Transcript Ch5

Chapter 5
The Nature of Light
Light is a wave and white light is composed of many different
wavelengths
White light is just a small part
of the entire electromagnetic
spectrum.
Continuous Emission
The Sun is a source of continuous emission at T=5800K
The Sun (and other Stars) radiate like Blackbodies
Wiens Law
Relates the wavelength of brightest emission
to the temperature
max T = 2.898 x 10-3
max (m) for a blackbody of temperature, T,
in Kelvins.
The Color of a Star is Related to
it’s Temperature
Saturn is a source of continuous emission by reflected sunlight
Continuous Infrared emission from Io’s Volcanoes
Continuous Radio Emission from a Galaxy
There are different mechanisms that
give rise to continuous emission but
they are all related to the atomic
nature of matter
Electrons orbiting the Nucleus of an Atom
Spectroscopy
The Sun (and other Stars) radiate like Blackbodies
An absorption line is like a silhouette
Classic absorption spectrum for the Sun
At the subatomic level, light behaves like a particle
Stellar Spectra
Emission & Absorption Lines
Electronic Transitions in the
Hydrogen Atom
Energy Level Diagram
Two important facts about
Hydrogen
• The ionization potential = 13.6 eV
• The wavelength of the Ha emission line is
6563Å
where 1Å = 10-10 m
You can figure out everything about Hydrogen from these two
facts and knowing that the energy difference between two
electronic states, DE, is proportional to
DE
a
1/n2
Where n is the principal quantum number
For Example
• The Ha absorption line results from the
electron jumping from
• the n=2 to n=3 level. We can use this fact
(#2) to calculate
• the constant of proportionality, R
DE = hc/= constant [ 1/ n12 – 1/n22 ]
So that,
1/ = R [ 1/ n12 – 1/n22 ]
Substitute n1= 2 and n2 = 3 and  = 6563Å
to yield R = 1.097 x 10-3
which is known as the Rydberg Constant
Now you can calculate the
wavelength for all other
electronic transitions
Since,
1/ = 1.097 x 10-3 [ 1/ n12 – 1/n22 ]
But, remember, that this equation yields  in Å
The origin of emission and absorption lines is related to the type
of background source.
Some of the most beautiful objects, however, are
emission (not absorption), nebulae
The red glow is due to the
emission from Hydrogen
atoms dropping from the
n=3 to n=2 level
Here, the blue glow
is from Oxygen atoms
Intervening dust grains scatter the blue light and preferentially
transmit the red light, which is why we have red sunsets.
The scattered light leads to blue reflection
nebulae
Dust also absorbs blue light making more
distant objects appear redder
One of the best known Hubble pictures includes elements of
all the light phenomena we have talked about