The Nature of Light

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The Nature of Light
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I. Light has a Dual Nature

• Light behaves as a wave
• Light behaves as a particle

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Light is a wave

• Light exhibits the properties of waves.
• Reflection
• Refraction
• Diffraction
• Interference

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Reflection

• When a wave encounters a new medium or a barrier,
  some or all of the energy will be reflected back.

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1. Reflection

• The angle between the direction of the wave and
  the normal of the reflective surface is called
  the Angle of Incidence (?i).
• Reflection

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Angle of Incidence
?i
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Reflection

• The angle between the reflected wave and the
  normal is called the Angle of Reflection (?r).
• Law of Reflection
• Angle of Incidence Angle of Reflection

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Angle of Incidence
?i
?r
Angle of Reflection
?i ?r
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Reflection

• If a blank sheet of paper reflects light, why
  cant you see your reflection in it?

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Surface of Paper
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• Specular vs. Diffuse Reflection
• Specular Reflection Light is reflected from a
  smooth surface at a regular angle
• Diffuse Reflection Light is relected from a
  rough surface in all directions
• Regular reflection

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Which of the pictures shows specular reflection
and which show diffuse reflection?
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2. Refraction

• Refraction is the bending of a wave as it enters
  a new medium.
• In the slinky lab you saw that a wave pulse will
  travel slower when it enters a more dense medium.

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2. Refraction

• When a wave front encounters a more dense medium,
  the change in velocity causes the wave front to
  bend.
• This phenomena is analogous to a line of soldiers
  walking quickly in formation. When each soldier
  crosses the red line, they are ordered to march
  slowly.

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SLOW
FAST
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SLOW
FAST
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Refraction

• Refraction of waves can be seen all the time.
• When you look at something in water like fish in
  a fish tank, they appear distorted if you look at
  it from different angles.

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Refraction

• Waves will always refract towards the more dense
  medium.
• Example When the sun rises in the morning, the
  light gets refracted by the Earths atmosphere.
  This makes the sun appear to rise about 5 minutes
  before it actually does.

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Apparent Angle of the Sun.
Sunlight entering Earths atmosphere
600 am
700 am
800 am
As the atmosphere scatters most of the sunlight,
we are left with a reddish color in the early
morning or late evening.
900 am
1000 am
1100 am
Red light scatters less than other colors of
light.
Noon
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3. Diffraction

• Diffraction is the bending of a wave around a
  barrier or through an opening.

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Diffraction
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Diffraction
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Diffraction
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Diffraction
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Diffraction
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Diffraction
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Diffraction
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Diffraction
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Diffraction
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Diffraction
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3. Diffraction
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4. Interference

• Light waves cause constructive and destructive
  interference with each other.
• Light movie
• Light interference

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Wave Theory

• If light is a wave, how can it travel through the
  vacuum of space??

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Electromagnetism

• 1800s James Maxwell
• Electricity produces a magnetic field.
• Magnets produce an electric field.
• Electricity and magnetism are two halves of the
  same thing.
• Both electricity and magnetism travel as waves.

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Electromagnetism

• Maxwell discovered that at very high speeds, the
  magnetic field and electric fields become
  self-propogating.
• He calculated that speed to be
• 3.00 x 108 m/s
• That speed just so happens to be the speed of
  light waves.

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Electric Field
Magnetic Field
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Assignment

• Read p. 622-627 of your textbook.
• Do 1-19 on chapter 31 packet

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Electromagnetism

• All light waves travel at exactly the speed of
  light. Only at that speed are the magnetic and
  electric components of the wave self-propagating.
  
• This includes more than just visible light it
  includes all parts of the electro-magnetic
  spectrum.

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Electromagnetic Spectrum
Visible light
Ultra-violet
Gamma rays
Infra-red
Radio waves
Micro-waves
X-rays
R
O
Y
V
G
B
I
longer ?
shorter ?
lower f
higher f
Higher Energy
Lower Energy
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5. Polarization

• Light is an electromagnetic wave.

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Polarization

• Aligning the vibrations of a transverse wave by
  filtering out light from other directions.

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Light Behaves like a particle

• Photoelectric Effect
• Light Exerts Pressure
• Light is affected by gravity

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Photoelectric Effect - Quantum Idea

• 1905 Einsteins miracle year
• Photoelectric Effect Einstein studied how an
  electric charge was produced when light was
  shined on a plate of potassium.

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Quantum Idea
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Quantum Idea

• Blue light, no matter how dim, always produced an
  electric current. The brighter the light, the
  greater the current.
• Red light, no matter how bright, would not
  produce any current.

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Quantum Idea

• WHY?
• Answer Blue light is made up of blue light
  particles. Each blue particle carries enough
  energy to stimulate an electron and dislodge an
  electron from its atom.

More blue particles
More electrons released

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Quantum Idea

• Each red particle had insufficient energy to
  release an electron from the potassium.
• Making red light brighter didnt increase the
  amplitude, it only increased the number of
  particles.

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Quantum Idea

• Einstein described each particle of light as
  having a QUANTA, or discrete amount, of energy.
• These light particles were termed photons
  (light particles).

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Quantum Idea

• Light demonstrates the behavior of both a
  particle AND a wave.
• - Wave-Particle Duality
• (But not always both at the same time.
  Sometimes it acts like a particle and sometimes
  it acts like a wave.)

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Light Exerts Pressure
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Light Exerts Pressure

• Photons of light have momentum.
• The momentum of the photons exert enough force to
  cause an object to move
• Example Solar sails

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Light is affected by gravity