Physics PHYS 102
General Astronomy

 

Problem Set #3

 

After you finish this problem set, you should be familiar with:

photons

refraction

resolution

electromagnetic waves

lenses

diffraction

electromagnetic spectrum

chromatic aberration

atmospheric blurring

wavelength

reflection

radio Telescopes

frequency

magnification

space telescope

absorption by atmosphere

light-gathering power

IR/UV/X-ray observatories

 

Please answer the following questions using sentences and paragraphs.  When it is helpful, please use diagrams and equations.

  1. As chief engineer of the starship Enterprise, it is Montgomery Scott's job to decide what type of telescope to use in the ship's optical scanners. As the on-board expert in telescope design, he comes to you for advice.           
     
    1. With the aid of a diagram, explain to Mr. Scott how a refracting telescope works.
    2. With the aid of another diagram, explain to Mr. Scott how a reflecting telescope works.
    3. Mr. Scott is confused by the fact that the images are inverted. Explain this phenomenon.
    4. Mr. Scott asks you what the important characteristics of a telescope are. Please tell him your opinion, listing each characteristic according to its importance, and explaining each characteristic fully.
    5. Explain some of the advantages of reflecting over refracting telescopes.
  1. Imagine you are a professional astronomer. In order to understand the phenomenon you are studying, you need to observe several astronomical objects using electromagnetic waves with a wavelength of around 1 m.
  1. What frequency are these waves? 
  2. These waves are in what part of the electromagnetic spectrum?
  3. How fast do these waves travel in a vacuum?
  4. Draw a pictorial representation of these electromagnetic waves. Be sure to label the wavelength.
  5. What type of “telescope" would you need to make your observations?
  6. Which of the world’s observatories would allow you to detect the faintest object at this wavelength?
  7. Which of the world observatories would give you the highest resolution?
  8. Now imagine you needed to make observations at wavelengths of about 1 fm. What type of device would you need to make your observations? Where would it necessarily be located? Does such a device exist?
  1. Here is an applet which simulates optical elements like lenses and mirrors: here

First try the simulation using a lens.  There is a candle-shaped object, located to the left of the lens.  You can click on and drag this object to any position. 

a.       Where is the image if the arrow object is farther from the lens than the focal point?  Is it inverted?  Is it magnified? 

b.      Where is the image if the arrow object is closer to the lens than the focal point?  Is it inverted?   Is it magnified?  

 

Next, click on the "LENS" button to change the lens to a mirror. Again, the candle is located to the left side of the screen.  You can drag the candle to any position. 

c.       Where is the image if the arrow object is farther from the lens than the focal point?  Is it inverted?  Is it magnified?

d.      Where is the image if the arrow object is closer to the lens than the focal point?  Is it inverted?   Is it magnified?  

 

Here are some practice "figure it out rather than memorize" type questions.  They will not be graded as part of the homework (so don't turn them in), but they may be useful as you prepare for the test. 

1.

The image formed by an object that is closer to a parabolic concave mirror than the focal point is

 

A)

inverted, real and smaller

 

B)

inverted, real and magnified

 

C)

upright, virtual and magnified

 

D)

none of these

 

2.

A particular reflecting telescope has a primary mirror 0.4 m in diameter and 4.0-m focal length and an eyepiece lens 1.0 cm in diameter and 2.0-cm focal length. What is the magnifying power of this telescope?

 

A)

20×

 

B)

200×

 

C)

40×

 

D)

400×

 

3.

If a photon with energy 2 eV has a wavelength of about 656 nm, what is the energy of a photon with four times that wavelength?

 

A)

8 eV

 

B)

4 eV

 

C)

0.5 eV

 

D)

0.25 eV

 

E)

none of these.