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WHAT IS LIGHT?
The nature of light is a question which has intrigued physicists for
centuries. To the physicist, light is a form of energy but does it also act like
matter? Sir Isaac Newton favored a particle model of light. Others such as
Huygens favored a wave model. Which is it? Is light a particle or a wave?
A look at both models poses a curious result. A wave model works
very well for reflection, refraction, dispersion, diffraction and interference.
A particle model is needed to explain the fact that light exerts pressure and
has momentum. Indeed, the photoelectric effect cannot be explained
without the particle model. Even more curious is the idea that particles
exhibit wave properties under certain conditions.
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BASIC WORDS TO KNOW:
Note: All angles are measured from a normal or perpendicular to the surface.
REFLECTION: Light reflects so that the angle of incidence equals the
angle of reflection.
REFRACTION: Light bends as it passes from one medium to another.
This bending occurs because the wave front travels slower in the more
optically dense medium. The angle of incidence is the angle measured
between the direction of propagation and the normal for the incident
wave. The angle of refraction is the angle measured between the
direction of propagation and the normal for the refracted wave.
Snell's Law gives the mathematical relationship between the two angles
and the media the light travels in. The index of refraction--
"n"-- is a constant for each medium and can be obtained by n = speed
of light in a vacuum / speed of light in the medium. n = c/v
See a diagram below of a light wave front passing from air to water!
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Notice that the wavelength
changes as the wave passes
from one medium to
another. The frequency does
not change! Using the wave
equation, you can see that
V1/V2 = wavelength 1/wavelength 2.
Also, n = wavelength in a vacuum/
wavelength in medium.
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n of air = 1.0
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n of water = 1.33
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MORE WORDS TO KNOW:
DIFFRACTION: The bending of a wavefront as it passes through an
opening is a phenomenon called diffraction.
The smaller the opening in comparison to the wavelength of the
wave, the more bending occurs.
DISPERSION: Light breaks into the colors of the spectrum when the
white light passes through a prism. This is known as dispersion.
POLARIZATION: An electromagnetic wave is a transverse one. When the
wave's vibration is constrained to one plane of vibration instead of the
space all around, then it has become polarized.
Study the diagrams below.
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An Important part of the wave-particle dual nature of light is the
DeBroglie Wavelength. Study the links below to learn about it.
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