A-LevelPhysicsSuperposition9702/22 · Feb/Mar 2020 · Q49 Marks

For a progressive wave, state what is meant by: the wavelength

(a)(i)For a progressive wave, state what is meant by: the wavelength[1]
(a)(ii)the amplitude.[1]
(b)(i)A beam of red laser light is incident normally on a diffraction grating. Diffraction of the light waves occurs at each slit of the grating. The light waves emerging from the slits are coherent. Explain what is meant by: 1. diffraction 2. coherent.[2]
(b)(ii)The wavelength of the laser light is 650nm. The angle between the third order diffraction maxima is 68°, as illustrated in Fig. 4.1. [Figure 4.1] Calculate the separation d between the centres of adjacent slits of the grating.[3]
(b)(iii)The red laser light is replaced with blue laser light. State and explain the change, if any, to the angle between the third order diffraction maxima.[2]

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Mark scheme

Marking points from the official mark scheme (Feb/Mar 2020). The mark or mark type is shown in brackets.

B1 distance moved by wavefront / energy during one cycle / vibration / oscillation / period (of source)
B1 or
B1 minimum distance between two wavefronts
B1 or
B1 distance between two adjacent wavefronts
B1 maximum displacement (of particle / point on wave)
B1 light / waves spread (at each slit)
B1 constant phase difference (between light / waves)
C1 nλ = d sinθ
C1 d = 3 x 650 × 10⁻⁹ / sin34°
A1 d = 3.5 × 10⁻⁶ m
M1 wavelength of blue light is shorter (than 650 nm / red light)
A1 so angle (between third order diffraction maxima) decreases

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