Lecture 27

Final Logistics• IMPORTANT: The Testing and Tutoring Center

will be closed during finals week except for these hours, so if you miss the exam, I will not be able to give you a make-up exam.

• The final exam will have about 80 questions.  The exam will have questions related to the topics covered in the problems in lecture, problems on the quizzes, and problems on the first three exams.  You will have 120 minutes to complete the exam (but I expect most students to finish in about 60 minutes).

Final – How to prepare

• Same as for previous exams• Work through the practice exams and

problems worked in class.• Make sure that you understand the

principle that the question is asking you about. (Try to memorize as little as possible.).

• Work through relevant problems at http://astro.unl.edu/classaction/

Grade Adjustments

• I have given 3 extra credit opportunities, and for borderline grades, I usually round in the student’s favor.

• I will not make any adjustments to your grade – please don’t ask!

Final Review

• I’ll post these slides today.

Moon Phases

Draw what the moon looks like from someone on Earth at positions 1-8

http://tierneylab.blogs.nytimes.com/2008/04/23/the-astronomy-quiz-answers/

Oblate

OblateDrill bit

racquetballWhat happens when the drill is turned on?

A B

Oblate

Which drill bit is spinning faster?

A B

Drill bit

racquetball

Flux

Flux• Flux is a measure of how much “stuff”

crosses or hits a small patch in a given amount of time.

• Can have flux = # of green photons that hit an area per second. Stuff = # of green photons

• Can have flux = Energy that hits and area per second. Stuff = Energy

Energy Flux?

If you know number of photons that hit per second, you can compute the energy per second into the solar panel

Each photon has a certain amount of energy in Joules that is given by Plank’s law.

1 meter x 1 meter squareBlue photon

Surface Area of Sphere

• It is an area, so it is proportional to the square of the radius

Cube with light source inside

L

• What is the volume of this cube?• What is the surface area of the outside of

the cube?• If the length of the sides of the cube are

increased by a factor of two, what would happen to– Its volume?– Its surface area?– The number of photons that hit each side?– The flux of photons on each side?

Side view of cube

L2L

Photons just before they hit inner wall.

Side view of cube

L2L

What happened to the separation distance d between hits on wall?

d

• What is the volume of this cube? L*L*L• What is the surface area of the outside of the

cube? 6*L*L• If the length of the sides of the cube are

increased by a factor of two, what would happen to– Its volume? Increases by factor of 8 (L*L*L changes

to 2*L*2*L*2*L– Its surface area? Increases by a factor of 4. 6*L*L

changes to 6*2*L*2*L– The number of photons that hit each side? Same– The flux of photons on each side? Decreases by

factor of 4. Same number of photons hit area that is 4x larger. Flux is (photons/s) / Area

Formulas

Lens

• Suppose you have a single lens from a pair of eyeglasses that is 3 inches in diameter. How many more photons hit this lens per second than would hit a lens that is only 1.5 inches in diameter?– 4 times– 2 times– 8 times– 16 times

Area

2 meters

1 m

eter

22

4dr

Area

Disk Disk

Increase r or d by a factor of 2 and Surface Area increases by 2x2 = 4

Lens

• Suppose you have a single lens from a pair of eyeglasses that is 3 inches in diameter. How many more photons hit this lens per second than would hit a lens that is only 1.5 inches in diameter?– 4 times– 2 times– 8 times– 16 times

Volume

2 meters

1 m

eter

3

6

1dVolume

Sphere Sphere

Increase d by a factor of 2 and Volume increases by 2x2x2 = 8

Volume

2 meters1

met

er

Sphere Sphere

What happens if radius increases by a factor of two?

Volume

2 meters

1 m

eter

33

6

1

3

4dr Volume

Sphere Sphere

Increase r by a factor of 2 and Volume increases by 2x2x2 = 8

Area

2 meters

1 m

eter

2rArea

Disk Disk

Increase r by a factor of 2 and Surface Area increases by 2x2 = 4

Blackbody

Wavelength

# of

pho

tons

em

itted

Object is heated. Its spectrum is found to go from the black to the red curve. Is the object a blackbody?

Waves

• Light and Spectra part of:

http://astro.unl.edu/classaction/

Y

X

X

X

X

Y

Y

Y

A

B

C

D

X

Y

Y

• Contellations drifting

• http://www.espace.gc.ca/eng/educators/resources/astronomy/multimedia/module3/solar_eclipses/solar_eclipses.swf?404=1&ref=