1. What is Light?

2. Newton – Corpuscular Theory of Light
Isaac Newton was so convinced that light was composed of particles that he composed the “Corpuscular Theory of Light” that explained all known phenomena of light by using particles.

3. Light as a particle
For most of recorded history, the majority of philosophers and scientists believed that light was composed of particles.

4. Light phenomena known to Newton:
Rectilinear propagation
Reflection
Refraction
Newton was able to explain all the known phenomena of light using his corpuscular theory. A key point to remember is that Newton’s explanation of refraction predicted that light would move faster in water.

5. Prediction
Light will be faster in water

6. Huygens- Wave Theory of Light
A contemporary of Newton, Christian Huygens, proposed that light was composed of waves. Huygens was able to explain the same phenomena using only waves.

7. Prediction
Light is a wave, and wave speed is indirectly related to density. This means light will be slower in water than in air.

8. Which is it?
Particle or Wave?
This set the stage for an argument that lasted for almost 150 years-- Was light composed of particles or waves? Prominent physicists disagreed.

9. Young’s Double Slit Experiment
Evidence for the Wave Theory
In 1801 Thomas Young performed an experiment that strengthened case for the wave theory. This experiment is known as Young’s Double Slit Experiment.

10. Foucault
Speed of light in water is slower – more evidence for the wave theory
The next blow to the Corpuscular Theory of Light came in 1850 when Jean Foucault measured the speed of light in water. Foucault showed that light was slower in water, and the majority of Physicists declared that light must be a wave.

11. Equations showed that light moved as electromagnetic waves.
Maxwell
Equations showed that light moved as electromagnetic waves.
The final blows to the Corpuscular Theory were the equations of James Clerk Maxwell. These equations showed that light moved as electromagnetic waves.

12. Problems for the Wave Theory
Hertz and the photoelectric effect
By the end of the 19th century the Corpuscular Theory was dead. Soon a problem arose, however. Heinrich Hertz and other physicists discovered something called the photoelectric effect.

13. Why a problem?
Photoelectric effect depended on frequency, not intensity as predicted by the Wave Theory.
The problem was that only certain frequencies of light caused the electrons to be emitted. This could not be explained by electromagnetic waves, which predicted that the intensity of the light should be related to the number of electrons emitted.

14. Einstein Explains It
Light is composed of photons
A photon is a “package” of energy.
In 1905 Albert Einstein wrote a theory to explain the photoelectric effect. He stated that light was composed of photons that were propelled by electromagnetic waves.

15. Photoelectric Equation

16. Light is both a particle and a wave.
Today-
Light is both a particle and a wave.
Einstein won the Nobel Prize for this theory, and today it is widely accepted that light is both particle and wave in nature.