This document discusses various properties of light, including its behavior as both particles (photons) and waves. It describes experiments by Newton showing visible light can be separated into different colors/wavelengths. It also discusses how different wavelengths of electromagnetic radiation can penetrate the atmosphere, allowing astronomers to study celestial objects across the electromagnetic spectrum. Finally, it summarizes laws like the Stefan-Boltzmann and Wien's laws that describe how the radiation of a blackbody relates to its temperature.

1. Properties of Light

2. Particle Theory
• Light can behave as particles called photons
• Photons are little packets of energy, they
have no mass, but their energy can be
calculated using
• E = hf
• h is Planck’s Constant 6.626E-34 J.s
• Higher energy means higher frequency.

3. Newton’s Light Experiments
• Newton showed visible
light can be separated into
different colors.
He did not know these were
wave-lengths.

4. Wave Theory of Light
• Each color corresponds to a different
wavelength.
The catch for astronomers: only certain parts of
the entire EMS can penetrate the earth’s
atmosphere. The windows are visible and radio.

5. Stars are basically gravitationally
bound balls of gas
• Since astronomers are no longer “stuck” on
earth they can use the entire spectrum to
figure out what chemical composition stars
are made of.
• Fact: A very dense gas or a solid will give off a
continuous spectrum that changes smoothly
in brightness from one color to the next.

6. Spectra of a cluster of stars

7. Blackbody Radiation
• Blackbodies are perfect absorbers. They will
absorb all incoming radiation and none is
reflected.
• They are also perfect emitters of their own
internal radiation.
• This depends only on
temperature, not its
chemical composition.

8. Wien’s Law
• Spectrum of a hot blackbody
• peaks at a shorter
• wavelength than that
• of a colder blackbody.
• The product of temperature and the wavelength
at which the spectrum peaks is a constant:
(always the same)
lpeakT = 2.9E7 A.K

9. Stefan-Boltzmann Law
• Blackbody radiation property 2: per unit of
surface area a hot blackbody emits much
more energy per second that a cold
blackbody.
• This energy is proportional to the fourth
power of the temperature:
• E = sT4
• Temp in kelvins, s = 5.67E-8 W/m2.K4
• Energy in J/area

10. Just a note:
• Brightness of a blackbody (L) luminosity can
be found by multiplying its surface area (S) by
energy emitted E
• L = SE
• Surface area = 4pR2
• Energy = sT4
• Then L = 4pR2sT4 and so if we know the
brightness of any star and its temperature,
then its radius can be found.

11. The bad news:
• Humans reflect visible light from the Sun or from
room lamps so we are NOT perfect
blackbodies.
But we do emit
thermal (blackbody)
radiation which is
most intense at
infrared wavelengths.
So we can be found:

12. Reflection
• Angle of incidence: this means the first ray that
hits the barrier such as a mirror will form and
angle with the barrier.
• Angle of reflection:
• the angle with which the second ray bounces
back.
• In reflection both angles of incidence and
reflection are equal and made with the NORMAL.

13. Refraction
• The direction of light rays can be changed at the
boundary of two media of different densities. (Ex:
air to water)
• General rule:
• Away from the perpendicular if medium 2 is less
dense than medium 1
• Toward the perpendicular if medium 2 is more
dense than medium 1
• Such effects form the basis of the refracting
telescope, and of optical devices using lenses in
general.

14. Diffraction
• The capability of light to bend around corners.
• For astronomers
diffraction

15. Diffraction can give spectrums.
• A diffraction grating can be used to separate
light into its constituent colors, and that
diffractive effects set an absolute limit on the
quality of an image observed through an
optical instrument such as a telescope. This
diffractive limit occurs because the lenses of
such objects are of finite size and diffract
light because they cut off part of the light
wave.

16. Sun spectrum

17. Fraunhofer Lines
• The dark lines
are from cooler
gas absorbing
radiation behind
a star such as the sun.
By seeing what wavelengths are absorbed will
tell what elements are in the gas.

18. Absorption and Emission Spectra
• https://www.khanacademy.org/partner-content/
nasa/measuringuniverse/spectroscop
y/a/absorptionemission-lines
• http://www.livescience.com/41548-
spectroscopy-science-fair-project.html

19. Doppler Effect and Orbits
Light also can be described as a wave, and relative motion of the
source of light waves leads to a corresponding Doppler effect for
light. In this case it is not the pitch but the color (that is, the
wavelength) that is shifted by the motion of the source.
The wavelength is shifted to larger values if the motion of the source
is away from the observer and to smaller values if the motion is
toward the observer.
The shift to larger wavelengths by motion away from the observer is
called a red shift by astronomers.
A shift to shorter wavelengths caused by motion toward the observer
is called a blue shift.
The terminology is borrowed from the visible part of the spectrum
where blue is toward the short wavelength end and red is toward the
long wavelength end, but the Doppler effect occurs for all
wavelengths of light, not just the visible spectrum.