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.