LightWave or Particle?
BothWe call this the “Dual Nature of Light”
Evidence
Wave PropertiesInterference of Light
Particle Properties
Interference of LightWhen two light waves hit they pass through each other.
While they occupy the same space the waves combine amplitudes and frequencies to produce a different wave. This is called interference.
If two particles hit they bounce off each other. They don’t interfere like waves.
Evidence
Wave PropertiesInterference
Refraction & Dispersion
Particle Properties
Refraction & Dispersion of Light
• When light enters glass it bends.• This is called refraction• How much light refracts depends on
the type of glass and the color of light.• A particle could refract, wave theory
works better.
• When white light enters glass the different frequencies bend differently.
• This causes the light to split into the rainbow.
• This is called dispersion.• Particles wouldn’t display dispersion.
Evidence
Wave PropertiesInterference
Refraction & Dispersion
The Doppler Effect
Particle Properties
The Doppler Effect for LightWhen a light source is moving the frequency of light it emits changes due to this
motion.A particle doesn’t do this, only a wave.
Evidence
Wave PropertiesInterference
Refraction & Dispersion
The Doppler Effect
Diffraction
Particle Properties
Diffraction of Light
If I fire this laser through a small slit, what will I see on the
wall when it hits?
A single point? Or a broken line?You get a broken line, called a diffraction pattern.
Basically the light bends as it passes through the slit causing an alternating series of constructive and destructive interference.
A particle wouldn’t do this.
Evidence
Wave PropertiesInterference
Refraction & Dispersion
The Doppler Effect
Diffraction
SO… Light is a Wave!!!!!!!!!!!!
Particle Properties
OR IS IT??
Evidence
Wave PropertiesInterference
Refraction & Dispersion
The Doppler Effect
Diffraction
SO… Light is a Wave!!!!!!!!!!!!
Particle PropertiesThe Photoelectric Effect
The Photoelectric Effect• When light hits certain metals it causes the metal to eject electrons.
• Treating light as a wave tells us that the brighter the light, the more electrons get ejected or the electrons get ejected at a faster speed.
• This would be true if it wasn’t false!!!
• It turns out the frequency of the light is what determines whether or not electrons are ejected from the metal.
• This suggests that light travels in quanta or packets called photons which behave like particles.
Evidence
Wave PropertiesInterference
Refraction & Dispersion
The Doppler Effect
Diffraction
SO… Light is a Wave!!!!!!!!!!!!
Particle PropertiesThe Photoelectric Effect
The Ultraviolet Catastrophe
The Ultraviolet Catastrophe• A blackbody radiator absorbs all
electromagnetic radiation that hits it and emits its own electromagnetic radiation based on its temperature.
• Wave theories predict that the wavelengths of electromagnetic waves a blackbody will emit should follow the relationship shown by the dashed curve in the graph.
• It turns out that experiments show that the wavelength of electromagnetic waves emitted by a blackbody actually follow the solid curves shown in the graph called blackbody curves.
• Which curve is determined by the temperature of the blackbody.
Our Sun’s Blackbody Curve
THE ULTRAVIOLET CATASTROPHE
This also suggests that light travels in quanta or packets called photons which behave like particles.
Evidence
Wave PropertiesInterference
Refraction & Dispersion
The Doppler Effect
Diffraction
SO… Light is a Wave!!!!!!!!!!!!
Particle PropertiesThe Photoelectric Effect
The Ultraviolet Catastrophe
SO… Light is a Particle!!!!!!!!!!!!
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THE DUAL NATURE OF LIGHT
Light is a wave Light is also a particle Light travels in packets of waves called
photons. These photons act like particles and waves
depending on what is being observed. The energy of each of these photons depends
on the frequency of the contained wave.