An explanation of what blue and redshifts are and sets up and solves a problem that uses knowledge of blueshift and the doppler effect

2.  A blueshift is any decrease in wavelength, due to an
increase in frequency, of electromagnetic waves.
 A redshift is the opposite of this. An example being the
redshift of the universe as evidence of its slowing
down.

3.  The other day I was flying along a galactic highway in
my spaceship. I passed through an intersection, and,
seeing a green light, proceeded through. A police
officer flashed his lights on and pulled me over,
informing me that I had just ran a red light, as well as
speeding.

4.  How fast must I have been travelling to perceive the
red light as a green light?

5.  𝜆 of red light: 685.0 𝑛𝑚
 𝜆 of green light: 532.5 𝑛𝑚
 𝑣 of sender: 0 𝑚/𝑠
 𝑣 of receiver: unknown
 𝑣 of light: 3 ∗ 108 𝑚/𝑠

6.  Find the frequency of red light:
 𝑣 = 𝑓𝑟 𝜆 𝑟
 3 ∗ 108 𝑚/𝑠 = 𝑓𝑟(6.85 ∗ 10−7 𝑚)
 𝑓𝑟 = 4.38 ∗ 1014 𝐻𝑧
 Find the frequency of green light:
 𝑣 = 𝑓𝑔 𝜆 𝑔
 3 ∗ 108 𝑚/𝑠 = 𝑓𝑔(5.325 ∗ 10−7 𝑚)
 𝑓𝑔 = 5.63 ∗ 1014 𝐻𝑧

7.  Start with regular Doppler formula
 𝑓𝑟 =
𝑣±𝑣 𝑟
𝑣∓𝑣 𝑠
𝑓𝑠
 Stationary sender, moving receiver, therefore
 𝑓𝑟 =
𝑣+𝑣 𝑟
𝑣
𝑓𝑠
 Solve for 𝑣𝑟
 5.63 ∗ 1014 𝐻𝑧 =
3∗
108 𝑚
𝑠
+𝑣 𝑟
3∗
108 𝑚
𝑠
4.38 ∗ 1014 𝐻𝑧
 𝑣𝑟 = 8.59 ∗ 107
𝑚/𝑠

8.  http://en.wikipedia.org/wiki/Blueshift