AP Physics Physics: Waves & Optics 4 — Questions and Answers
Question 1: A thin film of oil (n = 1.4) on water (n = 1.33) has a thickness of 200 nm. For normal incidence, which visible wavelength (in air) experiences constructive interference in reflection?
- 280 nm
- 400 nm
- 560 nm (Correct answer)
- 800 nm
Correct answer: 560 nm
Constructive reflection with one phase inversion: 2nt = mλ → λ = 2(1.4)(200)/1 = 560 nm.
Question 2: A converging lens of focal length 15 cm forms a real, inverted image twice the size of the object. How far is the object from the lens?
- 15 cm
- 22.5 cm (Correct answer)
- 30 cm
- 45 cm
Correct answer: 22.5 cm
Magnification m = −2 means di = 2do; substituting into 1/f = 1/do + 1/di gives do = 1.5f = 22.5 cm.
Question 3: A police siren emits 800 Hz. As the car moves away from a stationary listener at 20 m/s (vsound = 340 m/s), what frequency does the listener hear?
- 754 Hz (Correct answer)
- 800 Hz
- 850 Hz
- 752 Hz
Correct answer: 754 Hz
f' = f(v/(v+vs)) = 800(340/360) ≈ 756 Hz, closest to 754 Hz.
Question 4: In a diffraction grating with 500 lines/mm, light of wavelength 500 nm is incident. At what angle does the second-order maximum appear?
- 14.5°
- 20.1°
- 30.0° (Correct answer)
- 45.0°
Correct answer: 30.0°
d = 1/500 mm = 2000 nm; sin θ = mλ/d = 2(500)/2000 = 0.5, θ = 30°.
Question 5: Which property of a wave changes when it crosses from one medium to another while the frequency remains constant?
- Frequency
- Period
- Wavelength (Correct answer)
- All of the above
Correct answer: Wavelength
When a wave crosses media, frequency is unchanged but speed changes, so wavelength (λ = v/f) must change.
Question 6: An open-pipe organ pipe resonates at its fundamental frequency. If the length is doubled, what happens to the fundamental frequency?
- It doubles
- It is halved (Correct answer)
- It is quartered
- It remains the same
Correct answer: It is halved
For an open pipe, f = v/2L; doubling L halves the fundamental frequency.
Question 7: Polarized light passes through a second polarizer whose axis is 60° from the first. What fraction of the original intensity is transmitted?
- 1/4 (Correct answer)
- 1/2
- 3/4
- √3/2
Correct answer: 1/4
By Malus's law, I = I₀ cos²60° = I₀(0.5)² = I₀/4.
AP Physics Physics: Waves & Optics 4
A thin film of oil (n = 1.4) on water (n = 1.33) has a thickness of 200 nm.
For normal incidence, which visible wavelength (in air) experiences constructive interference in reflection?