Key Ideas:
Light the Messenger
Light is Electromagnetic Radiation
Light as Waves and Photons
Electromagnetic Spectrum
• Sequence of photon energies
Doppler Effect
• Relative motion between source & observer
• Way to measure speeds at a distance
RØmer’s observations of 1676
Speed of light c=300,000 km/s
Fizeau-Foucault experiment
Light: wave or particle?
Young’s double slit experiment
Wavelength: 
Length between crests of a wave

Light travels like
Waves.
Different colors—
Different
wavelengths

Length between troughs of a wave
Electromagnetic Wave
Light as electromagnetic wave
Electric
Field
Magnetic
Field
Wave Nature of Light
Electromagnetic Waves
• Periodic changes in the strengths of electric &
magnetic fields.
• Travels through a vacuum at the speed of light.
• Doesn’t need a medium to “wave” in.
Speed of light is a constant for all light waves:
c = 299,792.458 km/sec
Independent of wavelength or frequency.
Properties of Waves
Light waves are characterized by three numbers:
• Wavelength,  (size of the wave)
• Frequency, f
(number of waves/second)
• Wave speed, c
These are all related by:
c=f
Since c is the same for all light waves,
Longer  means Smaller f
The Electromagnetic Spectrum
The Electromagnetic Spectrum
Sequence of wavelengths from low to high for
light is called the Electromagnetic Spectrum
low frequency= long wavelength
Examples:
Radio Waves, Microwaves, Infrared
high frequency=short wavelength
Examples:
Ultraviolet, X-rays, Gamma Rays
Visible Spectrum
Temperature
Light we can see with our eyes:
-determines
Brightness
• Wavelengths: 400 – 700 nm
• Frequencies: 7.51014 – 4.31014 waves/sec
Basic Colors (lowest to highest energy):
Red Orange Yellow Green Blue Indigo Violet
700 nm ------------- 550 nm ------------ 400 nm
ROYGBIV
Measures of Temperature
-determines
Color ( at
which most
Of the light is
emitted)
Absolute Temperature
At high temperatures:
• Atoms & molecules move very rapidly.
At cooler temperatures:
• Atoms & molecules move more slowly.
If it gets cold enough, all motion stops!
• Absolute Zero
• Corresponds to –273º Celsius (–459º F)
• physically unobtainable ultimate lower limit (can
get close, but never get exactly zero).
Wien’s Law
Stefan-Boltzmann Law
Relates peak wavelength
and Temperature:
Flux F = Energy emitted per second per area by a
blackbody with Temperature (T):
 peak 
0.0029
m
T (K )
F =  T4
 is Boltzmann's constant (a number).
In Words:
“Hotter objects are BLUER”
“Cooler objects are REDDER”
In Words:
“Hotter objects are Brighter at All Wavelengths”
Examples:
Examples
Heat a piece of iron from 300K to 600K
Person: Body Temperature = 310 K
• Temperature increases by 2×
• Brightness increases by 24 = 16×
• Peak wavelength shifts towards the blue by 2×
from ~10m in the mid-Infrared to ~5m in the
near-Infrared.
Hotter objects get brighter at all
wavelengths and get bluer in color.
Blackbody
Spectrum
• Peak wavelength = 9400 nm (infrared)
• Typical adult emits about 100 Watts of infrared
light.
Sun: surface temperature = 5770 K
• Peak wavelength = 503 nm (visible light)
• Emits about 3.81026 Watts of mostly visible
light, infrared and ultraviolet.
Blackbody Radiation
……emitted by hot dense (solid) objects
they absorb everything
reflect nothing
light coming out depends on T and on T only.