1. Chapter 4: Light Discuss why the sky is blue, sunset, rainbows
bring prism, diffraction gratings, and telescopes
We take light for granted. Without light we would all be in the darkness, we would have no concept of color. Light seems so simple, but it is one of the most complicated and fascinating topics. Without light, we couldn’t see, but more fundamental than that, without light, life would not be able to exist. We need light in order to live. Plants need light to live. We need light in order to start a fire, to cook. Light is all around us. And light provides us with information.
And since astronomy is not like other sciences where you can see and touch what you are studying, astronomers depend on something that can traverse large distances and reach them. This special something is light. Astronomers gain all of the information from objects in space from the light that they emit. By studying light we can find out all sorts of information about the object. Since light is so important for gaining information, it is fitting that we devote a chapter to discussing the properties of this mysterious and fascinating thing we call life.

2. “Ever since we crawled out of that primordial
slime, that’s been our unifying cry, ’More light.’
Sunlight. Torchlight. Candlelight. Neon,
incandescent lights that banish the darkness from
our caves to illuminate our roads, the insides
of our refrigerators...Little tiny flashlights for
those books we read under the covers when
we’re suppose to be asleep. Light is more than
watts ... Light is metaphor. Light is knowledge.
Light is life...”
Since light is so important to us, I start out with this quote. How many of you have seen the show Northern Exposure? Its a show in the 80s about a group of people living in Alaska and this is a quote from that show. I feel somewhat old.
So light is important. It allows us to see. But not only that, it allows us to live, and it gives us knowledge about the celestial objects in the sky. Also because light does not travel infinitely fast, we are able to see the Universe as it looked in the past.
What information can you gain from light? Energy, temperature, age of stars, speed, direction, mass, chemical composition.
- Diane Frolov and Andrew Schneider
Northern Exposure,1993

3. What can we learn by analyzing starlight?
A star’s chemical composition
A star’s temperature
A star’s speed and direction of motion

4. How fast does light move?
186,000 miles per second! or
3 x 105 kilometers per second!
through a vacuum, all colors travel at the same speed.
Nothing travels faster than light!

5. Particle or Wave ?
1905, Einstein verified that light sometimes behaves as a wave and sometimes as particles.
This is called the wave-particle duality of light!
Just when it was beginning to be accepted that light behaves as a wave, Einstein showed conclusively that light behaves as particles. In 1905, Einstein proved that light behaves as both a particle and a wave with the photoelectric effect which has something to do with shining light on the surface of metals and that only short wavelengths of light can knock out the electrons from the metal, can only do that it light was made of different particles that had different energies.
Just like a platypus has both mammal like characteristics and reptilian characteristics!!
The strange thing about light was that when you put it under experiments to test a wave-like nature, light would behave as a wave, but when you performed other experiments to test whether it was a particle, it would behave like a particle. So it was hard to classify light, it was both a particle and a wave.

6. Particle or Wave ? Wave: Electromagnetic wave Particles:
Just when it was beginning to be accepted that light behaves as a wave, Einstein showed conclusively that light behaves as particles. In 1905, Einstein proved that light behaves as both a particle and a wave with the photoelectric effect which has something to do with shining light on the surface of metals and that only short wavelengths of light can knock out the electrons from the metal, can only do that it light was made of different particles that had different energies.
Just like a platypus has both mammal like characteristics and reptilian characteristics!!
The strange thing about light was that when you put it under experiments to test a wave-like nature, light would behave as a wave, but when you performed other experiments to test whether it was a particle, it would behave like a particle. So it was hard to classify light, it was both a particle and a wave.
Particles:
Photons (packets of energy)

7. Different wavelengths correspond to different colors!
Light as a Wave
not something that Mork would use; actually real units. Bring slinky as demo.
1 wavelength: distance between one peak to another peak.
Different wavelengths correspond to different colors!
Units: 1 nanometer= 10-9 meters!

8. Light as a Wave
frequency: how many cycles per second pass a certain point per second.

9. if wavelength↓, then frequency ↑
Some Important Relationships
if wavelength↓, then frequency ↑
if frequency ↑, then energy ↑
Slinky demo? Shake slinky to indicate low frequency corresponds to long wavelength and high frequency corresponds to short wavelength.
if wavelength ↓, then energy ↑

10. Which color light has more energy blue or red?
Concept Question
Which color light has more
energy blue or red?

11. Blue coats beat the red coats!

12. If you pass white light through a prism, it separates into its component colors.
Spectrum
Visible light is light that can be perceived by the human eye. When you look at the visible light of the sun, it appears to be colorless, which we call white. And although we can see this light, white is not considered to be part of the visible spectrum (Figure 2). This is because white light is not the light of a single color, or frequency. Instead, it is made up of many color frequencies. When sunlight passes through a glass of water to land on a wall, we see a rainbow on the wall. This would not happen unless white light were a mixture of all of the colors of the visible spectrum.
Isaac Newton was the first person to demonstrate this. Newton passed sunlight through a glass prism to separate the colors into a rainbow spectrum.
The light that we receive from the Sun is called visible light, or white light. But it actually is composed of many different colors. However because they are all traveling at the same velocity, we see them as a combined color, white sunlight. But I just said that light gets slowed down when it travels through a medium. This is what happens when you shine white light through a glass prism. The light gets slowed down, but not uniformly at all wavelengths.
Different wavelengths of light get slowed down by different amounts. Blue light gets slowed down more than red light. This slowing down happens because the light gets bent in the prism. Blue light gets bent more so it travels slower than red light. This bending of light is called diffraction of light. This diffraction causes us to see a rainbow of colors ranging from red to violet.
That is what happens when you see a rainbow in the sky after it rains. There sunlight which is called visible light or white light gets broken down into its component colors when it hits the water molecules in the sky. Because the light is hitting something, different wavelengths get slowed down by different amounts and the different colors that make up white light gets dispersed by different amounts.
Astronomers have developed a mnemonic to help you remember the colors that make up visible light. Starting from longer wavelength to shorter wavelength: It is Roy G. Biv. That is Red, Orange, Yellow, Green, Blue, Indigo, and Violet. These are all the colors of the rainbow. Of course in reality, each individual color is more finely divided and they show up as different tints of the color. spectra- when light gets dispersed into component colors or wavelengths.
Pass out diffraction gratings and ask them to list colors they see.
Bring spotlights to demonstrate that colors of rainbow make up white light.
I’m handing out what are called diffraction gratings. This is just some material with fine grooves on it so that when light hits it, it bends in different amounts. Just hold it over your head and then you should be able to see a rainbow of colors. This is what happens when white light gets diffracted.
Talk about Rainbows: Rainbow demo? Show demo that if you bring different colors together, they combine to form white light.
It is not a coincidence, but evolutionary that our eyes are tuned to the light that our Sun mainly emits which is visible light. It emits at other wavelengths too, but very little. So if our eyes were not tuned to visible light, we could barely see anything. Perhaps we can be like some animals which can detect infrared light. This is the light that occurs due to heat. Because we all have a temperature, our bodies are radiating in the infrared.
ROY G. BIV !!
long wavelengths
short wavelengths

15. A hot object or a hot, dense gas produces a continuous spectrum -- a complete rainbow of colors without any specific spectral lines.
Shine light bulb

16. A hot, less dense gas, when heated, produces an emission line spectrum - a series of bright spectral lines against a dark background.
Shine gas lamp

17. A cooler gas in front of a hot dense gas produces an absorption line spectrum - a series of dark spectral lines among the colors of the rainbow.

19. Each chemical element produces its own unique set of spectral lines when it burns

20. Spectral Lines By observing the location of the emission and or the
These are emission line spectrums for different types of gas. So as you can see, if you can identify the patterns of the line, you can figure out what gas produced it. You can also do this with absorption lines. So this is one kind of important information that you get from light. If observe the light from an object and you see emission line or absorption line pattern, you can figure out what gas or elements make up that object. These emission lines and absorption lines are just like fingerprints of the elements. Each element has a unique set of emission and absorption features. Even though you don’t know what element the object is made of, you can just study the light and find out.
Again, you would have to diffract (or bend) the light in order to see all the wavelengths. the type of instrument which does that is called a spectroscope because it lets you look at the spectra of the light. You can build a really simple one with a mirror, and a diffraction grating.
Show emission line demo
Sodium lamps
By observing the location of the emission and or the
absorption lines, one can identify the element!!!!

21. Twinkle, twinkle little star
I don’t wonder what you are.
For by spectroscopic ken
I know that you are hydrogen!
- anonymous

22. The Spectral Type of Stars
In a small room at the Harvard Observatory more than a century ago, a group of women spent their days searching the stars.
They weren't yet allowed to vote, but they quietly made history as they discovered new suns and deciphered their mysteries.
"It was really phenomenal to have a group of women scientists," said Barbara L. Welther, a historian of astronomy at the Harvard-Smithsonian Center for Astrophysics. "This was seen as crossing a threshold that previously had been accessible only to men."
The women found a place at Harvard because Observatory Director Edward C. Pickering needed a work force to analyze thousands of photographs bearing the images of starfields.
The women initially earned just 25 cents an hour, yet they were devoted to their work. They spent years doing complex computations to ascertain the positions of stars and analyze their spectra to determine their composition.
Harvard's female astronomers not only laid a foundation for modern astronomy, they also paved the way for women in science.
Most of the women who worked under Pickering are forgotten faces on old photographs, but a few became famous later on, including:
Henrietta Swan Leavitt ( )
Leavitt, an 1892 graduate of Radcliffe College
During her years at the Observatory, she discovered four new stars along with 2,400 variables. But her most important discovery was the "period-luminosity law" for variable stars -- a relationship between their brightness and length of period. This law was used to estimate distances to star clusters in our galaxy.
Annie Jump Cannon ( )
Cannon is, to this day, one of the best known of Harvard's early women astronomers. She specialized in using spectra to classify 50,000 to 60,000 stars a year.
"They aren't just streaks to me," Cannon once said about the lines of the spectra. "Each new spectrum is the gateway to a wonderful new world. It is almost as if the distant stars had really acquired speech and were able to tell of their constitution and physical condition."
Cannon became interested in astronomy as a young girl growing up in Dover, Del. She entered Wellesley College in 1880 at age 16, just five years after the school opened. She left higher education for several years but returned in 1894 to do graduate work at Wellesley before coming to Harvard as an assistant.
Nearly three decades later, she was appointed the William Cranch Bond Astronomer, a post named in honor of the Observatory founder. The appointment of a woman was so rare that the letter announcing her post was addressed to "Dear Sir."
Her discoveries earned her six honorary doctorate degrees, including the first degree from Oxford University awarded to a woman.
stars are classified by their spectral types:
O B A F G K M
hottest to coolest
most massive to least massive
shortest lifetime to longest lifetime

23. Stars are classified by their spectra as O, B, A, F,G, K, and M spectral types

24. Oh Be A Fine Guy (or Girl), Kiss Me

25. Electromagnetic Spectrum
Figure 3.5

26. Not all radiation can penetrate Earth’s atmosphere.
Because of our Earth’s atmosphere only some light from the entire electromagnetic spectrum is allowed to get through. Here is a diagram which indicates the window of transparency for our Earth. The x axis is the wavelength, the y axis is the altitude. You can see that for the most part, the only light that is allowed through to our surface is visible light, some infrared light (this is only at specific wavelengths) and radio waves. There is a little bit of leakage of UV light. Luckily for us, our atmosphere is pretty much opaque to UV light which is damaging to our skin and can cause cancer. So our atmosphere protects us from some certain kinds of light that is harmful to us.
But in terms of science and astronomy, our telescopes on Earth can be used to only study light in the visible which spans wavelengths of Ang, some infrared light, and radio light. In order to study light at other wavelengths, we need to get above the Earth’s atmosphere.

27. What is Color?
Color of objects due to how much light is reflected, absorbed, or emitted.
We learned that when we scatter visible light through a prism or a diffraction grating, it gets scattered into its component colors; the colors of the rainbow; You can remember them with the nmemonic Roy G. Biv. But do objects really have their own color? Technically they do not because objects don’t really possess their own color; the color you see is just a how much of the incoming radiation is reflected, absorbed, or transmitted. This depends upon how much light hits upon the object and the conditions of the object; what the object is made of. When light hits upon different objects; these objects are made of different types of atoms and molecules that reflect, absorb, and emit light in different ways. So an object appears to have a color because of what is happening when visible light (which is made up of all the colors of the rainbow) hits it. For example, the petals on a rose may appear to be red because the composition of the rose is such that it reflects red light, but not light at other colors; likewise green leaves have the ability to reflect green light but not other colors. Something that is black; like charcoal aborbs all the light that hits it and reflects known of it; when you absorb all the light; that object appears black; So why is it not good to wear black on a hot day? Wearing black is a bad idea because it absorbs all the sunlight hitting on it; all that heat; causes you to heat up. Whereas white colors reflect all the sunlight so you stay cooler. But most things in nature have a combination of reflected light, absorbed light, and emitted light; the combination of all these things give that object its color.
But the color that human beings perceive depends upon the physiological conditions of our eyes. When the light of different wavelengths hits our eyes, it triggers sensors in our brain which tells us that it is seeing a certain color. So to be correct, objects have no real color; they only scatter, absorb, or transmit light at different wavelengths and our brain perceives this as color.

28. Kelvin Temperature Scale
Celsius Temperature + 273

29. increasing temperature
Peak color (wavelength) shifts to shorter wavelengths as the temperature increases
So I said that objects have color if they emit radiation. Which object in our solar system emits the most radiation? Our Sun! But anything that has a temperature emits radiation; if it is very low temperatures, it emits in the infrared; our eye are not sensitive to it; but if we could see in the infrared; we would see everything glowing. So when you heat up objects at high temperatures; they can change their colors; temperatures are associated with energy. Different energies remember correspond to light at different wavelengths.
Hotter temperatures are bluer. So if you have a gas stove you can observe that the hottest central part of the flame is blue whereas the cooler parts are red. So it is kind of contradictory to what you were exposed to. The energy of the light depends upon the temperature. Again energy is associated with wavelength and frequency. Higher temperatures = greater energies and shorter wavelengths.
increasing temperature

30. Which is HOTTER???
Show Infrared video from Spitzer

31. Peak color (wavelength) shifts to shorter wavelengths as the temperature increases
So I said that objects have color if they emit radiation. Which object in our solar system emits the most radiation? Our Sun! But anything that has a temperature emits radiation; if it is very low temperatures, it emits in the infrared; our eye are not sensitive to it; but if we could see in the infrared; we would see everything glowing. So when you heat up objects at high temperatures; they can change their colors; temperatures are associated with energy. Different energies remember correspond to light at different wavelengths.
Hotter temperatures are bluer. So if you have a gas stove you can observe that the hottest central part of the flame is blue whereas the cooler parts are red. So it is kind of contradictory to what you were exposed to. The energy of the light depends upon the temperature. Again energy is associated with wavelength and frequency. Higher temperatures = greater energies and shorter wavelengths.

32. Humans emit infrared light!

33. The Doppler Shift Christian Doppler 1842:
the observed frequency of an object
is affected by its apparent motion
Doppler shift occurs when the source
of the waves is moving with respect to
the observer
occurs for all waves
(e.g. water, sound, light)
The Doppler effect is named after Christian Doppler who in 1842 pointed out that the observed wavelength of an object is affected by it’s motion. Put trumpeteers on a train.
The Doppler effect occurs when the source of the waves is moving with respect to the observer. If the source is stationary, then there is no Doppler effect occuring. This effects occur for all kinds of waves; e.g. water waves, sound waves, and light waves.

34. The Doppler Shift Sound Waves Doppler Ball demo high pitch low pitch
low frequency
long wavelength
high pitch
high frequency
short wavelength
show demonstration of Doppler effect from Redshift
The Doppler effect can also be observed in sound waves. Remember when you were driving your car on the highway and the police car turns on its siren? As the car approaches you, you hear a high pitch; you think to yourself, uh oh, what did I do? But as the car passes by you, you hear a low pitch from the siren; and you breathe a sigh of relief when you realize that the cop is chasing after someone else. That was an example of the Doppler effect in sound waves. In this diagram, the observer at A will hear a low pitch, or low frequency noise since the car is moving away from him. As the car moves away from him, the sound wave emitted from the car is emitted at a farther and farther distances so the frequency detected is lower and lower. The observer at B will hear a high pitch since the car is moving towards him. Here the sound wave from the car is emitted at closer and closer distances towards the observer so the waves will arrive towards him at higher frequency and he will hear a higher pitch.
Doppler Ball demo
It is important to remember that the pitch that the cop hears inside the police car is the same, it doesn’t change; this is because he is moving with the source at the same rate. It only changes for the different observers because the distance between the observers and the police car is changing. But if the cop car and both observers are stationary or all moving in the same direction with the same velocity, they would all hear the same pitch and frequency. Furthermore, if the cop car remains stationary and the observers were moving, the observers will once more detect changes in pitch. So it doesn’t have to be the source that is moving. As long as the distance changes between the observer and the source, the observer will detect a Doppler shift.
Do the Doppler Ball demonstration.

35. The Doppler Shift Light Waves Stationary Star A B
Star Moving to the Right A B
When we discuss the Doppler effect in light, we normally refer to it as a change in wavelength or color. because remember each color has a different wavlength; e.g. red has a longer wavelength than blue.
The main difference between light and sound is that the Doppler effect can only be detected in the light when the source is moving extremely fast. So in the case of the cop, we can not detect a change in the color of the cop because the speed of the car is very slow. However, astronomical objects move very fast; at speeds of hundreds to thousands of kilometers per second. here we can definitely detect the Doppler effect in the light waves.
Here is the wave on the diagram. The source that is producing the radiation is a faraway star. When the source is traveling away from the observer at A, the wavelength that is detected from the source is longer. That is the distance between each successive peak is longer. This is called a redshift. When the source is traveling towards the observer at B, the wavelength detected by the observer at B is shorter; the distance between each successive peak is shorter. This is called the blue shift. This terminology is named so because in the visible part of the electromagnetic spectrum, blue light has shorter wavelengths than red light. This doesn’t mean that the source necessarily changes its color to blue or red, but to colors with longer or shorter wavelengths. We just say that when the observed source is moving away from us, it gets redder and when it moves towards us, it gets bluer.
constant wavelength detected
by both A and B
A detects longer wavelength  redshift
B detects shorter wavelength blueshift

36. The Doppler Shift In Astronomy
Spectrum of Approaching Source
Spectrum of Stationary Source
Astronomers can use the Doppler effect in light waves to find out all sorts of useful information. By observing whether the object’s light is blue shifted or red shifted from us, they can determine whether the object if moving towards us or away from us. Furthermore, the speed that the object moves also affects the shift. If the object is moving at faster speeds, there will be a higher shift detected. Thus, by observing how great the Doppler shift is, they can also calculate the speed of the object.
Here is a schematic of how they go about getting this information by observing the Doppler effect. These represent three spectrums of the source that we are observing. for simplicity, I have only showed one emission line in the spectrum, but in reality, the spectrum may have several emission lines. These emission lines represent the same element in the star. The wavelength of the spectrum increases to the right, so at this end, the wavelengths are shorter than at this end. So by looking at these spectrum, which one do you think represents the source that is moving towards us? Yes, it is the top one. Why? Because the emission line is shifted towards the direction of shorter wavelength. This is called a blue shift and that indicates that the source is moving towards us. The other spectrum represents the source that is moving away from us. We know this because we see that the emission line is shifted to the right towards longer wavelengths; that is, it is redshifted. Thus, we know that when an object’s spectrum is redshifted, it is moving away from us.
Thus, by observing the position of these emission lines with respect to the one that is stationary, Astronomers can understand whether the source is moving towards us or away from us. You can usually get a spectrum in the lab as the spectrum of a stationary source. We just look at the spectrum of different elements in the lab as it gets heated up; then it emits emission lines. And then we compare it with the emission of the source possessing the same element, usually a star or galaxy and see how big the shift is. Also the larger the shift is, the faster the objects is traveling. Thus, if the emission line was located here, the object is traveling at a faster velocity. By observing where the emission line is with respect to the
stationary line, astronomers can also determine the speed of the source.
The shift here is greatly exaggerated; for objects such as stars, these shifts would be much smaller. But for objects such as distant galaxies; they are traveling at such huge velocities, that this could be a realistic shift and their observed colors could change from blue to red.
Show Doppler shift simulation from redshift
Spectrum of Receding Source
400nm nm nm

37. Detection of Extrasolar Planets: Stellar Wobble
gravity of the planet causes
the star to wobble back
and forth
1990s, used Doppler effect
to detect stellar wobbles
Our Solar system is not unique, in fact we should expect to see planets around other stars and we have found almost one hundred of them. These planets were discovered in the 1990s. The reason why it took so long to find these planets is because planets are very faint. Remember they shine predominantly by reflected starlight, thus we can not detect them directly.
They have been found by detecting the wobble of the Star which the planet is orbitting around. If a star wobbled from side to side, it would reveal that a planet was wobbling invisibly the other way.
In the 1990s, techniques were developed to use the Doppler effect. Doppler effect is the change in wavelengths of the light that is emitted caused by motion of the source along the line of sight. At this time we could measure the Doppler effect very precisely. The observed spectrum of the star can be compared to a simulated spectrum of a stationary star. This method allows small Doppler shifts to be detected and the speeds of stars towards and away from us can be measured.
a) The orbits of some of the exoplanets overlaid on the same scale. We see circular orbits close in and eccentric orbits further out.
b) The distribution of the masses of the planets. (Lower limits) since we don’t know the inclination
The discovery of several planetary systems instead of just our own will change the models for how planetary systems are formed. Many theorists stick to their own ideas of how giant planets form far away from their parent stars just as Jupiter and the other giant planets are far out in our Solar system. These planets may be jostled loose from their orbits and put in orbits that bring them closer to their parent stars. Perhaps orbits shrink as they encounter the debris in the dusty disk from which they formed, or shrink along with an overall swirling of the whole disk of orbiting material.
Perhaps highly elliptical orbit didn’t start that way, but were produced by gravitational interactions that ejected the planets from the system. The gravitational interactions between two planets can lead to the ejection of one planet leaving the other in a very eccentric orbit. Or interactions between the planet and the protoplanetary disk can cause high eccentricities. But once a planet has part of its orbit close to the parent star, tidal forces can circularize the orbit. Other planets can spiral into the star and be destroyed.

38. Concept Question If a star is moving away from us, which
statement best describes what is happening
to the star’s light?
A) the light is blueshifted;
we perceive that the wavelength increases
B) the light is redshifted;
C) the light is blueshifted;
we perceive that the wavelength decreases
D) the light is redshifted;
Doppler Shift Simulation:
Redshift 1

39. What can we learn by analyzing starlight?
A star’s chemical composition
by spectrum
A star’s temperature
by color (peak wavelength)
A star’s speed and direction of motion
by spectrum and Doppler Shift