Wave and photon models of lightEdexcel International A Level Physics: Subtopic test

1. 1
   A teacher is comparing the wave model and the photon model of light. She demonstrates Young's double-slit experiment with a laser and then discusses how light is emitted and absorbed by atoms.

   (a)

   Which observation is best explained by the wave model of light?

   [1 mark]

   • ALight casts sharp shadows of large objects
   • BLight is reflected from a plane mirror
   • CLight carries energy from the Sun to the Earth
   • DBright and dark fringes are seen when light passes through two narrow slits

   (b)

   Which statement is part of the photon model of electromagnetic radiation?

   [1 mark]

   • ARadiation is emitted and absorbed in discrete packets of energy
   • BRadiation spreads its energy continuously over each wavefront
   • CThe energy of a photon is proportional to its wavelength
   • DPhotons carry an electric charge

   (c)

   Explain why a particle model of light cannot account for the fringes observed in the double-slit experiment.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A laser pointer emits red light of wavelength 650 nm. A second laser pointer emits green light of wavelength 532 nm with the same power output. Use the Planck constant h = 6.63 × 10⁻³⁴ J s and the speed of light c = 3.00 × 10⁸ m s⁻¹.

   (a)

   Which statement about the photons emitted by the red laser pointer is correct?

   [1 mark]

   • AThe energy of each photon is greater when the beam is brighter
   • BEvery photon in the beam has the same energy
   • CThe photons have a range of different energies
   • DThe energy of each photon is inversely proportional to its frequency

   (b)

   Which pointer emits more photons per second?

   [1 mark]

   • AThe green pointer, as its photons carry more energy
   • BThe two emit the same number, as their powers are equal
   • CThe red pointer, as each of its photons carries less energy
   • DThe green pointer, as its light travels faster

   (c)

   Calculate the energy of one photon from the red laser pointer.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A sodium street lamp emits yellow light of wavelength 589 nm. The power emitted by the lamp as light is 20 W. Use the Planck constant h = 6.63 × 10⁻³⁴ J s and the speed of light c = 3.00 × 10⁸ m s⁻¹.

   (a)

   Calculate the energy of one photon emitted by the lamp.

   [3 marks]

   (b)

   Calculate the number of photons emitted by the lamp each second, and comment on why the light appears to be continuous.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Ideas about the nature of light have changed over several centuries. Today, electromagnetic radiation is described by a wave model and a photon model, and a physicist compares radio waves of frequency 1.0 × 10⁸ Hz with X-rays of frequency 1.0 × 10¹⁸ Hz. Use the Planck constant h = 6.63 × 10⁻³⁴ J s.

   (a)

   Describe how ideas about the nature of light have developed over time.

   [6 marks]

   (b)

   Calculate the energy of one photon of each radiation, and explain why the wave model is sufficient for radio waves but the photon model is needed to describe the X-rays.

   [6 marks]

   Total for question 4: 12 marks

End of questions