Peoples Physics Concepts

16.1. Speed of Light http://www.ck12.org

16.1 Speed of Light

• Define the speed of light and a light year.


• Solve problems involving light.

Students will gain an understanding of the speed of light and work problems involving light. The distance of a light
year is also explained and practiced.

Key Equations

D=ct; distance is equal to the speed of light multiplied by the time it has traveled

Guidance

• Light travels at 3× 108 m/s. We call this valuec. Light can never travel at any other speed. Although when
  light travels through materials, due to scattering and absorption it appears that light is going slightly slower
  (see refraction lesson for more detail).


• A light year is the amount of distance light covers in a time of one year. The value of one light year is simply
  the speed of light multiplied by the number of seconds in a year and is roughly equal to 9. 4 × 1015 m.


• Viewing distant stars is looking back in time. The stars we see are many thousands of light years away, which
  means the light takes many thousands of years to reach us. Thus the stars we see in the sky are how they
  looked thousands of years ago.

Example 1

QuestionHow long after texting your friend on Mars will he receive it?

AnswerMars is a distance of 3. 1 × 1011 m. Thus, using the equationt=Dc=^3.^1 ×^10

(^11) m
3 × 108 m/s= 1033 seconds or about 17
minutes.
Explore More

1. Our sun is 8 “light minutes” from earth.
   a. Using the speed of light asccalculate the earth-sun distance inm, then convert it to miles (one mile =
   1609 m)
   b. The nearest galaxy to our Milky Way is the Andromeda Galaxy, 2 million light years away. How far
   away is the Andromeda Galaxy in miles?


2. The Canis Major Dwarf galaxy is 2. 364 × 1020 m.
   a. How far is it in light years?
   b. If the galaxy were to disappear, how long until we notice it vanish on Earth?


3. Light does not travel infinitely fast –its speed in a vacuum is 3× 108 m/s. We measure very large astronomical
   distances in “light years” (LY) –the distance light travels in one year. The closest star beyond our sun is Alpha
   Centauri, 4 LY away. Some distant quasars are 2 billion LY away. Why do astronomers say that to look at
   these distant objects is to look back in time? Explain briefly.