DP Physics (last assessment 2024)

Question 21M.2.HL.TZ1.6b

Date	May 2021	Marks available	[Maximum mark: 3]	Reference code	21M.2.HL.TZ1.6b
Level	HL	Paper	2	Time zone	TZ1
Command term	Explain	Question number	b	Adapted from	N/A

[Maximum mark: 3]

21M.2.HL.TZ1.6b

A painting is protected behind a transparent glass sheet of refractive index n_glass. A coating of thickness w is added to the glass sheet to reduce reflection. The refractive index of the coating n_coating is such that n_glass > n_coating > 1.

The diagram illustrates rays normally incident on the coating. Incident angles on the diagram are drawn away from the normal for clarity.

(b)

Explain the condition for w that eliminates reflection for a particular light wavelength in air $λ_{air}$ .

[3]

Markscheme

«to eliminate reflection» destructive interference is required ✓

phase change is the same at both boundaries / no relative phase change due to reflections ✓

therefore $2 w n_{coating} = (m + \frac{1}{2}) λ_{air}$

$w = \frac{λ_{coating}}{4}$

$w = \frac{λ_{air}}{4 n_{coating}}$ ✓

Syllabus sections

Additional higher level (AHL)

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

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

Question 21M.2.HL.TZ1.6b

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