1. Making Light
How do we make light?

2. Making Light How do we make light? LED’s (light emitting diodes)
Heat and Light: Incandescent Lighting
(3-5% efficient)
Atoms and Light: Fluorescent Lighting
(20-40% efficient)
LED’s (light emitting diodes)
(60-80% efficient)
We’ll review Heat and Light first. Later in this part we will consider Atoms and Light.

3. Heat and Light
The way we see most things is by shining light on them, and then looking at the light reflected from the object.
The way we see stars is not this way. We see the light that comes solely from the object itself rather than light reflected from some other source. This type of radiation is called “blackbody radiation” since there is no reflected light involved, and things that don’t reflect light normally look black.

4. Heat and Light
A good absorber is also a good emitter, and a poor absorber is a poor emitter. We use the symbol  to indicate the blackness (=1) or the whiteness (=0) of an object.

5. Heat and Light
What are the parameters associated with the making of light from warm objects?
Temperature of the object, T.
Surface area of the object, A.
Color (whiteness) of the object, 
[Temperature must be in Kelvin, where size of one Kelvin is same as size of one degree Celsius, but T=0 K is absolute zero, and T = 0o C (freezing) = 273 K.]

6. Heat and Light: Experimental Results
At 310 Kelvin (= 37o C = 98.6o F), only get IR
Intensity
blue yellow red IR UV
wavelength

7. Experimental Results At much higher temperatures, get visible.
Look at blue/red ratio to get temperature.
Intensity
blue yellow red IR UV
wavelength

8. Experimental Results Ptotal = AT4 where  = 5.67 x 10-8 W/m2 *K4
peak = b/T where b = 2.9 x 10-3 m*K
Intensity
blue yellow red IR UV
wavelength

9. Example
If you eat 2,000 calories per day, that is equivalent to about 100 joules per second or about 100 Watts - which must be emitted.
Let’s see how much radiation you emit when the temperature is comfortable, say 75oF=24oC=297K, and pick a surface area, say 1.5m2, that is at a temperature of 93oF=34oC=307K: Pemitted = AT4
= (5.67x10-8W/m2K4)*(.97)*(1.5m2)*(307K)4 = 733 Watts emitted! You do “glow”!

10. Example continued
But this is not the whole story: besides emitting radiation, we receive radiation from the outside: Pabsorbed = AT4
= (5.67x10-8W/m2K4)*(.97)*(1.5m2)*(297K)4 = 642 Watts absorbed!
Hence, the net power emitted by the body via radiation is: Pnet = 733 Watts Watts = 91 Watts. This is consistent with eating 2,000 calories per day (burning energy at the rate of 100 Watts).

11. Example continued The peak of this radiation is at:
peak = b/T = 2.9x10-3m*K / 307K = 9.5m which is in the infrared (as expected). You do “glow”, but only in the IR, not in the visible.
There are night vision devices that can detect this IR radiation, so the army can “see at night”. However, this IR radiation has a much longer wavelength than visible radiation and so results in much bigger “diffraction fuzzy dots” which makes the resolution of detail not nearly as good as regular eyesight using visible light.

12. Heat and Light: Wave Theory
wave theory: UV catastrophe
intensity
experiment
wavelength

13. Heat and Light: Planck’s idea
Planck found that he could match the curve and DERIVE the empirical relations:
P = AT4 where  = 5.67 x 10-8 m2 *K4
max = b/T where b = 2.9 x 10-3 m*K
with the simplest relation:
E = (constant) * f
if the constant = 6.63 x J*sec = h.
The constant, h, is called Planck’s constant.

14. How to Make Light
The wave theory combined with the equipartition of energy theory failed to explain blackbody radiation (heat and light).
Planck kept the wave idea of standing waves but introduced E = hf, the idea of light coming in discrete packets (or photons) rather than continuously as the wave theory predicted.

15. How to Make Light
From this theory we now have a way of relating the photon idea to color and type of E&M waves: E = hf .
Note that high frequency (small wavelength) light has high photon energy, and that low frequency (large wavelength) light has low photon energy.

16. How to Make Light
E = hf
High frequency light tends to be more dangerous than low frequency light (UV versus IR, x-ray versus radio). The photon theory gives a good account of why the frequency of the light makes a difference in the danger. Individual photons cannot break bonds if their energy is too low while big photons can!

17. Photons and Colors Electron volts are useful size units of energy
1 eV = 1.6 x Coul * 1V = 1.6 x J.
radio photon: hf = 6.63 x J*s * 1 x 106 /s = 6.63 x J = 4 x eV
red photon: f = c/3 x 108 m/s / 7 x 10-7 m =
4.3 x 1014 Hz, red photon energy = eV
blue: = 400 nm; photon energy = eV .

18. Temperature of the Sun
When we look at the visible spectra of the sun, we see that it’s intensity peaks at about 500 nm (green light). From the equation:
 = b/T (where b = 2.9 x 10-3m*K)
we get: T = b/ = (2.9 x 10-3m*K) / 500 x 10-9m
 6,000 K (about 10,000 oF).
In Part 4 of the course we’ll use this technique to determine the surface temperatures of the various stars.

19. Power output of the sun From the relation:
P = AT4 where  = 5.67 x 10-8 m2 *K4
and the size of the sun (radius = 700,000 km = 7 x 108 m; A = area = 4r2 = 6 x 1018m2), we get: P = (1) * (5.67 x 10-8 m2*K4) * (6x1018m2) * (6,000K)  4 x 1026 Watts.
This will become important when we consider how long the sun, as well as other types of stars, can continue to “shine”.

20. Intensity of sunlight at the earth’s orbit
At the earth’s distance from the sun
(93 million miles = 1.5 x 1011m),
the intensity of sunlight we receive is about
I = P/A = (4 x 1026Watts)/(4**[1.5x1011m]2)
 1,600 W/m2.
However, due to reflection off the atmosphere and due to day/night and slanting angle of sunlight during the day, the average intensity striking the earth is only about 250 to 300 Watts for each square meter of surface.

21. Solar Energy
Solar energy is great for some things, not so great for others. It is especially good for things that require small amounts of power and are removed from easy access to the power grid (like satellites). Max power from the sun at noon on a summer day without clouds is about 1,200 W for each square meter of collector, but the collectors are only about 10-20% efficient in converting the solar energy into electrical energy. This then provides a maximum electrical power of about 150 W (peak) for each square meter of collector and an average of about W/m2.

22. Solar Energy
Each square meter of solar collector can supply only about 150 W of electrical power at most – on sunny summer days around noon. For comparison: One horsepower is about 746 W. A typical car has an engine that provides about 150 hp, or about 120,000 W.

23. Sun’s intensity at other planets
Mercury: at a distance of .38*earth’s distance 11,000 W/m2 Venus: at a distance of .72*earth’s distance 3,000 W/m2 Earth: 1,600 W/m2 Mars: at a distance of 1.5*earth’s distance 700 W/m2 Jupiter: at a distance of 5*earth’s distance 64 W/m2 Saturn: at a distance of 10*earth’s distance 16 W/m2 Uranus: at a distance of 20*earth’s distance 4 W/m2 Neptune: at a distance of 30*earth’s distance 2 W/m2

24. Atoms and Light
While heat emits a continuous spectrum of light (all the colors), when we excite atoms with energy, we find that the atoms emit light. However, they do not emit light in a continuous spectrum (all colors) like hot objects do. Rather, they emit only certain colors of light, which we call a discrete spectrum, or an emission spectrum. Each element emits its own individual spectrum. Several examples will be demonstrated in class and/or lab.

25. Atoms and Light
Hydrogen’s spectrum (in the visible) consists of just three lines: purple, blue-green, and red.
Helium has quite a bit different set of lines in its spectrum.

26. Absorption Spectra
In addition to the continuous spectrum (all the colors) of hot objects, and the discrete or emission spectra of hot gases of atoms, we find a third type of spectrum: an absorption spectrum. This occurs when a hot object shines light through a relatively cold cloud of gas, and the cloud absorbs just those colors that the gas emits when it is hot. An absorption spectrum is just a continuous spectrum with certain lines MISSING.

27. Spectra
We find that our atmosphere and the atmospheres of stars (including the sun) give us absorption spectra.
We also find emission spectra from the hotter areas of the atmospheres of stars (including the sun).

28. Doppler Effect
We are all familiar with the change in pitch of a train horn as the train approaches and then passes us at a railroad crossing.
Does a similar change in color (light’s “pitch”) occur when a light source approaches or recedes?
We don’t notice this for light – but that is because the speed of light is so much faster than that of sound that the effect is so much smaller. However, that effect does exist for light. In fact, we can use it in radar to detect speeds – in traffic speed traps.

29. Doppler Effect for light
If a source is approaching us, the pitch of sound is higher which means the frequency of the sound is higher.
For light, this effect also shifts the frequency and hence the color of the light. For a spectral line in the middle of the visible spectrum (yellow), the shift is toward the blue. Thus, this shifting of frequency to a higher value is called a “Doppler blue shift”. If the source is receding, the yellow spectral line will be shifted towards the red, and so this shifting of frequency to a lower value is called a “Doppler red shift”.

30. Red shift and blue shift
Regular spectrum for hydrogen:
Blue-shifted spectrum for hydrogen:
(first line is in UV)
Red-shifted spectrum for hydrogen:
(third line is in IR)
Notice that the colors change, but the spacing remains the same.

31. Doppler Effect for light
Note that a line in the blue will be shifted towards the ultraviolet if the source is moving towards us, but this is still called a “blue shift”. A line in the red will be shifted towards the infrared if the source is moving away from us, and again this is still called a “red shift”.

32. Doppler Shifts
Because we know the emission spectra of many elements when they are at rest in the lab, we can look at emission spectra from objects outside the lab and measure how far the spectral lines have been shifted.
This will be important in Part 5 when we look way far away.
We can even see the difference in lines coming from one side of an object compared to the opposite side of the object if that object is spinning!

33. Doppler shifts due to rotations
Spinning object
Red shifted light
Blue shifted light