Exam-style Questions: Diffraction

1. a) Explain the meaning of the term diffraction.

(2 Marks)

b) i) Describe how transverse water waves with a plane wavefront may be produced in a ripple tank.

(2 Marks)

ii) State how the wavelength of the waves could be shortened.

(1 Mark)

c) Fig.5.1 shows plane water waves in a ripple tank approaching a narrow gap, the size of which is approximately the same as the wavelength of the waves.

i) On Fig. 5.1, draw the pattern of the wavefronts emerging from the gap.

(2 Marks)

ii) Describe how the pattern of wavefronts emerging from the gap would change if the size of the gap were significantly increased.

(2 Marks)

iii) State why, under normal circumstances, light seems to travel in a straight line and does not appear to be diffracted.

(1 Mark)

(Marks available: 10)

2. A single slit is illuminated by a monochromatic (single colour) light source. A screne is placed 5.0 m away from this slit and two very narrow, parallel slits, 0.5 mm apart, are placed half way between the single slit and the screen. Interference fringes are visible on the screen and 10 fringe spaces measure 20 mm on the screen.

a) What is the wavelength of the light?

(3 Marks)

b) What happens if you use double slits with half the spacing between them?

(1 Mark)

c) What happens if you cover one of the double slits?

(2 Marks)

(Marks available: 6)

Answer outline and marking scheme for question:

1. a) the spreading out of the wavefront/waves

{do not allow 'the spreading out of light/sound' or 'bending of waves'}

when they pass through a gap (OR pass an obstacle)

(2 Marks)

b) i) a straight strip (OR bar OR ruler)

is vibrated vertically OR up and down (in the water)

(2 Marks)

ii) increase the frequency (of the waves/wave source)

OR use a shallower depth of water

(1 Mark)

c) i) semicircular wavefronts drawn

no change in : i.e. approx. same before & after gap

(2 Marks)

ii) less diffraction occurs

wavefronts only slightly curved at edges (OR shown in a diagram)

{full marks may be scored from a valid diagram}

(2 Marks)

iii) Wavelength of light much smaller than most gaps

(1 Mark)

(Marks available: 10)

2. a)

 λ = ax (1 Mark) D

where,

λ = wavelength

a = double slit spacing = 0.5 m m

x = fringe spacing = 20/10 = 2.0 mm

D = double slit to screen distance = 2.5 m

 So, λ = 0.5 x 10-3 x 2.0 x 10-3 = 4.0 x 10-7 m (1 Mark) 2.5 (1 Mark)

(3 Marks)

b) If a is halved and D and λ remain the same, the fringe spacing (x) must be doubled.

(1 Mark)

c) The fringes disappear (1 Mark) and you can see a single slit pattern. This is a bright central line with parallel, fainter lines visible on each side.

(2 Marks)

(Marks available: 6)