Waves Module Summary Elliott Describing waves basic Describing

Waves Module Summary Elliott

Describing waves (basic) •

Describing waves (continued) • The phase of a wave describes the position in its one period cycle in degrees or radians. The phase difference describes how “out of step” two waves are. • Waves can be; • Longitudinal – oscillate parallel to the direction of travel e. g. sound waves or • Transverse – oscillate at right angles to the direction of travel e. g. electromagnetic radiation (light).

Coherence • Two waves that have the same frequency and 0 phase difference are called coherent. • Waves can be; • Spatially coherent – coherent (in phase) at a given point. • Temporally coherent – coherent at any given time.

Polarisation • Polarisation is when a wave which oscillates in more than one direction (a transverse progressive wave) has its oscillations reduced to just one plane. • Compounds that are optically active will rotate the angle of the polarisation.

Polarisation (continued) • Polarisation can be linear or circular. Circularly polarised waves rotate the plane on which their oscillation is restricted. If the magnitude of the oscillations varies with this rotation then it is elliptically polarised. • Circularly polarised waves consist of two perpendicular electromagnetic plane waves that have the same magnitude but are 90 o out of phase. • To determine whether polarisation is left or right handed, use your curled hand with your thumb in the direction of propagation.

Polarisation methods • Polarisation can occur by dichroism, reflection or birefringence. • Dichroism occurs because certain dichroic materials (often crystals) absorb light of different planes of incidence by different amounts. • When light is reflected, it becomes polarised at the incident angle, at Brewster’s angle it is completely linearly polarised. • Birefringent materials have different refractive indexes (n) for different polarisations of light.

Diffraction • If any wave passes by an obstacle or aperture similar in size to its wavelength the direction and intensity of it will change. This is called diffraction. • The lines in these diagrams are wavefronts. They represent the peaks or troughs of a wave.

Double slit diffraction • In double slit diffraction the peaks and the troughs of the two diffracted waves interact. • This produces an interference pattern (discussed on slide 11). • The effect is that areas of high and low light intensity can be measured on this screen.

Double slit experiment •

Interference • When waves that are “in-sync” react their amplitudes are added. Resultant wave Wave 1 Wave 2 • If they are exactly opposing they will sum to zero. Resultant wave Wave 1 Wave 2 • The effect of waves not exactly in or out of sync can be calculated by the principle of superposition (next slide). This is known as constructive interference This is known as destructive interference

The principle of superposition • When two waves pass a point, the total displacement is the sum of the displacements caused by each individual wave. • Examples:

Diffraction gratings •

Path difference • The diagram shows constructive interference at point C where the path difference is 2λ.

Stationary waves • Stationary waves occur when an oscillation is reflected at a closed end. At closed ends you will get a node, at open ends an antinode. • A node is a point where the wave doesn’t move at all, the points with maximum displacement are the antinodes. • Musical instruments can have closed or open ends. The wave that gets produced can be the fundamental or a frequency an integer number of times higher, a harmonic. Two closed ends (node at each end)

Stationary waves (continued) • Two open ends • One open, one closed • Two closed ends