1. Aristotle and the Ether (02:42)

About 2,400 years ago the greek philosopher Aristotle proposed that an invisible fluid he called ether exists all around us. He went on to explain light as disturbances that travel across the ether.osed that an invisible fluid he called ether exists all around us. He went on to explain light as disturbances that travel across the ether.

2. Ancient Geometry: Rays (02:22)

About 100 years before Aristotle described his ether theory, the Greek philosopher Empedocles proposed that light streams from the eyes. Euclid refined the idea of light rays and used geometry to explain some of light's observable behaviors.

3. Modern Geometry: Waves (02:01)

Cartesian geometry provides a description of light as periodic waves propagating from bright sources. Aristotle's description of light as a disturbance in the ether was gradually reborn as a wave theory of light.

4. Huygens' Principle (03:40)

In the late 17th century Dutch scientist Christian Huygens developed a wave model that proposed waves bend around corners. Huygens' Principle, the idea that every wave front is a point source for new waves, seemed a good explanation for sound waves but did not explain light and shadows.

5. The Other Theory: Particles (03:26)

In the late 17th century Sir Isaac Newton revived an older particle theory. He proposed that tiny particles, traveling in the same straight lines as imaginary light rays, could explain how light behaves and what light is.

6. Father Grimaldi: Diffraction (04:07)

In the mid-17th century Father Francesco Grimaldi conducted an experiment measuring light and shadows. His findings seemed to provide clear evidence that light bends; diffraction was the term Grimaldi applied to describe the bending of light he observed.

7. Interference (03:30)

When waves intersect they momentarily interfere with each other then emerge beyond the interference fundamentally unchanged. Under certain circumstances, the result of constructive and destructive interference between wave sets can be an unusually constant pattern.

8. Young's Double Slit Experiment (04:57)

In the early 19th century Thomas Young conducted a famous experiment to explore the behavior and nature of light. The light interference pattern produced by Young's experiment offers compelling proof that light behaves as some form of wave.

9. Transverse and Longitudinal Waves (03:16)

Transverse waves disturb the medium at right angles to the direction of propagation. In longitudinal waves particles of the medium are disturbed backwards and forwards in the same direction the wave propagates.

10. Polarization (06:10)

Polarized light is a transverse light wave propagating in a single plane along an axis. Polarization is a phenomenon that occurs everywhere in nature.

11. Electromagnetic Waves (07:25)

In 1832 Scottish physicist James Clerk Maxwell published four equations that defined an electromagnetic wave. Maxwell's electromagnetic energy has been shown to largely describe and predict the nature of light.

12. The Speed of Light (04:23)

The first measurement of the speed of light took place in the 17th century and was based on four newly discovered moons of Jupiter. James Clerk Maxwell's discovery of electromagnetic waves provided a mathematical equation for predicting the speed of light.

13. The Electromagnetic Spectrum (09:13)

German scientist Heinrich Hertz first demonstrated by experiment that the electromagnetic waves described by Maxwell actually exist. From radio waves to gamma rays and beyond, an infinite number of electromagnetic wavelengths is possible.

14. Single Slit Interference (05:18)

Single slit interference is caused as light waves are disturbed by the edges of the single slit; the pattern formed can be explained using Huygens' Principle. Fraunhofer interference describes the kind of single slit interference created by distant, or parallel, light waves.

15. Dispersion (02:36)

The dispersion of light passed through a prism suggests that white light is nothing more than a reaction of the eyes and brain to what is actually a full spectrum of colors. Early proponents of both particle and wave theories of light developed explanations for dispersion.

16. Diffraction Gratings (05:38)

Diffraction gratings are useful for identifying different kinds of atoms. Spectroscopes use diffraction gratings to process light emitted by atoms; diffraction gratings show a unique emission spectrum for each atom.

17. Thin Films (02:17)

The interference of light in thin films often causes colorful patterns. As the viewing angle of reflected light changes, different wavelengths reinforce destructively to produce the kaleidoscope of light seen reflecting from thin films.

18. What is Light? (02:41)

James Clerk Maxwell's theories about the electromagnetic nature of light closely describe and predict its behavior. Even so, as much as light behaves as a continuous energy wave, at times it still seems to behave more like a particle.