1.1. To consolidate some terms and definitions To consolidate some terms and definitions before moving onbefore moving on

2.2. To describe a standing wave practical related To describe a standing wave practical related to microwaves which is the basis of common to microwaves which is the basis of common exam questionsexam questions

Book Reference : Pages 199-201Book Reference : Pages 199-201

A monochromatic light source provides light of a A monochromatic light source provides light of a single wavelength i.e. A single colour.single wavelength i.e. A single colour.

The definition can be extended with the words The definition can be extended with the words “highly monochromatic” “highly monochromatic” and and “mostly “mostly monochromatic”monochromatic”

For Young’s double slit experiment we use two For Young’s double slit experiment we use two slits to provide two slits to provide two coherentcoherent light sources. light sources.

The practical does The practical does notnot work with light from two work with light from two nearby lamps since the light waves are emitted at nearby lamps since the light waves are emitted at random therefore any interference between the random therefore any interference between the waves would also be random and we would waves would also be random and we would notnot see the characteristic interference patternsee the characteristic interference pattern

Waves are said to be coherent if there is a Waves are said to be coherent if there is a constant phase difference, (can be 0) constant phase difference, (can be 0) between between them them andand they are the same wavelength they are the same wavelength

A collimated light source produces light where A collimated light source produces light where the rays exhibit a high degree of parallelism. the rays exhibit a high degree of parallelism.

They diverge/converge slowly as the wave They diverge/converge slowly as the wave propagates propagates

Many wave/optics experiments require light Many wave/optics experiments require light sources with specific properties in terms of sources with specific properties in terms of coherency and colour composition. Here are coherency and colour composition. Here are some of the options:some of the options:

White lightWhite light

Either natural sunlight or light from a from a Either natural sunlight or light from a from a lamp. This contains the full spectrum of lamp. This contains the full spectrum of colours, (350-650nm). Can be passed through a colours, (350-650nm). Can be passed through a coloured filter to provide coloured filter to provide “mostly “mostly monochromatic” monochromatic” light, i.e. Light dominated by light, i.e. Light dominated by a single coloura single colour

Vapour Lamps & Discharge TubesVapour Lamps & Discharge Tubes

Typically produce light with a dominate colour Typically produce light with a dominate colour “mostly monochromatic”. “mostly monochromatic”.

LasersLasers

Lasers are a convenient source of Lasers are a convenient source of “highly “highly monochromatic”monochromatic” light (within 1nm). The colour light (within 1nm). The colour produced will depend upon the composition of produced will depend upon the composition of the laser (e.g. Helium-neon red light 635nm)the laser (e.g. Helium-neon red light 635nm)

The light is very parallel, (The light is very parallel, (collimatedcollimated) and can be ) and can be readily focussed to a concentrated spot with a readily focussed to a concentrated spot with a very small area. (hence the capacity to burn and very small area. (hence the capacity to burn and the H&S issues) the H&S issues)

Laser light is Laser light is coherentcoherent since the emission of since the emission of photons is not random unlike other light sourcesphotons is not random unlike other light sources

Coherent LightCoherent Light

1.same wavelength or frequency 2.same phase or constant phase difference

Laser LightLaser Light1.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 Recall the standing wave on a string diagrams...diagrams...

The same properties apply to microwaves...The same properties apply to microwaves...

NoteNote

Microwaves are reflected by metalMicrowaves are reflected by metal

See HW questions and virtual practicalSee HW questions and virtual practical