SOC PHYSICS: Young's Double Slit Experiment

Saturday, August 7, 2010

Young's Double Slit Experiment

A light source (with a narrow band of wavelengths eg red light) illuminates a thin plate with two parallel slits cut in it, and the red light passing through the slits strikes a screen behind them. The wave nature of light causes the light waves passing through both slits to interfere, creating an interference pattern of bright red and dark bands on the screen.

View the Walter Fendt applett at
http://www.walter-fendt.de/ph14e/doubleslit.htm

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Experimental evidence for the Particle Nature of Light

The photoelectric effect is the emission of photo-electrons from a clean metal surface due to incident light whose frequency is greater than a threshold frequency. This phenomenon cannot be explained by using a wave approach to light, and its explanation led to the development of the photon or particle-like model of light.

In the photon model, a beam of light consists of a stream of photons, each carrying an energy E = hf. The total energy of the beam will be Nhf where N = number of photons in the beam (assumed to be of the same frequency or colour).

h = planck's constant = 6.63 x 10( to the power -34) Js

The wave model failed to explain why:
- the maximum kinetic energy remained the same when the intensity of the beam changed.
- the maximum kinetic energy changed when the frequency of the light changed.
- there was a threshold frequency for each metal used.
- the time elapsed was in nanoseconds before electrons were emitted even for the lowest intensity light.

The wave model would have expected light intensity to be related to its amplitude, which measures energy of the wave. Therefore an electron should absorb any amount of light energy, depending on the time interval it is exposed to the light wave.

Experimental evidence supporting the Wave Model for LIght

Young's Double Slit experiment: the interference pattern seen in a double slit experiment consists of alternating bright and dark fringes resulting from constructive and destructive interference of monochromatic (single frequency) light. The small wavelength of light requires that the slits are very close to each other and that the screen is a significant distance from the slits.

Diffraction of Light: the extent of Diffraction of light is porportional to the ratio of wavelength over slit size (or object width).