Question
(a) For a progressive wave, state what is meant by wavelength.
…………………………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………………… [1]
(b) A light wave from a laser has a wavelength of 460nm in a vacuum.
Calculate the period of the wave.
period = ……………………………………………… s [3]
(c) The light from the laser is incident normally on a diffraction grating.
Describe the diffraction of the light waves at the grating.
…………………………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………………… [2]
(d) A diffraction grating is used with different wavelengths of visible light. The angle θ of the
fourth-order maximum from the zero-order (central) maximum is measured for each
wavelength. The variation with wavelength λ of sinθ is shown in Fig. 4.1.
(i) The gradient of the graph is G.
Determine an expression, in terms of G, for the distance d between the centres of two
adjacent slits in the diffraction grating.
d = …………………………………………………
(ii) On Fig. 4.1, sketch a graph to show the results that would be obtained for the
second-order maxima.
Answer/Explanation
Ans:
(a) distance moved by wavefront/energy during one cycle/oscillation/period (of source)
or
minimum distance between two wavefronts
or
distance between two adjacent wavefronts
(b) v =λ / T
or
v = fλ and f = 1 / T
\(T = 460 \times 10^{–9} / 3.00 \times 10^8 \)
\(= 1.5\times 10^{–15} s\)
(c) waves pass through/enter the slit(s)
waves spread (into geometric shadow) B1
(d)(i) n λ = d sin θ
G = sin θ / λ
d = 4 / G
(d)(ii) straight line from 400 nm to 700 nm that is always below printed line
straight line has smaller gradient than printed line and is 5 small squares high at wavelength of 700 nm
Question
(a) State the principle of superposition. [2]
(b) A transmitter produces microwaves that travel in air towards a metal plate, as shown in
Fig. 4.1.
The microwaves have a wavelength of 0.040m. A stationary wave is formed between the
transmitter and the plate.
(i) Explain the function of the metal plate. [1]
(ii) Calculate the frequency, in GHz, of the microwaves.
frequency = …………………………………………. GHz [3]
(iii) A microwave receiver is initially placed at position X where it detects an intensity
minimum. The receiver is then slowly moved away from X directly towards the plate.
1. Determine the shortest distance from X of the receiver when it detects another
intensity minimum.
distance = ………………………………………………….. m
2. Determine the number of intensity maxima that are detected by the receiver as it
moves from X to a position that is 9.1cm away from X.
number = ………………………………………………………
[2]
[Total: 8]
Answer/Explanation
Ans
(a) (two or more) waves meet (at a point) B1
(resultant) displacement is the sum of the individual displacements
(b) (i) it is a (wave) reflector / it reflects (the wave)
(b) (ii) v = fλ or c = fλ
f = 3.0 × 108 / 0.040
= 7.5 × 109 (Hz)
= 7.5 × 109 / 109 (GHz)
= 7.5 GHz
(b) (iii) 1 distance = 0.020 m
2 number = 5