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Date May 2015 Marks available 2 Reference code 15M.3.HL.TZ1.13
Level Higher level Paper Paper 3 Time zone Time zone 1
Command term Calculate Question number 13 Adapted from N/A

Question

This question is about thin-film interference.


A thin film of oil lies on a puddle of water. White light from above shines on the film at normal incidence.

Outline the process by which coloured fringes are formed.

[3]
a.

The following data are available:

Refractive index of oil = 1.4
Refractive index of water = 1.3
Thickness of the oil film = 250 nm

Calculate the maximum wavelength of the incident light for which destructive interference occurs.

[2]
b.

Markscheme

light reflects from the top surface of the oil and the top surface of the water;
mention of interference/superposition;
path difference exists between both reflected rays;
different wavelengths interfere constructively for different positions/angles (hence colours appear/shift);

a.

\(\lambda \left( { = t\frac{{2n}}{m}} \right) = 250 \times {10^{ - 9}}\frac{{2 \times 1.4}}{1}\);
\(\lambda  = 700\left( {{\rm{nm}}} \right)\);

b.

Examiners report

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

Syllabus sections

Additional higher level (AHL) » Topic 9: Wave phenomena » 9.3 – Interference
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