The Nature of Light Waves vs. Particles

Particle (Corpuscular) Theory Advocated by Newton ( ) 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

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

Particles or Waves? Isaac NewtonChristian Huygens

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

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.

Fresnel

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

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 >10 25 Hz

Electromagnetic Spectrum

Electromagnetic Theory All e-m waves travel (in a vacuum) at 3.00 x 10 8 m/s (c) Wavelengths range from 3 x 10 7 m to less than 3 x m Often expressed in nanometers or Angstroms Visible region is about Angstroms or 400 – 760 nm

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

Albert Michelson

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: 9.460x10 12 km

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

Thermal Radiation Hot objects emit e-m waves whose frequency depends on the temperature Infrared radiation is felt as heat Hotter objects emit some visible too, first red, ( o C) then higher frequencies until white hot (~2500 o C)

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

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

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)