Transcript The dual nature of light involving light:

The dual nature of light
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wave theory of light explains most phenomena
involving light:
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wave theory does not explain:
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propagation in straight line
reflection
refraction
superposition, interference, diffraction
polarization
Doppler effect
frequency dependence of thermal radiation
photoelectric effect
IS LIGHT A WAVE OR A PARTICLE?
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answer:
it is both, depending on what question you ask:
it has a “wave'’ aspect and a “particle” aspect
Note:
according to quantum theory, “particles” (e.g.
electrons, protons,..) have also a “wave” aspect!
(depends on what question you ask)
Thermal radiation
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experimental observations:
 atoms of a hot solid emit radiation;
 increase in temperature  more radiation, and
component of maximum intensity shifted towards
higher frequency (shorter wavelength)
“classical” explanation:
the hotter the solid, the more vibrational energy
 higher frequency of vibration of atoms/electrons
 higher frequency of radiation
but frequency spectrum of this radiation (“black body
radiation” calculated within framework of
electromagnetism and thermodynamics did not agree
with measured spectrum;
predicted “ultraviolet catastrophe” I  f4
Max Planck's hypothesis (1900):
energy is quantized;
“oscillators” (oscillating atoms) can only have
certain amounts of energy
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relation between energy and frequency of oscillator:
E = h f,
where h = “Planck’s constant” = 6.63x10-34 Js
calculation of black body spectrum using Planck's
hypothesis gives formula (“Planck formula”) which
describes measured spectra.
= first evidence that energy is quantized
Photoelectric effect
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(first observed by Heinrich Hertz in 1887)
electrons are emitted when certain metallic materials exposed to
light (now used in photocells in cameras, and solar energy cells)
some aspects of photoelectric effect could not be explained by
classical theory:
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classical theory: if light continuos flow of e.m. energy takes
some (calculable) time for wave to supply sufficient energy
for electron to be emitted;
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find experimentally: current flows almost immediately upon
exposure to light;
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classical theory: light of any frequency could cause
photoelectric effect - need only sufficient intensity
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find experimentally: only light with frequency above certain
minimum frequency causes electrons to be emitted
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classical theory: energy of electrons depends on light
intensity
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find experimentally: energy of electrons depends on
frequency
Albert Einstein's explanation:
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assume that not only energy in atoms is quantized, but also
energy carried by light
light comes in “packets of energy” called light quanta or
photons
energy of one photon = h f, where f = frequency of the light.
with this assumption, all aspects of photoelectric effect could
be explained
photon energy vs color of light: E = hf = hc/
 blue light has more energy than red light