AP Physics Physics: Waves & Optics 5 — Questions and Answers
Question 1: A pipe closed at one end and open at the other has a length of 0.85 m. What is its fundamental frequency? (speed of sound = 340 m/s)
- 100 Hz (Correct answer)
- 200 Hz
- 400 Hz
- 50 Hz
Correct answer: 100 Hz
For a closed pipe, f = v/4L = 340/(4×0.85) = 100 Hz.
Question 2: A virtual image is formed by which of the following?
- A concave mirror when the object is beyond the center of curvature
- A convex lens when the object is beyond the focal point
- A plane mirror (Correct answer)
- A concave mirror when the object is at the focal point
Correct answer: A plane mirror
A plane mirror always produces a virtual, upright image located behind the mirror.
Question 3: When light travels from air into a medium with index of refraction n = √2, and the angle of incidence is 45°, what is the angle of refraction?
- 20°
- 30° (Correct answer)
- 45°
- 60°
Correct answer: 30°
Snell's law: sin 45° = √2 sin θr → sin θr = (√2/2)/√2 = 0.5, so θr = 30°.
Question 4: The intensity of a sound wave decreases as 1/r² from a point source. If you move from 2 m to 6 m from the source, by what factor does the intensity decrease?
- 3
- 6
- 9 (Correct answer)
- 12
Correct answer: 9
I ∝ 1/r²; ratio = (6/2)² = 9, so intensity decreases by a factor of 9.
Question 5: In Young's double-slit experiment, which change would produce brighter but fewer fringes per unit length on the screen?
- Decreasing the wavelength
- Moving the screen closer
- Increasing the slit separation
- Decreasing the slit separation (Correct answer)
Correct answer: Decreasing the slit separation
Decreasing slit separation increases fringe spacing (Δy = λL/d), producing wider, more spread-out fringes with no change in brightness per fringe.
Question 6: Which term describes the superposition of two waves that results in a wave of reduced or zero amplitude?
- Constructive interference
- Resonance
- Destructive interference (Correct answer)
- Diffraction
Correct answer: Destructive interference
Destructive interference occurs when the crest of one wave aligns with the trough of another, reducing the resultant amplitude.
Question 7: A photon of light has energy E = hf. If the frequency of the light is tripled, what happens to its wavelength?
- It triples
- It is halved
- It is reduced to one-third (Correct answer)
- It doubles
Correct answer: It is reduced to one-third
Since λ = c/f, tripling f reduces λ to one-third (λ → λ/3).
AP Physics Physics: Waves & Optics 5
A pipe closed at one end and open at the other has a length of 0.85 m.
What is its fundamental frequency? (speed of sound = 340 m/s)