Light - Particle vs Wave Quiz | Sci / Tech

Quiz Answer Key and Fun Facts

1. This 17th century bout began as the two participants entered the ring. In the blue corner was Christiaan Huygens, a Dutch scientist, who argued that light was a wave. Which scientist, who believed light to be a particle, was in the red corner?

Answer: Isaac Newton

Isaac Newton is one of the world's most familiar and influential scientists. He had his finger in many pies, including calculus, the theories of gravity, optics and mechanics. One of Newton's biggest claims to fame was his three laws of motion as outlined in his "Philosophae Naturalis Principia Mathematica", which was published in 1687.

These laws are still taught internationally in schools and are the base of many physical principals. Indeed science itself was greatly influenced by Newton.

2. In the pre-match press conference, Newton put forward some convincing arguments for his particle theory. Which word did Newton continuously use to describe these particles of light?

Answer: Corpuscles

As with many theories of Newton, the particle theory was well documented. At this time (17th century) reflection of light could be demonstrated. This is a "wave property" and so allowed many to challenge Newton and potentially weaken the argument that light was a particle. Newton claimed that reflection was caused by a "force" on the surface on which light landed that caused the corpuscles to change direction.

3. There was some pre-existing tension to this contest as Newton and Huygens also disagreed over the way light behaved in a material that was more dense than air. With reference to air, which of the following best describes Newton's theory of the behaviour of light?

Answer: Light travels faster in more dense material

Both scientists explained their theories of light behaviour in denser materials by displaying the refraction of light.

While Newton believed that light travelled faster in a more dense material, Huygens stated that light travelled slower in denser materials.

When entering a more dense material, light will "bend" towards the normal (90 degrees to the change of medium). Newton stated that the parallel component of velocity of the light would increase when entering a more dense material and as the perpendicular component remains unchanged, the light would therefore appear to "bend" towards the normal. Huygens on the other hand explained the "bend" towards the normal as being caused by light travelling slower in the denser material. He used the principal that the greater the difference in refractive index (change in speed) between the two materials the greater the refraction of light. In this case Newton was incorrect.

4. In this confrontation over whether light was a wave or a particle, the bookies had Newton as a clear favourite to win over his opponent, Huygens. Why was this?

Answer: Because of Newton's pre-eminence in science

Newton's works such as "Philosophae Naturalis Principia Mathematica" were funded by other scientists such as Halley. He was therefore able to demonstrate his views to the masses, and, along with his many other associations in the field of science, he became a household name. Huygens, on the other hand, lacked the reputation for revolutionary scientific theories which Newton had achieved and so faded into the background.

5. With the crowd behind Newton, he began to pull ahead. However, assistance was given to Huygens by a fellow scientist. This assistance came in the form of the double slit experiment. Which scientist was behind this famous experiment?

Answer: Thomas Young

Thomas Young is also the scientist behind Young's Modulus - a scientific quantity which is determined by the elastic nature of a material.

The result obtained by Young's double slit experiment was based on the theory of diffraction of light - a wave property. The reason that the double slit experiment could not be performed until the early 19th century was because two coherent sources of light could not be obtained. Young combated this by using a single slit to diffract light from a source allowing the light to spread, thus illuminating the double slit. The light emitted by each slit in the double slit would therefore have the same wavelength and have a fixed phase difference between them (i.e. are coherent).

6. The people in the crowd could hardly believe their eyes as the double slit experiment demonstrated two wave properties of light. One of these wave properties was diffraction. What was the other?

Answer: Interference

Waves have a special property of being able to overlap. This is called interference and when the two waves which interfere are coherent they produce interference patterns. The pattern produced in Young's double slit experiment consisted of alternating bright and dark fringes, with the brightest fringe being at the centre.

This occurs as when the phase difference between the two waves is half a wavelength, the interference is destructive and a dark fringe is formed. When the waves have a phase difference of a whole wavelength, the interference is constructive and a bright fringe is formed.

7. Ding ding! Round one was over and Huygens' wave front theory became widely accepted in the world of science. Originally thought to be a longitudinal wave, light was proven to have the same speed as all electromagnetic waves in a vacuum and so was shown to be a transverse wave. Which scientist proved that all electromagnetic waves travelled at the speed 3 x 10^8 m/s?

Answer: James Clerk Maxwell

The realisation that light was a transverse wave came after scientists determined the speed of light to be 3 x 10^8 m/s and saw the clear similarity to the value found in Maxwell's equations.

Electromagnetic waves are transverse as the propagation of the wave is perpendicular to the oscillations of the electrical and magnetic components which make it up. The propagation of longitudinal waves is in the same direction as the oscillations which cause it.

Light was also shown to be a transverse wave by the fact it could be polarised. This could be demonstrated by using a polarising slide which only transmits wave propagations in one plane.

8. In round two of the confrontation to decide whether light was a wave or a particle, Newton now began to fight back. The photoelectric effect proved that light could behave as photons - an idea more consistent with the theory that light was a particle. When these photons hit a metal surface, what could be emitted?

Answer: Electrons

During the early 20th century it was shown that light with higher frequencies was more likely to cause electron emission from the metal surface. This is because light with higher frequency has more energy and so can provide sufficient energy to the electrons, allowing them to overcome the electrostatic force of attraction between themselves and the positive nuclei.

This showed that, to an extent, the particle theory was correct as it could not be explained by the wave theory. If Huygens' theory was applied and light arrived at the metal surface as a continuous stream of waves, then eventually electrons would always be emitted, regardless of frequency.

9. After the tremendous blows dealt to Newton by his opponent (Huygens) in the first round it meant that he was only able to recover with help from his corner. Which scientist was responsible for explaining the photoelectric effect, thus winning a Nobel Prize and supporting Newton's idea that light was a particle?

Answer: Albert Einstein

Einstein's particle theory had been predictably rejected by scientists such as Planck and Bohr. This was mainly due to the history of the debate and the seemingly overwhelming evidence that light was, and could only be, a wave.

10. So, where does this leave us? Is light a wave or a particle? It went to a judge's decision and in the end it was up to one judge to deliver the result. The answer is that it is both! A draw! Which scientist suggested that all subatomic particles could show what he called "wave-particle duality"?

Answer: Louis De Broglie

The ability of light to behave as both of these can be shown by the equation which allows the momentum to be calculated for a particle (mass x velocity) and for a wave (Planck's constant / wavelength).

Note that light cannot behave as a wave and a particle at the same time.

As well as light, electrons can behave as both waves and particles. This can be shown by the fact that electrons can be both deflected (particle property) and diffracted (wave property).

Source: Author doublemm

This quiz was reviewed by FunTrivia editor crisw before going online.
Any errors found in FunTrivia content are routinely corrected through our feedback system.

Light - Particle vs Wave Trivia Quiz

Here it is! The post match analysis of the showdown to decide once and for all whether light is a particle or a wave. Who will be the victor? Play this quiz and be prepared to be taken through the 350 year face-off!

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