Lecture Notes:
Light: General Properties, Temperature
text: Chapter 3, Sections 3.1 and 3.2
Introduction
Much of the information we get in astronomy is carried by
"light"
What information does "light" contain ?
- location of light in the sky ..... gives position of a
planet, moon, or star
- color ..... gives temperature, speed and direction of
motion, chemical composition
- brightness ..... gives distance, composition, periods of
motion (from changes)
- size of light ..... gives size and distance
information
Visible light, radio waves, X-rays . . .
are all different forms of the same thing => all
"electromagnetic radiation"
Properties of Light (electromagnetic
radiation)
speed of light in empty space is constant
c = 3 x 108 m/s = 670 billion mph
this finite speed has a real effect on information carried
by light
- it takes 1 1/4 seconds for light to reach us from the
moon
- it takes 8 1/3 minutes for light to reach us from the
sun
- it takes 4 1/3 years for light to reach us from the
nearest star
Think of light as a wave
wavelength = the distance between wave crests
the wavelength is indicated by the Greek symbol
"l" in the figure above.
frequency = number of wave crests per second passing
a point
We assume that each wave is traveling at the same
speed in this figure.
color is determined by wavelength (or frequency)
Here
is a LINK to a page from another University that describes the
wave nature of light and gives some exercise you can try to test
your understanding.
Spectrum and more properties
When we divide up all the types of electromagnetic radiation by
wavelength (color), we get a "spectrum".
The study of light at different wavelengths is called
"spectroscopy". It is a very powerful tool.
RED . . . . . BLUE
|
RADIO
|
INFRARED
|
VISIBLE
|
ULTRAVIOLET
|
X-RAYS
|
GAMMA RAYS
|
low ---------------------------------------> frequency
-----------------------------------------------> high
long <-------------------------------------- wavelength
<---------------------------------------------- short
See Figure 3.5 in the textbook for
another description of a spectrum. Here
is a link to the figure.
Visible light is actually a very small part of the spectrum.
Here
is a more detailed discription of the spectrum.
We can see the colors within the sunlight because each color is
bent differently when it passes through a prism. This process is
described more completely at this LINK.
Why do we see "visible" light ?
- Sun emits most strongly at these wavelengths
- Earth's atmosphere blocks many other wavelengths
- Human eye efficiently detects available light.
Is light really a wave ?
- sometimes light acts like a particle - "photon"
- sometimes light acts like a wave
- it is really both - or neither - we just don't have the right
words to describe it completely
Energy carried by light depends on wavelength (color,
frequency):
- short wavelength => more energy
- long wavelength => less energy
Mathematically, we can express this as:
E = hc / l
where E is the energy, l is the
wavelength, and h anc c are constant numbers.
Temperature
see page 55 in the text book (pages 50-51 in the 2nd ed.)
We measure temperature in the Kelvin scale
|
|
Fahrenheit
|
Celsius
|
Kelvin
|
|
absolute zero
|
-460
|
-273
|
0
|
|
air (nitrogen) becomes liquid
|
-321
|
-196
|
77
|
|
water freezes
|
32
|
0
|
273
|
|
room temperature
|
70
|
22
|
295
|
|
water boils
|
212
|
100
|
373
|
|
surface of the sun
|
10,340
|
5727
|
6000
|
Here is a conversion calculator for temperatures.
(Rankine is another temperature scale that is seldom used in
science. Just enter the value you want to convert in the
appropriate box and click on the Calculate button. All the
other boxes will display the equivalent values in the units
provided. A few units may sometimes display ****** when the value
gets to be too large.
Light and Heat
All objects emit electromagnetic radiation from heat.
- warm = dull red
- hot = bright red
- really hot = "white hot"
Properties of this electromagnetic radiation from heat:
- covers a range of wavelengths
- emits more at one single wavelength (peak wavelength)
- peak wavelength gets shorter for higher temperatures
See figure 3.6 in the textbook for a
better figure ! Here
is a link to the figure.
hot star ............... emits mainly in ultraviolet
Sun ................... emits mainly in the visible
(yellow-green)
planets, people ...... emit mainly in the Infrared
Here
is a more detailed discussion of radiation from heat.
Here
is an interactive exercise which allows you to see how this
works.
This
interactive exercise allows you to plot several different
temperatures on the same graph.
Do not confuse this emitted light with the
reflected light which allows our eyes to see other people,
planets, etc.
Previous Lecture - - - INDEX
OF LECTURES - - - Next
Lecture 
RETURN to Lecture Notes Outline