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Wave property of light
Waves can carry energy
Wavelength (l) : distance between successive crests (or
troughs)
Frequency (f): # of waves passing a point in 1 second
Wave speed (v): wavelength x frequency
v=lxf
vsound = 340 meters/second
vlight = 3x108 meters/second
More Newton…
Showed that light is composed of many colors
Different l means different color (and vice versa)
Visible light has short wavelengths:
Blue: l = 4 x 107 meters (400 nm)
Green: l = 5.5 x 107 meters (550 nm)
Red: l = 7 x 107 meters (700 nm)
1 nanometer = 10-9 meters
Visible light range = 400 to 700 nm
Electromagnetic Spectrum
Visible: 400-700 nm
Usable: 0.001 nm – 10 km!
Atmospheric Windows:
•Earth’s atmosphere absorbs and reflects radiation at
several wavelengths
•From the ground, we only detect visible and radio
•For other wavelengths, we must observe in a place
above most or all of the atmosphere:
Mountains: Near IR
Planes: Far IR
Balloons: UV, X-ray
Space: everything including gamma ray
Measuring Light
Luminosity
•Total rate of energy emission
•Intrinsic to the star
L = energy/time “ergs per second”
Brightness
•Depends on distance from object
•Rate of energy passage through a fixed area
B = (ergs/second) / # of cm2 covered = L/4pR2
B ~ L/R2 … Inverse Square Law of Light
The particle nature of light
Late 1800s – Early 1900s: Quantum Mechanics
Light can be thought of as being made up of particles
called PHOTONS
The energy of a photon is proportional to frequency:
Ephoton ~ f
High frequency = high energy photon
Low frequency = low energy photon
Bright light = lots of photons
Continuous Spectrum
Emitted by hot bodies (also called thermal
radiation or black body radiation)
A blackbody is a perfect radiator:
•Emits photons at all energies (a continuous
spectrum)
•Shape of the spectrum is solely determined by
the object’s temperature
Properties of thermal radiation
Increase temperature:
particles move faster
interact at higher energies
more of the higher energy photons produced
Wien’s Law:
Hotter objects emit higher energy (bluer) photons
lmax = 3x106 nm / T
(T in Kelvin)
Properties of thermal radiation
Hotter objects emit more photons, so hotter objects
are brighter objects
Energy emitted per unit surface area ~ T4
Double an object’s temperature, and it emits 16 times as much
energy! (16 = 24)
Triple the temperature, and it emits 81 times as much energy!!
(81 = 34)
Luminosity of an object depends on?
Properties of thermal radiation
Stefan-Boltzmann Law
Luminosity depends on temperature and surface
area
L ~ R2T4