Light
Early Ideas About Light
•
•
•
•
•
Early Greeks thought light came from eye
Newton said it was made of particles
Huygens said it was a wave
Wave-particle debate continued
Discovery of light interference in 1801
seemed to confirm wave theory
The Photoelectric Effect
• Hertz and others noticed electrons are
emitted by metal if light was shining on it
The Photoelectric Effect
• Energy of the emitted electrons didn’t
depend on intensity of light but on its
frequency
• If frequency too low, no emission occurs no
matter how bright the light
• Could not be explained by wave theory,
only particle theory
• Explained by Einstein (1905)
Modern Theory of Light
• Light has dual nature, with both wave and
particle properties
• Contains stream of particles called photons,
each having energy depending on the
frequency of the light wave
• Experiments can reveal wave or particle
nature but never both at same time
Speed of Light
• Ancients thought light instantaneous
• Galileo realized it had very fast, but finite
speed
• First measured by Roemer in 1675 by
timing orbit of Jupiter’s moon, Io
• Michelson first to measure accurately in
1880
Speed of Light
• Accepted value is now 299,792,458 m/s ;
we will use 3.00 x 108 m/s
• Symbol for speed of light is c
• Light takes about 8 min to arrive from the
sun and 4 years from nearest star
• Interstellar distances measured in light
years, the distance light travels in one year
Electromagnetic
Waves
• Light is energy emitted by vibrating
electrons
• Travels in wave with both electric and
magnetic components
• Visible light is only small portion of broad
spectrum of electromagnetic radiation
• All e-m waves travel at same speed, c
Electromagnetic Spectrum
General Categories: Increasing Frequency and Energy Left to Right
Electromagnetic
Spectrum
Power
Radio
TV
Microwave
Infrared
(heat)
Visible
light
roygbiv
Ultra violet
(uv)
X-ray
gamma ray

Light Sources
• Sources of light are luminous: the sun, stars,
light bulbs, candle flames, etc.
• Objects that reflect light falling on them are
illuminated: the moon, the planets, anything
we can see that is not luminous
Light Emission
• Atoms absorb energy from heat or electricity
and electrons move to higher energy levels
• Atom is said to be in an excited state and is
unstable
• Electrons fall back to lower energy levels and
emit a packet of light wave energy called a
photon
• Light frequency (and color) depends on
amount of energy lost by electron
Continuous Spectrum
• When atoms are crowded together in a solid
or dense gas, available energy levels are so
numerous, all light frequencies are emitted
• White light is seen and when dispersed, all
colors are seen
Continuous Spectrum
Atomic Spectra
• When atoms are not crowded, (low pressure
gas), only certain energy levels are available
for electrons
• These energy levels are different for each
element
• Spectroscope: instrument for measuring
wavelengths of light emissions
Atomic Spectra
• Dispersion of the light through prism or
diffraction grating reveals a spectrum of
discrete lines, a characteristic pattern of
colors emitted when atoms excited by heat
or electricity
• Pattern of colored lines is unique to element
• Called bright line spectrum, can identify
elements
Atomic Spectra Examples
Measurement of Light
• Luminous flux is the rate light energy is
emitted by an object, measured in lumens.
• Illuminance is the rate that light energy falls
on a surface, measured in lux (lumens per
square meter)
• Illuminance varies inversely with the square
of the distance from the light source
Transparent Materials
• Transparent materials allow visible light to
pass through
• Energy passed from atom to atom and
emitted out other side
• Slows passage of light through material
• In water, light travels at 3/4 c in glass at 2/3
c
Transparent Materials
• Ultraviolet (uv) light causes resonance in
glass blocking transmission
• Infrared (ir) waves vibrate overall structure
of glass so it is blocked also
• Glass is transparent to visible light, opaque
to uv and ir waves
Opaque Materials
• Opaque materials will block transmission of
light or other e-m radiation
• Light either absorbed or reflected
• Due to free electrons, metals re-emit light
and appear shiny
• Atmosphere is transparent to visible light, ir
and low energy uv but opaque to high
energy uv, X-ray and gamma ray
Shadows
• Where light rays don’t reach behind opaque
objects
• Sharp shadows caused by small light source
(or very distant)
• Large light source creates shadow with dark
central region, the umbra, and surrounding
gray region, the penumbra
Shadows
• From penumbra, part of light source is
visible
• In partial solar eclipse, observer is in
penumbra of moon’s shadow
Polarization
• Ordinary light vibrates in all directions
• Polarized light all vibrates in same plane
• Usually caused by filters that absorb
components of light perpendicular to
polarization axis, transmit component
parallel to axis
Polarization
• Two polarizing filters with axes at right
angles will block all light transmission
• Reflection will also polarize light, reflecting
mostly light polarized parallel to reflecting
surface - glare
• Polarizing sunglasses have vertical filters
that block reflected glare
Polarization by Reflection
Scattering
• Light is scattered by small particles or
molecules with which it interacts
• High frequency (blue, violet) light interacts
and is scattered by air molecules -blue sky
• Dust particles and water droplets scatter
lower frequencies also - white color of
clouds and dusty sky
Scattering
• Sunset and sunrises scatter more blue light
because more atmosphere to go through red-orange sky, yellow sun
• Light is partially polarized by scattering
Dispersion
• Dispersion is spreading out wave according
to frequency/wavelength
• Prisms and water disperse light into
rainbow
• Different wavelengths move at different
speeds, short waves slowed more and are
refracted more