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Modern Physics
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When matter is struck by electromagnetic waves with relatively short wavelengths, such as ultraviolet light or visible
light, its atoms can emit electrons. This process is known as the photoelectric effect or, less commonly, the Hertz
effect, and it occurs because electromagnetic waves possess energy that is capable of dislodging the electrons in
an atom. The observation of the photoelectric effect helped to clarify some issues about the nature of light and
about the nature of atoms. Light, it was discovered, can act both as a wave and as a particle; light travels in wave
motion but can physically impact surfaces and even cause mechanical change by dislodging electrons from atoms.
The photoelectric effect is usually observed when light is shined on metallic surfaces. The beam of light that is
shone in a metal surface is referred to as the photocathode, and the electrons that it ejects from an atom are
called photoelectrons. Shining light on a conductive metal surface can actually cause an electrical current, called
a photocurrent , to form. A material that is sensitive to light, such as the metals which can carry an electrical
current because of light, are referred to as photosensitive substances
The number of electrons ejected as a result of the photoelectric effect is closely related to the frequency and the
intensity of the light shone on the metal surface. Low frequency light, which has a long wavelength, tends to
dislodge few, if any, electrons from a metallic surface. This is true if the light is of high intensity or low intensity.
At high frequency, however, light tends to dislodge far more electrons, especially if the light is particularly
intense. This basically means that, at any intensity, red light will release very few electrons but blue light will
dislodge many !!!!!
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The phenomenon of electrons being emitted from a metal when struck by incident electromagnetic radiation. Einstein extended Planck's quantum hypothesis by postulating that quantization was not a property of the emission mechanism, but rather an intrinsic property of the electromagnetic field. Using this hypothesis, Einstein was able to explain the observed phenomenon that the maximum kinetic energy K of emitted electrons varied with frequency
of incident radiation as
where h is Planck's constant,
is a characteristic energy associated with a given metal and called the work function, c is the speed of light, and 
is the wavelength. This is exactly the result expected if photons are quantized with energies 
, which is exactly what Einstein proposed.