Light
Students will learn about light.
Light
• Light is a transverse electromagnetic wave.
Consider the electric field portion as
transverse up and down and the magnetic
field portion as perpendicular side to side.
• Light has many wave properties: reflection,
refraction, diffraction, interference patterns,
Doppler effect, color dependent on frequency,
and intensity (or brightness) dependent on
the amplitude of the waves.
Speed of light
• The speed of light in a vacuum (and also in air)
is 3 x 10^8 m/s.
• Albert Einstein showed that nothing,
nowhere, no how can ever every go faster
than the speed of light = 3 x 10^8 m/s.
Wave Theory of Light
• Wave Theory supported by observations that
light exhibits diffraction (through one slit
approximately one wavelength) and
interference (Young’s double-slit experiments)
• Wave Theory explains refraction of light and
the fact that light travels more slowly in
denser media
Light as a particle
• Sometimes light acts as a particle
• Light “particles” are called “photons”
• Light colliding with a mirror makes very small
“pings”
• Photoelectric Effect: Light delivers energy
through its frequency in order to excite and
give electrons kinetic energy. Light energy (as
photons): E = hf
– H = 6.63 x 10^-34 Joule seconds
Photoelectric Experiment (1887 Hertz;
1905 Planck; Einstein; 1913 Millikan)
• Vibrational energy is “quantized” or “discrete”
(example: piano or flute or staircase with
steps; not smooth or continuous)
• Light is transmitted as tiny particles or
“photons” (the most basic quantum of light
energy).
• Photoelectric effect is when light shining on
metal causes electrons to be emitted from the
surface with various kinetic energies
Wave Theory Predictions
• 1. Red light does not cause electrons to be
emitted. So, let us increase the intensity of
the red light, then the number of electrons
and their kinetic energy should be increased.
• 2. Frequency should not affect the kinetic
energy of the ejected electrons.
Photon Theory
• 1. An increase in intensity of the light means
more photons so more electrons will be ejected,
but the kinetic energy of the electrons will not be
changed.
• 2. If the frequency of the light is increased
(example: from red to blue light) the maximum
kinetic energy of the electrons increases linearly:
E = hf
• 3. There exists a minimal cutoff frequency where
no electrons would be emitted.
Electromagnetic Spectrum
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Gamma rays
X rays
Ultraviolet
Visible
Infrared
Microwave
FM/TV
AM
10^19 – 10^21 Hertz
10^17 – 10^20 Hertz
10^15 – 10^18 Hertz
10^14 – 10^15 Hertz
10^12 – 10^14 Hertz
10^9 – 10^12 Hertz
10^8 Hertz
10^6 Hertz
10^-12 m
10^-10 m
10^-7 m
10^-6 m
10^-5 m
.01 m
3m
200 m
Dispersion
• Dispersion is the spreading of white light into
the full spectrum. A prism separates white
light into a rainbow of colors. This happens
because the index of refraction of the material
depends on the wavelength. Different
wavelengths are bent to varying degrees.
• Violet is bent the most; red is bent the least.
• The sky is blue because it is bent the most as
sunlight comes through earth’s atmosphere.
Examples of Dispersion
• Prism
• Rainbow
• Diamonds
Polarized light
• Ordinary light is unpolarized. It has many
vibrations in many planes at once. The
electric field vectors vibrate at all angles.
• Light can be “filtered” or “polarized” by
passing it through parallel slits. Long
molecules can be arranged as long slits to
achieve this purpose.
• Example: polarizing lenses in Polaroid
sunglasses filter out “glare”