NESA Physics Light: Wave Model
10 sample questions with marking guides and sample answers · Avg. score: 75%
After coherent light has been passed through a double slit, the observation of an interference pattern on a screen is explained by the
wave nature of light.
equal width of the slits.
discrete packets of photons.
distance from the slits to the screen.
Reveal Answer
wave nature of light.
Interference is a fundamental property of waves where overlapping wavefronts add constructively or destructively; this experiment is the classic evidence for the wave nature of light.
equal width of the slits.
While slit width affects the diffraction envelope and contrast, the interference pattern itself is caused by wave superposition, which can occur even if the slits are not perfectly equal in width.
discrete packets of photons.
Discrete packets (photons) refer to the particle nature of light; if light behaved strictly as classical particles, it would form two distinct bands rather than an interference pattern.
distance from the slits to the screen.
The distance to the screen affects the spacing of the fringes (scale), but it is not the fundamental cause of the interference phenomenon itself.
Discuss the nature of light by describing evidence from two key experiments.
Reveal Answer
Young's double slit experiment and black-body radiation both provide evidence for the behaviour of light.
In Young's double slit experiment, the interference patterns formed as light passed between the two slits demonstrates the wave nature of light.
In contrast, black-body radiation demonstrates the quantised nature of light as electrons can only absorb or emit energy in discrete amounts.
Therefore, light has some wave properties and some particle properties.
| Descriptor | Marks |
|---|---|
Identifies evidence for the nature of light comes from Young's double slit experiment | 1 |
Identifies evidence for the nature of light comes from black-body radiation | 1 |
Describes evidence for wave nature of light | 1 |
Describes evidence for photons | 1 |
Concludes light has the properties of both waves and particles | 1 |
Which one of the following phenomena best demonstrates that light waves are transverse?
polarisation
interference
dispersion
diffraction
Reveal Answer
polarisation
Polarisation restricts the oscillations of a wave to a single plane perpendicular to the direction of travel, a property unique to transverse waves. Longitudinal waves cannot be polarised because their oscillations are parallel to the direction of propagation.
interference
Interference is the superposition of waves to form a resultant wave. This phenomenon occurs in both transverse and longitudinal waves (like sound), so it does not prove light is transverse.
dispersion
Dispersion is the separation of light into different colors due to varying refractive indices for different frequencies. While it demonstrates wave properties, it does not specifically prove the wave is transverse.
diffraction
Diffraction is the bending of waves around obstacles or through gaps. Because both transverse and longitudinal waves exhibit diffraction, it cannot be used to prove light is transverse.
Young’s double slit experiment demonstrates that light
behaves differently in different frames of reference.
shares characteristics with mechanical waves.
is a longitudinal wave.
acts like a particle.
Reveal Answer
behaves differently in different frames of reference.
This statement relates to the theory of relativity rather than Young's experiment. Young's double slit experiment specifically demonstrates interference and diffraction, which are wave phenomena.
shares characteristics with mechanical waves.
The experiment produces an interference pattern, a phenomenon characteristic of all waves (including mechanical waves like sound or water), thereby demonstrating that light behaves as a wave.
is a longitudinal wave.
Light is a transverse electromagnetic wave, not a longitudinal one. Furthermore, polarization experiments are required to distinguish between transverse and longitudinal waves, while this experiment primarily proves wave nature in general.
acts like a particle.
Young's experiment provided historical evidence against the particle theory of light by demonstrating interference, which classical particles do not exhibit. The particle nature of light is demonstrated by the photoelectric effect.
Two experiments were conducted, and the following observations were made.
| Experiment 1 | Light passing through a double slit produces a diffraction pattern. |
| Experiment 2 | Above a specific frequency, light incident on a metallic surface produces photoelectrons with discrete amounts of energy. |
Which statement can be supported by the observations?
A wave theory of light can completely describe the nature of light.
The bending of light is a result of light behaving as a particle.
The particle model only describes some properties of light.
Only light waves can travel in a vacuum.
Reveal Answer
A wave theory of light can completely describe the nature of light.
This is incorrect because Experiment 2 (the photoelectric effect) demonstrates particle-like behavior that cannot be explained by the wave theory of light.
The bending of light is a result of light behaving as a particle.
This is incorrect because the bending of light (diffraction) observed in Experiment 1 is a characteristic behavior of waves, not particles.
The particle model only describes some properties of light.
This is correct because the particle model explains the photoelectric effect in Experiment 2 but cannot explain the diffraction pattern in Experiment 1, showing it is an incomplete description.
Only light waves can travel in a vacuum.
This is incorrect because the observations provided do not address travel in a vacuum, and light travels through a vacuum regardless of the model used to describe it.
When light from an incandescent lamp is passed through a plane polarising filter, the intensity of the light is reduced.
Explain this phenomenon.
Reveal Answer
Light from an incandescent lamp is unpolarised.
A polarising filter absorbs the component of the electric field of the electromagnetic wave that are not parallel to the polarisation direction of the filter.
The intensity of the light would be reduced as only the parallel components will pass through the filter.
| Descriptor | Marks |
|---|---|
Relates the interaction between the filter and differing planes of oscillation of the electromagnetic waves to the reduction of light intensity | 4 |
Relates the differing planes of polarisation of the light to the absorption of radiation by the polarising filter | 3 |
| 2 |
Provides some relevant information | 1 |
None of the above | 0 |
Explain how Young's double slit experiment provides evidence for the wave model of light.
Reveal Answer
Young’s double slit experiment consisted of light shining through two thin slits. This produced light and dark spots on a screen behind the slits, caused by constructive and destructive interference of the light. This interference is a behaviour seen in mechanical waves and provides evidence for the wave nature of light.
| Descriptor | Marks |
|---|---|
Describes the results of Young’s double slit experiment | 1 |
Recognises that Young’s experiment involves constructive and destructive interference | 1 |
States that the result is similar to that of a mechanical wave | 1 |
Which of the following provides evidence for the model of light proposed by Huygens?
Emission spectra
Diffraction of light
Black body radiation
The photoelectric effect
Reveal Answer
Emission spectra
Emission spectra provide evidence for quantized energy levels in atoms, which relates to the quantum model of light rather than Huygens' classical wave model.
Diffraction of light
Huygens proposed the wave model of light, and diffraction is a characteristic behavior of waves that cannot be explained by a classical particle model.
Black body radiation
Black body radiation provides evidence for the quantization of energy, which led to the particle-like photon model of light rather than the wave model.
The photoelectric effect
The photoelectric effect demonstrates that light behaves as discrete particles (photons), which contradicts Huygens' continuous wave model.
Polarisation of visible light provides evidence that electromagnetic radiation can be explained using a
standing wave model for light.
transverse wave model for light.
mechanical wave model for light.
longitudinal wave model for light.
Reveal Answer
standing wave model for light.
Incorrect. Standing waves are formed by the interference of two waves traveling in opposite directions, which does not explain the phenomenon of polarization.
transverse wave model for light.
Correct. Polarization restricts the oscillations of a wave to a single plane, which is only possible if the wave oscillates perpendicular to its direction of travel, proving light is a transverse wave.
mechanical wave model for light.
Incorrect. Mechanical waves require a physical medium to propagate, whereas light is an electromagnetic wave that can travel through a vacuum. Polarization does not provide evidence for a mechanical nature.
longitudinal wave model for light.
Incorrect. Longitudinal waves oscillate parallel to the direction of propagation, meaning they cannot be restricted to a single plane and therefore cannot be polarized.
Scientists are conducting experiments to compare the circular diffraction patterns formed by X-ray photons and electrons when they pass through small circular apertures. The X-ray photons have an energy of and pass through an aperture of diameter . The electrons are moving at .
Show that the de Broglie wavelength of the electrons is equal to .
Reveal Answer
| Descriptor | Marks |
|---|---|
Calculates the correct de Broglie wavelength of the electrons () by substituting correct values into the de Broglie equation | 1 |
The scientists want an aperture for the electrons that forms diffraction patterns with the same spacing as the diffraction patterns formed by the X-ray photons.
Calculate the diameter of the aperture that the scientists should choose. Show your working.
Reveal Answer
The width of the diffraction pattern can be found from the ratio. The wavelength of the X-rays is found by:
This gives a ratio of:
The electrons, with a de Broglie wavelength of , will also have to have the same ratio of .
| Descriptor | Marks |
|---|---|
Calculates the wavelength of the X-rays () | 1 |
Calculates the ratio of wavelength to slit width for the X-rays () | 1 |
Equates the ratio for the electrons to the ratio for the X-rays | 1 |
Calculates the correct slit width for the electrons () | 1 |