IB Physics SL C.3.10 Thin-film interference
Practise tracking reflected rays in thin films to decide phase reversals and calculate thickness or missing reflected wavelengths.
- Syllabus
- First assessment 2025
- Objective
- —
- Level
- SL
Exam points
- identify on a ray diagram which reflection gives a 180° phase change by comparing refractive indices
- use 2nt = mλ, or the quarter-wave form, at normal incidence to find film thickness or a missing wavelength
- explain a missing reflected colour using two reflected rays, phase reversal, and destructive interference
C.3.10—Thin-film interference question 1
[Maximum number: 6]
C1. This question is about digital devices.
(a) Both a CD and a long playing record (LP) are used to store and reproduce musical sounds.
Outline the difference between these two methods of storing musical sounds.
CD:
LP:
(b) In a particular CD, the wavelength of the laser light used to retrieve the musical sounds stored is 720 nm .
Determine, explaining your answer, the depth d of a pit on the surface of the CD.
(c) A charge-coupled device (CCD), unlike an audio CD, stores optical images. The surface of a CCD is divided into small regions called pixels. Each pixel behaves like a capacitor with capacitance C.
(i) Define capacitance.
(ii) A pixel of a particular CCD has capacitance C=20 pF and a quantum efficiency of 80 %. The pixel is illuminated with light for a short period of time, such that the electric potential of the pixel changes by 0.18 mV .
Estimate the number of photons incident on the pixel in this time period.
C1(a): information is stored digitally on the CD and in analogue form on the LP. CD: sound is first encoded/converted to binary numbers; the binary numbers are converted to pits and lands on the CD surface. LP: the groove shape corresponds to the musical sound. To award [3], candidates must obtain the first marking point plus two others.
C1(b): for destructive interference, path difference between light reflected from a pit and a land is lambda/2 or 360 nm; path difference is 2d; therefore d = 180 nm. Award [2 max] for a clearly labelled diagram and [1 max] for calculation.
C1(c)(i): capacitance is the ratio of charge stored to potential difference / formula with symbols explained.
C1(c)(ii): charge liberated = CV = 20 x 10^-12 x 1.8 x 10^-4 = 3.6 x 10^-15 C; number of photoelectrons = 3.6 x 10^-15 / 1.6 x 10^-19 = 2.25 x 10^4; number of photons = 2.25 x 10^4 / 0.8 = 2.8 x 10^4.
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