This document discusses evidence for both the wave and particle nature of light. For the wave nature, it mentions phenomena like reflection, refraction, diffraction, and interference that were observed in experiments. For the particle nature, it discusses observations that supported Planck's quantization of energy and Einstein's model of light as discrete photon particles, including the photoelectric effect, atomic spectra, and blackbody radiation. The photoelectric effect in particular provided strong evidence that light behaves as a particle by instantaneously ejecting electrons from metals above a threshold frequency, rather than depending on intensity as waves were thought to. This helped establish the dual wave-particle nature of light.

1. Light – Wave or Particle
Evidence Supporting Wave Nature of Light
• Reflection
• Refraction
• Diffraction
• Interference
Where did we observe these phenomena?
• Polarization
Where did we observe this phenomena?

2. Light – Wave or Particle
Evidence Supporting Particle Nature of
Light
Physics had yet to explain the following as of 1900:
•Atomic Spectra
Characteristic Color Pattern
• Blackbody Radiation
Absorption and Emission of Energy
• Photoelectric Effect
Photoemission of Electrons

3. Every element has its own signature
emission and absorption spectrum

4. Blackbody Radiation
A blackbody is an object/system that absorbs
all radiation in which it comes in contact.
Blackbody Radiation is the radiation emitted
from a blackbody based only on temperature.
(usually associated with stars, we can measure
temperature but not radiation levels)

5. Energy is Quantized
Definition
Energy comes in DISCRETE units
called quanta (photons).
(Birth of Quantum Mechanics)

6. Photoelectric Effect

7. Photoelectric Effect
Classical Physics vs. Quantum Mechanics
Classical
Quantum Mechanics
Physics (light as a
(light as a particle)
wave)
Whether electrons are The intensity of the The frequency of the
ejected depends on… light. light.
The kinetic energy of
The intensity of the The frequency of the
ejected electrons
light. light.
depends on
At low intensities, Occurs almost
Takes time. instantaneously above
electron ejection … a certain frequency.
Proves duality of light.

8. Photoelectric Effect
Classical vs. Quantum Mechanics
• These observations support light being a
particle rather than a wave.
• It requires that electromagnetic energy be
quantized as Max Planck proposed earlier in
his explanation of blackbody radiation.
• Albert Einstein extended Planck’s quantized
energy to a stream of photons.

9. Work Function
Definition
The minimum amount of energy required for an
electron to escape from a metal.
The value of the work function depends ONLY
on the metal.

10. Work Function
Threshold Frequency
The frequency of incident radiation (photons)
below which there is no ejection of electrons
from a metal.
Symbol – ft
Units – Hz

11. Photoelectric Effect Explained:
Light is going to hit the metal.
If the frequency of the light is above the
threshold frequency, then the loosely bound
electrons will be emitted (released from the metal).
Metals with more loosely bound electrons
will have a lower Threshold frequency,
so it will require less energy to release the
Electrons.