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Question 18M.2.SL.TZ1.3

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Date May 2018 Marks available [Maximum mark: 8] Reference code 18M.2.SL.TZ1.3
Level SL Paper 2 Time zone TZ1
Command term Calculate, Explain, State Question number 3 Adapted from N/A
3.
[Maximum mark: 8]
18M.2.SL.TZ1.3

A beam of coherent monochromatic light from a distant galaxy is used in an optics experiment on Earth.

The beam is incident normally on a double slit. The distance between the slits is 0.300 mm. A screen is at a distance D from the slits. The diffraction angle θ is labelled.

M18/4/PHYSI/SP2/ENG/TZ1/03.a

(a.i)

A series of dark and bright fringes appears on the screen. Explain how a dark fringe is formed.

[3]

Markscheme

superposition of light from each slit / interference of light from both slits

with path/phase difference of any half-odd multiple of wavelength/any odd multiple of π  (in words or symbols)

producing destructive interference

 

Ignore any reference to crests and troughs.

[3 marks]

(a.ii)

The wavelength of the beam as observed on Earth is 633.0 nm. The separation between a dark and a bright fringe on the screen is 4.50 mm. Calculate D.

[2]

Markscheme

evidence of solving for D «D = s d λ »

« 4.50 × 10 − 3 × 0.300 × 10 − 3 633.0 × 10 − 9 × 2» = 4.27 «m»

 

Award [1] max for 2.13 m.

[2 marks]

The air between the slits and the screen is replaced with water. The refractive index of water is 1.33.

(b.i)

Calculate the wavelength of the light in water.

[1]

Markscheme

633.0 1.33 = 476 «nm»

[1 mark]

(b.ii)

State two ways in which the intensity pattern on the screen changes.

[2]

Markscheme

distance between peaks decreases

intensity decreases

[2 marks]

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