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Date May 2018 Marks available 2 Reference code 18M.2.SL.TZ1.3
Level Standard level Paper Paper 2 Time zone Time zone 1
Command term State Question number 3 Adapted from N/A

Question

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

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

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

[3]
a.i.

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]
a.ii.

Calculate the wavelength of the light in water.

[1]
b.i.

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

[2]
b.ii.

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 \(\pi \) (in words or symbols)

producing destructive interference

 

Ignore any reference to crests and troughs.

[3 marks]

a.i.

evidence of solving for D «D = \(\frac{{sd}}{\lambda }\)»

«\(\frac{{4.50 \times {{10}^{ - 3}} \times 0.300 \times {{10}^{ - 3}}}}{{633.0 \times {{10}^{ - 9}}}}\) × 2» = 4.27 «m»

 

Award [1] max for 2.13 m.

[2 marks]

a.ii.

\(\frac{{633.0}}{{1.33}}\) = 476 «nm»

[1 mark]

b.i.

distance between peaks decreases

intensity decreases

[2 marks]

b.ii.

Examiners report

[N/A]
a.i.
[N/A]
a.ii.
[N/A]
b.i.
[N/A]
b.ii.

Syllabus sections

Core » Topic 4: Waves » 4.4 – Wave behaviour
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