Question

what contributions did the following experiments make to the theory of light?

a) Young's double slit experiment(1804)

b)The photoelectric effect (1905)

Answer 1

Answers :

Part (a) :

With the double-slit experiment Young challenged the theories of Isaac Newton and proved that light is a wave, because light suffers the phenomenon of interference that is typical of the waves.

The double-slit experiment is a wonderfully simple experiment, which allowed Thomas Young to demonstrate convincingly the wave nature of light for the first time. When the waves emerging from two narrow slits are superimposed on a screen placed at some distance parallel to the line connecting these slits, a pattern of bright and dark fringes regularly spaced appears on the screen (interference pattern). This is the first clear proof that light added to light can produce darkness. Interference is accompanied by a spatial redistribution of the optical intensity without violation of power conservation.

This phenomenon is known as interference and thanks to this experiment the intuitive ideas of Huygens regarding the wave nature of light were confirmed.

Part (b) :

In 1905 Albert Einstein proposed that light be described as quanta of energy that behave as particles. A photon is a particle of electromagnetic radiation that has zero mass and carries a quantum of energy. The energy of photons of light is quantized according to the E = hv equation. For many years light had been described using only wave concepts, and scientists trained in classical physics found this wave-particle duality of light to be a difficult idea to accept. A key experiment that was explained by Einstein using light’s particle nature was called the photoelectric effect.

The photoelectric effect is a phenomenon that occurs when light shined onto a metal surface causes the ejection of electrons from that metal. It was observed that only certain frequencies of light are able to cause the ejection of electrons. If the frequency of the incident light is too low (red light, for example), then no electrons were ejected even if the intensity of the light was very high or it was shone onto the surface for a long time. If the frequency of the light was higher (green light, for example), then electrons were able to be ejected from the metal surface even if the intensity of the light was very low or it was shone for only a short time. This minimum frequency needed to cause electron ejection is referred to as the threshold frequency.