#### Transcript Lec5_2D

```Fundamentals of
Light
Light as a Wave
Light (or electromagnetic radiation) can be thought of as either a
particle or a wave. As a wave, light has
• a wavelength,  (distance between waves)
• a frequency,  (number of waves passing you each second)
• an energy, E = h  (where h is just a constant)
• a speed, c =   (this is always the same: 300,000 km/s)
Note that because the speed of light is a constant, , , and E are
linked: if you know one, you know the other two.
The Electromagnetic Spectrum
Light waves can be
any size, from the
size of an atomic
nucleus to larger than
a football field. The
eye only detects a
small range of the
electromagnetic
spectrum.
Atmospheric Windows
Not all light from space makes it through the Earth’s atmosphere. In
fact, only visible light, radio waves, and some infrared light makes it
to the ground. The rest of the electromagnetic spectrum can only be
observed from space.
The Doppler Shift
The wavelength emitted by an object is not always the wavelength you
observe. If you are moving towards an object, you will see more
waves per second (i.e., a higher frequency, like swimming upstream).
Yellow light will appear bluer and be blueshifted. Conversely, if you
are moving away from an object, the light will be redshifted.
The Doppler Shift
The wavelength emitted by an object is not always the wavelength you
observe. If you are moving towards an object, you will see more
waves per second (i.e., a higher frequency, like swimming upstream).
Yellow light will appear bluer and be blueshifted. Conversely, if you
are moving away from an object, the light will be redshifted.

v


c
The faster the relative
motion, the larger the
red or blue shift.
Light as a Particle
Light can also behave as particle. Each packet of light is called a
photon, and each photon carries a specific amount of energy
(associated with the photon’s wavelength or frequency).
Photons emitted from a source
will spread out in all directions
at the speed of light. Since the
amount of area surrounding a
source increases as the distance
squared, the density of photons
will decrease as 1 / r2. This is
the inverse square law of light.
Scattering of Light
Dust (and molecules) in the Earth’s atmosphere (or in space) can
scatter light. In general, short wavelength (blue) light gets
scattered more than red light. That’s why the sky is blue.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
When you look away from the Sun, you see photons that have been
scattered towards you. These are mostly blue.
Scattering of Light
Dust (and molecules) in the Earth’s atmosphere (or in space) can
scatter light. In general, short wavelength (blue) light gets
scattered more than red light. That’s why the Sun is red at sunset.
The long path through the atmosphere means all the blue photons
are scattered away.
Ways of Creating Light
There are 3 ways to produce light:
• Through the blackbody process (a.k.a. thermal
emission)
• Through line emission
• Through synchrotron emission
(This last way is only for a few peculiar objects with strong magnetic
fields. We will be ignoring this mechanism in this class.)
The Blackbody Process
Anything that is hot (i.e., above absolute zero) produces light at
all wavelengths – a continuous spectrum. But the amount of
light given off at each wavelength is very sensitive to an
object’s temperature. Specifically,
The hotter the object:
• the more high-energy
photons are created
• the more light is
created (MUCH more)
L  T4
The Blackbody Temperatures
The color of an object reflects its temperature -- cool
objects will produce more red light than blue light,
while the opposite will be true for hot objects.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
The Blackbody Temperatures
The color of an object reflects its temperature -- cool
objects will produce more red light than blue light,
while the opposite will be true for hot objects.
QuickTime™ and a
Graphics decompressor
are needed to see this picture.
The Blackbody Temperatures
The color of an object reflects its temperature -- cool
objects will produce more red light than blue light,
while the opposite will be true for hot objects.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
The Blackbody Temperatures
Temperatures
Sun: 6000° (optical)
People: 300° (IR)
Rigel: 44,000° (UV)
1,000,000° gas: x-ray
(all temperatures are
in Kelvin)
```