The interesting is that light will behave like a wave when it shines upon a single slit or a double slit. That is, it doesn't act like a stream of particles (although I note an interesting point later). Planck was only able to understand the results of the experiments by making the radical assumption that the light came in discrete bundles of energy of size Energy = h x f = h x c / W where h is known as Planck's constant, W is the wavelength of the radiation, f is the frequency of the wave, and c is the speed of light. Note that f x W = c. These packets of light (energy) are referred to as photons. A wave can have any energy, and so this notion is foreign to a wave. There are other examples of experiments where light behaves as a discrete particle rather than as a smoothly varying, spread-out wave, e.g., in the Photo-electric Effect. where some threshold for the ejection of particles seems to exist. In this experiment, the low intensity beam of radiation seems to interact with the screen discretely (one at a time), i.e., like a particle. However, if we wait long enough, the pattern which gets built up on the screen is the one predicted by wave mechanics, and so the light also seems to behave like a wave, Light cannot simultaneously be both a wave and a particle. However, we note that we can make some comments about how light behaves. What apparently is happening is that light seems to travel through space (propogate through the Universe) as if it were a wave, but when it comes time for the light to interact with matter (exchange energy and momentum with matter), it seems to behave as a particle in that it exchamges energy and momentum in discrete chunks. This is apparently true and what it is telling us is that light may be something other than a wave or a particle (something which we don't know about).

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