Lecture 12 ASTR 111 – Section 002

Outline Quiz Discussion Finish a few slides from last lecture Light (Reading is Chapter 5)

Quiz Discussion 75% Computing your grade – will not cover in class 66% Photons through a hole – will cover in class Can we finish going over lecture 12 in class? Are you going to post the answers to lecture 12? When is the next exam scheduled? What acts as a nature's prism to create a rainbow in the sky?   If enacted, will clickers be mandatory?   I think we should use iclickers. Wouldn't it be easier than texting? How many pets do you really have? Can you review fully before the next exam? Does it bother you when people come 45 minutes late to lecture and slam there stuff around and make a lot of noise? Because it really bothers me.   Why is this class getting exponentially more difficult?

Outline Quiz Discussion Finish a few slides from last lecture Light (Reading is Chapter 5)

Measurements in Astronomy In astronomy, we need to make remote and indirect measurements Think of an example of a remote and indirect measurement from everyday life This may be a good time to have them work in groups to come up with answers. You may also want to try to get them to think and debate the distinction between what defines a direct versus an indirect measurement.

Using Light Light has many properties that we can use to learn about what happens far away Light interacts with matter in a special way

Example with colored balls and a child who only likes certain colors Example with colored balls and a child who only likes certain colors. He never stops to grab certain colors that roll by. When he catches certain colors, he holds on to them for a while before tossing them in a random direction.

X Only photons with special wavelengths will interact with atom How will this affect what a person will see at point X? When is the atom “hotter”? From Universe 7e Section 5.2 online material http://bcs.whfreeman.com/universe7e/pages/bcs-main.asp?v=chapter&s=05000&n=00020&i=05020.02 X From Universe 7e Section 5.2 online material http://bcs.whfreeman.com/universe7e/pages/bcs-main.asp?v=chapter&s=05000&n=00020&i=05020.02

Why is UV light usually blamed for skin cancer Why is UV light usually blamed for skin cancer? What is special about it compared to other light sources?

DNA “absorbs” or “is excited by” UVB radiation. This causes a chemical reaction to take place that modifies DNA. Why doesn’t UVA affect DNA like this? http://earthobservatory.nasa.gov/Features/UVB/Images/dna_mutation.gif

Cloud of Gas A cloud of gas will emit only certain frequencies. Better to bring colored balls and throw them to demonstrate. What will you see on the wall? A prism bends photons more or less depending on their wavelength

Cloud of Gas A cloud of gas will emit only certain frequencies. Better to bring colored balls and throw them to demonstrate. What will you see on the wall? A prism bends photons more or less depending on their wavelength

What will the spectrum look like here?

Emission line spectrum

Continuous Spectrum A blackbody emits photons with many energies (wavelengths) – a continuous spectrum

What will the spectrum look like here? Note that the original image said “Cloud of cool gas”. I covered up the “cool” because some people thought it would mean that the gas was so cold that we would not see anything on the spectrum (they were actually thinking about the radiated power amplitude according to the blackbody curve being very low in the visible range for a cold gas). What will the spectrum look like here?

Absorption Spectrum

Three types of spcetra

What type of spectrum is produced when the light emitted from a hot, dense object passes through a prism? What type of spectrum is produced when the light emitted directly from a cloud of gas passes through a prism? Describe the source of light and the path the light must take to produce an absorption spectrum There are dark lines in the absorption spectrum that represent missing light. What happened to this light that is missing in the absorption line spectrum? From Lecture Tutorials for Introductory Astronomy, page 61.

Each chemical element produces its own unique set of spectral lines

Stars like our Sun have low-density, gaseous atmospheres surrounding their hot, dense cores. If you were looking at the spectra of light coming from the Sun (or any star), which of the three types of spectra would be observed? If a star existed that was only a hot dense core and did not have a low-density atmosphere surrounding it, what type of spectrum would you expect this particular star to give off? Two students are looking at a brightly lit full Moon, illuminated by reflected light from the Sun. Consider the following discussion between two students about what the spectrum of moonlight would look like: I think moonlight is just reflected sunlight, so we will see the Sun’s absorption line spectrum. I disagree, an absorption spectrum has to come from a hot, dense object. Since thie Moon is not a hot, dense object, it can’t give off an absorption line spectrum. Do you agree or disagree with either or both of these students? Explain your reasoning. c c c

Stars like our Sun have low-density, gaseous atmospheres surrounding their hot, dense cores. If you were looking at the spectra of light coming from the Sun (or any star), which of the three types of spectra would be observed? If a star existed that was only a hot dense core and did not have a low-density atmosphere surrounding it, what type of spectrum would you expect this particular star to give off? Two students are looking at a brightly lit full Moon, illuminated by reflected light from the Sun. Consider the following discussion between two students about what the spectrum of moonlight would look like: I think moonlight is just reflected sunlight, so we will see the Sun’s absorption line spectrum. I disagree, an absorption spectrum has to come from a hot, dense object. Since thie Moon is not a hot, dense object, it can’t give off an absorption line spectrum. Do you agree or disagree with either or both of these students? Explain your reasoning. c c c

Stars like our Sun have low-density, gaseous atmospheres surrounding their hot, dense cores. If you were looking at the spectra of light coming from the Sun (or any star), which of the three types of spectra would be observed? If a star existed that was only a hot dense core and did not have a low-density atmosphere surrounding it, what type of spectrum would you expect this particular star to give off? Two students are looking at a brightly lit full Moon, illuminated by reflected light from the Sun. Consider the following discussion between two students about what the spectrum of moonlight would look like: I think moonlight is just reflected sunlight, so we will see the Sun’s absorption line spectrum. I disagree, an absorption spectrum has to come from a hot, dense object. Since thie Moon is not a hot, dense object, it can’t give off an absorption line spectrum. Do you agree or disagree with either or both of these students? Explain your reasoning. c c c

Stars like our Sun have low-density, gaseous atmospheres surrounding their hot, dense cores. If you were looking at the spectra of light coming from the Sun (or any star), which of the three types of spectra would be observed? If a star existed that was only a hot dense core and did not have a low-density atmosphere surrounding it, what type of spectrum would you expect this particular star to give off? Two students are looking at a brightly lit full Moon, illuminated by reflected light from the Sun. Consider the following discussion between two students about what the spectrum of moonlight would look like: I think moonlight is just reflected sunlight, so we will see the Sun’s absorption line spectrum. I disagree, an absorption spectrum has to come from a hot, dense object. Since thie Moon is not a hot, dense object, it can’t give off an absorption line spectrum. Do you agree or disagree with either or both of these students? Explain your reasoning.

Imagine that your are looking at two different spectra of the Sun Imagine that your are looking at two different spectra of the Sun. Spectrum #1 is obtained using a telescope that is in a high orbit far above Earth’s atmosphere. Spectrum #2 is obtained using a telescope located on the surface of Earth. Label each spectrum below as either Spectrum #1 or Spectrum #2.

Spectrum #2 (Near surface) Spectrum #1 (High above surface) Imagine that your are looking at two different spectra of the Sun. Spectrum #1 is obtained using a telescope that is in a high orbit far above Earth’s atmosphere. Spectrum #2 is obtained using a telescope located on the surface of Earth. Label each spectrum below as either Spectrum #1 or Spectrum #2. Spectrum #2 (Near surface) Spectrum #1 (High above surface)

Would this make sense? This dark line was removed Spectrum #2 (Near surface) Spectrum #1 (High above surface)

Energy and electromagnetic radiation Planck’s law relates the energy of a photon to its frequency or wavelength E = energy of a photon h = Planck’s constant c = speed of light l = wavelength of light The value of the constant h in this equation, called Planck’s constant, has been shown in laboratory experiments to be h = 6.625 x 10–34 J s

Which electromagnetic wave has a higher energy: one with f=10 cycles per second or f=1 cycles per second?

Three Temperature Scales

Color and Temperature

An opaque object emits electromagnetic radiation according to its temperature

! (An aside)

Blue: Hot or Not? http://www.straightdope.com/mailbag/mhotflame.html

If blue light has higher energy, and energy is proportional to temperature, why are my cold spots blue?

If it is not opaque (or a perfect blackbody), relationship between color that you see and temperature are more complicated.

Why do we associate blue with cold and red with hot? Lips turn blue when cold Ice takes on a blue-ish tint Face turns red when hot Red is the first thing you see when something is heated (usually don’t see much blue)

What you see depends on if it is a result of Absorption (light reflected off your face or light reflected by a plant) Emission (light from a flame or a heated bar)

What you see depends on if it is a result of Absorption and reflection (light reflected off your face or light reflected by a plant) Scattering (Absorption and then emission at a random angle as in a gas) Emission (light from a flame or a heated bar) Blackbody has simple absorption and emission spectra Elements have simple absorption and emission spectra Other objects have complex absorption and emission spectra, and to explain how the color relates to the temperature, you usually need to combine Absorption, Emission, and Scattering effects. General trend is emitting object get bluer when heated and absorbing objects get bluer when cooled.

!

Blackbody Definition Does not reflect incoming radiation, only absorbs Emits radiation, depending on temperature Temperature and emitted radiation intensity follow a special relationship One way of creating a blackbody The “hole” is the blackbody. Photon enters If hole is very small, what is probability that it exits?

Blackbody Definition Does not reflect incoming radiation, only absorbs Emits radiation, depending on temperature Temperature and emitted radiation intensity follow a special relationship One way of creating a blackbody The “hole” is the blackbody. Photon enters Eventually photon excites an atom or molecule in the wall.

Blackbody Definition Does not reflect incoming radiation, only absorbs Emits radiation, depending on temperature Temperature and emitted radiation intensity follow a special relationship One way of creating a blackbody The “hole” is the blackbody. Photon enters Excited molecule emits photons at different wavelengths (according to its spectra). Does not need to re-emit the same wavelength!

Wien’s law and the Stefan-Boltzmann law are useful tools for analyzing glowing objects like stars A blackbody is a hypothetical object that is a perfect absorber of electromagnetic radiation at all wavelengths Stars closely approximate the behavior of blackbodies, as do other hot, dense objects

Blackbodies do not always appear black! The sun is close to being a “perfect” blackbody Blackbodies appear black only if their temperature very low

Special Relationship For Intensity, think photons/second on a small area Intensity Wavelength

Question Why is photon/second similar to energy/second? How are they related?

Watt? Energy Flux? Using

Flux Flux is a measure of how much “stuff” crosses a small patch in a given amount of time. Can have flux of green photons, red photons, etc.

Blackbodies and Astronomy

Blackbody Laws Stefan-Boltzmann Law – relates energy output of a blackbody to its temperature Wein’s law – relates peak wavelength output by a blackbody to its temperature

Special Relationship For Intensity, think photons/second on a small area Energy Flux Intensity Wavelength

Stefan-Boltzmann Law A blackbody radiates electromagnetic waves with a total energy flux F directly proportional to the fourth power of the Kelvin temperature T of the object:

Special Relationship Stefan-Boltzmann Law tells us that if we add up the energy from all wavelengths, then the total energy Flux Energy Flux Intensity Wavelength

Special Relationship Wien’s law tells us that lmax depends on temperature Max intensity at lmax Energy Flux Intensity Wavelength lmax

Special Relationship Sketch this curve for larger and smaller T Energy Flux Intensity Wavelength

Wavelength of peak decreases as temperature increases At high wavelengths, intensity goes to zero Overall amplitude increases with Temperature As wavelength goes to zero, intensity goes to zero

Color and Temperature

What would this object look like at these three temperatures?

Why does it glow white before blue?

Can this figure help us explain?

Can this figure help us explain? Near this temperature, this special combination of intensities is what we call white. Also, the real curve is a little flatter near the peak Can this figure help us explain?

The Sun does not emit radiation with intensities that exactly follow the blackbody curve

If “white” was actually defined by the ideal blackbody curve, perhaps we could add a little green to white …

So, what color is the sun in space? Solid green square

So, what color is the sun in space? Add a little green to white background by making solid green square mostly transparent

If “white” was actually defined by the ideal blackbody curve, this would (sort of*) make sense … * A stream of photons of wavelength that we call green would actually be perceived as a mixture of red, green, and blue by our eye, so calculation is more complicated … But what we call white is actually not the ideal blackbody curve. See http://casa.colorado.edu/~ajsh/colour/Tspectrum.html

So, what color is the sun in space? Left side is white Right side is (should be) a little “pinker” http://casa.colorado.edu/~ajsh/colour/Tspectrum.html

D65 is reference “white” It is determined using a detector that measures the Flux of photons

5 A B Figure 1 4 C 3 Energy Flux 2 1

Which curve represents an ideal blackbody? Curve A Curve B Curve C

Which curve represents an ideal blackbody? Curve A Curve B Curve C

If the object in Figure 1 were increased in temperature, what would happen to curves A, B, and C?

If the object in Figure 1 were increased in temperature, what would happen to curves A, B, and C? All would increase in amplitude. Peak would shift to left.

Curve C is more jagged. The locations where the curve C is small correspond to Spectral lines of a blackbody Spectral lines of atmospheric molecules Instrumentation error Diffraction lines Spectral lines of the lens used to the light into colors

Curve C is more jagged. The locations where the curve C is small correspond to Spectral lines of a blackbody Spectral lines of atmospheric molecules Instrumentation error Diffraction lines Spectral lines of the lens used to the light into colors

What is the intensity of curve B at 550 nm? Impossible to tell; 550 nm is not shown in this figure Nearest 4 Nearest 3 Nearest 1 Nearest 0.5

What is the intensity of curve B at 550 nm? Impossible to tell; 550 nm is not shown in this figure Nearest 4 Nearest 3 Nearest 1 Nearest 0.5

The moon has no atmosphere The moon has no atmosphere. If you measure the spectrum of the same object as that measured in Figure 1 from its surface instead of from Earth’s, Curves B and C would not change Curve C would look more like A Curve C would look more like B Curve B would look more like A Curve B would look more like C

Venus has no atmosphere. If you measure the spectrum from its surface, Curves B and C would not change Curve C would look more like A Curve C would look more like B Curve B would look more like A Curve B would look more like C

White light is composed of Equal intensities of all colors of the rainbow Unequal intensities of all colors of the rainbow Equal number of photons of all colors of the rainbow Unequal number of photons of all colors of the rainbow Equal numbers of red, green, and blue photons

White light is composed of Equal intensities of all colors of the rainbow Unequal intensities of all colors of the rainbow Equal number of photons of all colors of the rainbow Unequal number of photons of all colors of the rainbow Equal numbers of red, green, and blue photons

Does a blackbody have color? Yes, and they all appear the color of the sun No, you cannot see a blackbody Yes, but its depends on its temperature Maybe, it depends on if it is an ideal blackbody

Does a blackbody have color? Yes, and they all appear the color of the sun No, you cannot see a blackbody Yes, but its depends on its temperature Maybe, it depends on if it is an ideal blackbody

Why is the best reason for putting a telescope in orbit? Closer to stars Better view of celestial sphere The speed of light is higher in space Less atmospheric interference Cost

Why is the best reason for putting a telescope in orbit? Closer to stars Better view of celestial sphere The speed of light is higher in space Less atmospheric interference Cost

Bonus How would blackbody curve change as you moved closer to the object?