The Evolution of Light: From Particles to Waves and Electromagnetic Spectrum

1. The Nature of Light Waves vs. Particles

2. Particle (Corpuscular) Theory • Advocated by Newton (1642-1727) • Said energy is carried by tiny particles from source • Could explain rectilinear propagation, ray model of light, reflection, refraction; all observed in 17th century • Predicted light would travel faster in water than in air

3. Wave Theory • Advocated by Christian Huygens (1629-1695), Dutch astronomer, physicist • Explained reflection • Explained refraction if light traveled slower in water than in air • Had trouble explaining rectilinear propagation

4. Particles or Waves? Isaac Newton Christian Huygens

5. Wave Theory • Huygens’ principle: Each point on a wave front may be regarded as a new source of wave disturbance • Interference of light not known until 19th century • Diffraction couldn’t be explained without wave theory

6. Wave Theory • Discovery of interference of light in 1801 by Thomas Young and explanation of diffraction in 1816 by Fresnel proved light had wave properties • Fresnel found speed of light slower in water -- further proof of wave theory • Since all known waves needed a medium, mysterious substance called ether (or aether) proposed that enabled light to travel through space.

7. Fresnel

8. Electromagnetic Theory • Faraday proposed lines of force to visualize electric and magnetic fields (1830’s) • Maxwell (1870’s) developed series of equations explaining light & heat were electromagnetic waves all moving at speed of light • No medium was required for propagation of this type wave

9. Electromagnetic Theory • Each e-m wave has electrical and magnetic components perpendicular to each other and to direction of wave travel • Hertz discovered radio waves also e-m, moving at same speed with different frequency and wavelength • Electromagnetic spectrum includes waves from ~10 Hz to >1025 Hz

11. Electromagnetic Spectrum

12. Electromagnetic Theory • All e-m waves travel (in a vacuum) at 3.00 x 108 m/s (c) • Wavelengths range from 3 x 107 m to less than 3 x 10-17 m • Often expressed in nanometers or Angstroms • Visible region is about 4000 - 7600 Angstroms or 400 – 760 nm

13. The Speed of Light • Galileo first to suggest finite speed • Roemer used variation in orbital period of Jupiter’s moon to get approximate value • 1st accurate measurement by Michelson using rotating octagonal mirror and light reflected between two observatories

14. Albert Michelson 1852-1931

15. Speed of Light • Later measured in vacuum, c = 299,792,458 m/s, a fundamental constant • Speed of light is the universal speed limit • Light Year: a measure of distance, the distance light will travel in one year: 9.460x1012 km

16. Luminous vs. Illuminated • Luminous: giving off light because of energy emitted by electrons • Illuminated: able to be seen because of light reflected and scattered • Transparent: transmits light readily • Translucent: transmits but diffuses light • Opaque: will not transmit light

17. Shadows • If light source is large compared to opaque object, shadow consists of a dark central region called the umbra surrounded by lighterregion called the penumbra • Within the penumbra, part of light source is visible

18. Light Measurements • Photometry: the quantitative study of light • Luminous intensity: related to power output, measured in candelas, a fundamental SI unit • Luminous flux: a flow rate of energy through an area, measured in lumens

19. Light Intensity • Illuminance: density of flux on a surface; often called light intensity • Intensity varies inversely with square of distance from light source • Intensity measured with instrument called photometer • Modern photometers usually electronic sensors (CCD devices)