Waves, optics and modern physics

Written by Dr. Hamza Farooq (Physics)

ECAT · Physics · 7 questions

Name: ____________________ Date: ____________

In Young's double slit experiment, if the wavelength of light is doubled, the fringe spacing:
- A Stays the same
- B Doubles
- C Halves
- D Becomes zero
Two tuning forks of 256 Hz and 260 Hz are sounded together. What is the beat frequency?
- A 2 Hz
- B 516 Hz
- C 4 Hz
- D 258 Hz
If the momentum of a particle is doubled, its de Broglie wavelength:
- A Becomes four times larger
- B Is halved
- C Doubles
- D Stays the same
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?
- A 7 eV
- B 3 eV
- C 2 eV
- D 5 eV
What is the fundamental frequency of a pipe of length 0.5 m closed at one end? (Speed of sound 340 m/s)
- A 85 Hz
- B 680 Hz
- C 170 Hz
- D 340 Hz
A sample has 800 radioactive nuclei. How many remain undecayed after 2 half-lives?
- A 600
- B 200
- C 100
- D 400
Polarisation of light shows that light is:
- A A sound wave
- B A longitudinal wave
- C A transverse wave
- D A particle only

Answer key

1. B, 2. C, 3. B, 4. B, 5. C, 6. B, 7. C

Fringe spacing = wavelength x D / d, so it is directly proportional to the wavelength.
The beat frequency is the difference between the two frequencies: 260 - 256 = 4 Hz.
The de Broglie wavelength is h / p, so doubling p halves the wavelength.
Maximum kinetic energy = photon energy - work function = 5 - 2 = 3 eV.
For a closed pipe f = v / 4L = 340 / 2 = 170 Hz.
After one half-life 400 remain, and after a second 200 remain.
Only transverse waves can be polarised, because they vibrate at right angles to the direction of travel.