AS Physics

Few more exam questions that need checking.

4) Arrangement where microwaves leave a transmitter T and move in a direction TP which is perpendicular to a metal plate P.

a) When a microwave detector D is slowly moved from T towards P the pattern of the signal strength received by D is high, low, high, low...etc

Explain
- why these maxima and minima of intensity occur (2)
Microwaves from T reflect off P to form two waves travelling in opposite directions, a standing wave. Amplitudes are reinforced at various points when waves are in phase between T and P causing constructive interference (maxima). Amplitudes cancel out when waves are approximately 0.5 wavelengths out of phase, causing destructive interference (minima)

- how you would measure the wavelength of the microwaves (2)
Distance between area of minima intensity and another area of minima intensity = 0.5 wavelengths. Multiply by 2 to get wavelength.

- how you would determine their frequency (2)
Electromagnetic waves travel at the same speed in air/free space.
Therefore use, f = v/lamda
v = 3*10^8ms^-1, lamda = wavelength calculated

4)b Describe how you could test whether the microwaves leaving the transmitter are plane polarised. (2)
Place a grid with horizontal dividers (is there a name for this?) in front of an area of maxima intensity. If maxima intensity is still measured at this point, waves are horizontally polarised. Rotate grid 90 degrees, if area of maxima occurs when this has been done, waves a vertically polarised. E.g. the waves vibrate in the same plane.

5)b Arrangement for viewing a visible interference pattern on a screen.
In a darkened room, a double slit (S1S2) is placed in front of a narrow single slit situated in front of a monochromatic light source.

Explain how the arrangement shown ensures that the slits S1 and S2 act as coherent light sources. (2)
Light is from a monochromatic source. Frequency is the same when passed through both slits, S1 and S2. S1 and s2 are equidistant from light source therefore waves passing through slits will be in phase, ie no time delay between waves meeting at same point.

Are these answers okay? Can anyone else add anything?