Download The Nature of Light

The Nature of Light
Waves vs. Particles
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
Wave Theory
• Advocated by Christian Huygens (16291695), Dutch astronomer, physicist
• Explained reflection
• Explained refraction if light traveled slower
in water than in air
• Had trouble explaining rectilinear
propagation
Particles
Isaac Newton
or Waves?
Christian 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 >1025 Hz
Electromagnetic Spectrum
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
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
1852-1931
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
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
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)