# Lecture 7 ASTR 111 – Section 002.

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Lecture 7 ASTR 111 – Section 002

Outline Discuss Quiz 5 Light
Suggested reading: Chapter and of textbook

Light travels through empty space at a speed of 300,000 km/s
In 1676, Danish astronomer Olaus Rømer discovered that the exact time of eclipses of Jupiter’s moons depended on the distance of Jupiter to Earth

Light travels at 300,000 km/sec
About how fast does your car travel in km/hour when you are on the freeway? About how fast does your car travel in km/sec when you are on the freeway? If it takes light 8 minutes to travel from the sun to Earth, how long would it take you to drive?

Approximately what was the difference in time of the eclipses that Olaus Rømer observed?

Determining the Speed of Light
Galileo tried unsuccessfully to determine the speed of light using an assistant with a lantern on a distant hilltop

In 1850 Fizeau and Foucalt also experimented with light by bouncing it off a rotating mirror and measuring time The light returned to its source at a slightly different position because the mirror has moved during the time light was traveling d=rt again gave c

Light is electromagnetic radiation and is characterized by its wavelength ()

Frequency and Wavelength
The Greek letter “nu” and not the letter “v”

Cars are traveling at 100 km/hr to the right
What would you need to know in order be able to tell how often a car will pass the finish line? Finish line velocity = 100 km/hr

Replace cars with lines
Finish line v = 100 km/hr

Cars are traveling at 100 km/hr to the right
What would you need to know in order be able to tell how often a peak will pass the finish line? Finish line v = 100 km/hr

How often peak passes finish line Distance between peaks
How fast wave moves to right

Interference - destructive
A B

Interference - destructive

Interference A ? B

Interference - simple C ? ? D

Video

Interference

The electromagnetic spectrum

Because of its electric and magnetic properties, light is also called electromagnetic radiation
Visible light falls in the 400 to 700 nm range Stars, galaxies and other objects emit light in all wavelengths

Which of the following has the highest frequency?
Visible light Radio waves Microwaves X-Rays Infrared light Ultraviolet light Gamma rays

Which of the following has the highest wavelength?
Visible light Radio waves Microwaves X-Rays Infrared light Ultraviolet light Gamma rays

Which of the following has the highest speed?
Visible light Radio waves Microwaves X-Rays Infrared light Ultraviolet light Gamma rays

Which of the following has the highest energy E (h is a constant)?
Visible light Radio waves Microwaves X-Rays Infrared light Ultraviolet light Gamma rays

The dual nature of light
Particle Wave

Particle

What would you expect if instead of a laser beam you used yellow spray paint beam? Draw it!

Wave

The atom and light

Probing the atom An atom has a small dense nucleus composed of protons (and neutrons) Rutherford’s experiments with alpha particles shot at gold foil helped determine the structure

Spectral lines are produced when an electron jumps from one energy level to another within an atom
The nucleus of an atom is surrounded by electrons that occupy only certain orbits or energy levels When an electron jumps from one energy level to another, it emits or absorbs a photon of appropriate energy (and hence of a specific wavelength). The spectral lines of a particular element correspond to the various electron transitions between energy levels in atoms of that element. Bohr’s model of the atom correctly predicts the wavelengths of hydrogen’s spectral lines.

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

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 Section 5.2 X

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?

What will the spectrum look like here?

Emission line spectrum

Continuous Spectrum At this point I go back to the dryer lint question and ask what it has to do with light. If they do not get it, I ask if their dryer lint ever came out red and why. When they get the answer, ask why it is gray instead of white (among other reasons, because you usually wash whites separate). A blackbody emits photons with many energies (wavelengths) – a continuous spectrum A prism bends photons more or less depending on their wavelength

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

Absorption vs. Emission

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.

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.

Would this make sense? This dark line was removed

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 = x 10–34 J s

Three Temperature Scales

Color and Temperature

An opaque object emits electromagnetic radiation according to its temperature

Blue: Hot or Not?

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