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Waves, optics and modern physics
7 ECAT Physics questions with answers and explanations.
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In Young's double slit experiment, if the wavelength of light is doubled, the fringe spacing:
- 1 Halves
- 2 Doubles
- 3 Stays the same
- 4 Becomes zero
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Correct answer: Doubles
Fringe spacing = wavelength x D / d, so it is directly proportional to the wavelength.
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Two tuning forks of 256 Hz and 260 Hz are sounded together. What is the beat frequency?
- 1 516 Hz
- 2 4 Hz
- 3 2 Hz
- 4 258 Hz
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Correct answer: 4 Hz
The beat frequency is the difference between the two frequencies: 260 - 256 = 4 Hz.
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If the momentum of a particle is doubled, its de Broglie wavelength:
- 1 Doubles
- 2 Is halved
- 3 Stays the same
- 4 Becomes four times larger
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Correct answer: Is halved
The de Broglie wavelength is h / p, so doubling p halves the wavelength.
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A metal has a work function of 2 eV and is lit by photons of 5 eV. What is the maximum kinetic energy of the emitted electrons?
- 1 7 eV
- 2 2 eV
- 3 5 eV
- 4 3 eV
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Correct answer: 3 eV
Maximum kinetic energy = photon energy - work function = 5 - 2 = 3 eV.
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What is the fundamental frequency of a pipe of length 0.5 m closed at one end? (Speed of sound 340 m/s)
- 1 340 Hz
- 2 170 Hz
- 3 680 Hz
- 4 85 Hz
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Correct answer: 170 Hz
For a closed pipe f = v / 4L = 340 / 2 = 170 Hz.
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A sample has 800 radioactive nuclei. How many remain undecayed after 2 half-lives?
- 1 400
- 2 200
- 3 100
- 4 600
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Correct answer: 200
After one half-life 400 remain, and after a second 200 remain.
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Polarisation of light shows that light is:
- 1 A longitudinal wave
- 2 A transverse wave
- 3 A particle only
- 4 A sound wave
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Correct answer: A transverse wave
Only transverse waves can be polarised, because they vibrate at right angles to the direction of travel.