# Light and Average Distance

**Topics:**Light, Electromagnetic radiation, Wavelength

**Pages:**2 (259 words)

**Published:**January 22, 2013

All answers should be given with three significant digits (e.g., 1.50x103 or 2.57x10-3). You should show your work (the steps which led to your answers) and also units (e.g., years or seconds), when appropriate.

1. Please write the following numbers using scientific or "powers of 10" notation.

a) 125 hundred million light-years

1.25 x 10 ^8

b) 125 billionths of a meter

1.25 x 10^-7

c) the average distance to our Moon in units of parsecs (Consult an appropriate Appendix of the text.)

d) the average distance to our Moon in units of nanometers

2. How long does it take light photons to travel the following distances? (Assume light is traveling in a vacuum, like space.)

a) from the Moon to the Earth

b) from Houston to the San Antonio

3. A certain light source is emitting radiation with a frequency of 5.55x1015 Hz.

a) What is the wavelength of the emitted photons?

b) In what part of the electromagnetic spectrum are the emitted photons?

4. Suppose a dark hydrogen line in a star's spectrum is produced at a wavelength of 6563 A, BUT this line has a MEASURED wavelength of 6560 A.

(a) Would this star be moving toward us or away?

(b) How fast would it be moving along our line of sight in units of km/s?

(c) How fast would it be moving in miles/hour?

(d) How fast would it be moving in units of c?

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