 Vapour Lamps & Discharge Tubes Typically produce light with a dominate colour “mostly monochromatic”.

Practical Light Sources 3 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

White Light / Laser Comparison:

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

Standing Waves : Practicals & Exam Questions Recall the standing wave on a string diagrams...

Standing Waves : Practicals & Exam Questions The same properties apply to microwaves... Note Microwaves are reflected by metal See HW questions and virtual practical