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Shine a light on a metal surface and electrons pop off. Shine a brighter light and more electrons come off but with the same energy. Make the light bluer and they have more energy. Albert Einstein explained this: Light, which everyone knew to be electromagnetic waves, also behaved like particles. What was it - waves or particles or "wavicles"? Albert Einstein got the Nobel Prize in 1921 for his explanation of the photoelectric effect, not for relativity. Later it was found that electrons also behave as waves and particles. Helium atoms and even large molecules can behave as waves - how weird is that?
The Hungarian-British physicist Denis Gabor was trying to improve a microscope using electron waves (an electron microscope) with wavelengths much shorter than that of light, hoping to eventually see individual atoms. But electron lenses were not good; the images were hopelessly blurred. In 1947 he had a genius idea! Don't use a lens but record a diffraction pattern between the scattered electrons and a beam of "coherent electron waves." Coherent means the wave peaks and valleys are all in step. Have you seen "the Wave" by fans in a stadium? It was very difficult with electrons, but he tried it with light and it worked, but not very well since he did not have a coherent light beam. Gabor's holography remained a curiosity with no applications.
Everything changed with the invention of the laser in 1960. It became easy to make wide coherent light beams, split them in two with a half-silvered mirror, and illuminate an object and a photographic plate together. The plate looks a mess, but when you illuminate it with an identical light beam an image of the object appears, floating in space and transparent like a ghost. Cut the plate in pieces and each piece has the full image, only fuzzier. Many applications followed: high density data storage, imaging miniscule vibrations, security on credit cards, art and entertainment, and many more.
Last year 2021 was the 50th anniversary of Gabor's Nobel Prize for his invention. Light is waves. And particles! |