OAT Physics: Optics and Waves 2 — Questions and Answers
Question 1: A converging lens has a focal length of 20 cm. An object is placed 30 cm from the lens. Where is the image formed?
- 60 cm on the same side as the object
- 60 cm on the opposite side of the object (Correct answer)
- 12 cm on the opposite side
- 40 cm on the opposite side
Correct answer: 60 cm on the opposite side of the object
Using 1/f = 1/do + 1/di: 1/20 = 1/30 + 1/di → di = 60 cm on the opposite side (real image).
Question 2: Which phenomenon explains why the sky appears blue?
- Reflection of sunlight off the ocean
- Rayleigh scattering of shorter wavelengths (Correct answer)
- Absorption of red wavelengths by the atmosphere
- Diffraction of light around clouds
Correct answer: Rayleigh scattering of shorter wavelengths
Rayleigh scattering preferentially scatters shorter (blue) wavelengths of sunlight in all directions, making the sky appear blue.
Question 3: Two waves with equal amplitude A are superimposed. If they are exactly out of phase (180°), the resultant amplitude is:
- 2A
- A√2
- 0 (Correct answer)
- A/2
Correct answer: 0
Destructive interference occurs when waves are 180° out of phase, causing complete cancellation and a resultant amplitude of zero.
Question 4: The critical angle for total internal reflection at a glass-air interface is 42°. What is the index of refraction of the glass?
- 1.33
- 1.49 (Correct answer)
- 1.52
- 1.65
Correct answer: 1.49
At critical angle: sin(θc) = n_air/n_glass → n_glass = 1/sin(42°) ≈ 1.49.
Question 5: Which type of wave requires a medium to propagate?
- X-rays
- Radio waves
- Sound waves (Correct answer)
- Light waves in vacuum
Correct answer: Sound waves
Sound waves are mechanical waves that require a medium (solid, liquid, or gas) to propagate, unlike electromagnetic waves.
Question 6: A plane mirror forms an image that is:
- Real, inverted, and same size
- Virtual, upright, and magnified
- Virtual, upright, and same size (Correct answer)
- Real, upright, and same size
Correct answer: Virtual, upright, and same size
A plane mirror always produces a virtual, upright image at the same distance behind the mirror, with magnification = 1.
Question 7: The wavelength of light in a medium with index of refraction n = 1.5, if the wavelength in vacuum is 600 nm, is:
- 900 nm
- 400 nm (Correct answer)
- 600 nm
- 300 nm
Correct answer: 400 nm
The wavelength in a medium is λ_medium = λ_vacuum / n = 600 nm / 1.5 = 400 nm.
OAT Physics: Optics and Waves 2
A converging lens has a focal length of 20 cm.
An object is placed 30 cm from the lens.
Where is the image formed?