OAT Physics: Optics and Waves 3 — Questions and Answers
Question 1: A concave mirror has a radius of curvature of 30 cm. An object is placed 45 cm in front of it. The image distance is:
- 22.5 cm in front (Correct answer)
- 90 cm behind
- 45 cm in front
- 22.5 cm behind
Correct answer: 22.5 cm in front
Focal length = R/2 = 15 cm; 1/f = 1/do + 1/di → 1/15 = 1/45 + 1/di → di = 22.5 cm in front (real image).
Question 2: In Young's double-slit experiment, if the slit separation is doubled while keeping all else constant, the fringe spacing:
- Doubles
- Is halved (Correct answer)
- Is quadrupled
- Remains the same
Correct answer: Is halved
Fringe spacing Δy = λL/d, so doubling slit separation d halves the fringe spacing.
Question 3: The Doppler effect for sound predicts that as a source moves toward a stationary observer, the observed frequency:
- Decreases
- Remains unchanged
- Increases (Correct answer)
- Becomes zero
Correct answer: Increases
As the source approaches, wavefronts are compressed, increasing the observed frequency (blue shift for sound).
Question 4: Which color of visible light has the greatest energy per photon?
- Red
- Yellow
- Green
- Violet (Correct answer)
Correct answer: Violet
Photon energy E = hf = hc/λ; violet light has the shortest wavelength (~400 nm) and thus the highest frequency and energy.
Question 5: A light ray travels from water (n = 1.33) into glass (n = 1.52). The ray bends:
- Away from the normal
- Toward the normal (Correct answer)
- Parallel to the surface
- No bending occurs
Correct answer: Toward the normal
When light enters a denser medium (higher n), it bends toward the normal per Snell's law.
Question 6: Standing waves in a string fixed at both ends have nodes at:
- Only at the center
- Both fixed endpoints only
- Both endpoints and at integer multiples of half-wavelengths (Correct answer)
- Only at antinodes
Correct answer: Both endpoints and at integer multiples of half-wavelengths
Fixed ends are always nodes; additional nodes occur at every half-wavelength along the string's length.
Question 7: The index of refraction of a material is defined as:
- The ratio of light's speed in the material to speed in vacuum
- The ratio of speed in vacuum to speed in the material (Correct answer)
- The angle of refraction divided by angle of incidence
- The wavelength in vacuum divided by frequency
Correct answer: The ratio of speed in vacuum to speed in the material
Index of refraction n = c/v, where c is the speed of light in vacuum and v is its speed in the medium.
OAT Physics: Optics and Waves 3
A concave mirror has a radius of curvature of 30 cm.
An object is placed 45 cm in front of it.
The image distance is: