Year 9 Physics Assignment 1
Name:……………
The Speed of Light
Maximum Marks: 25
Light travels at a constant speed. The symbol for the speed of light is c. The speed of
light depends on the substance through which it is traveling.
c = 299 792 458 m/s (this is in a vacuum, i.e. empty space)
For our calculations, we will use 3 sig figs, c = 3.00 × 108 m/s. Note that this value
is also used as the speed of light in air.
Remember that for constant speed, 𝑠𝑝𝑒𝑒𝑑 =
1.
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
𝑡𝑖𝑚𝑒
or 𝑐 =
How long does it take light to travel 10 000 km?
𝑑
𝑡
(3 marks)
...………………………………………………………………………………………
…………………………………………………………………………………………
…………………………………………………………………………………………
…………………………………………………………………………………………
2.
When measuring astronomical distances, we often use the unit of “light-year”.
One light-year is the distance that light covers in one year. Calculate the distance
(in km) corresponding to one light year.
(3 marks)
...………………………………………………………………………………………
…………………………………………………………………………………………
…………………………………………………………………………………………
…………………………………………………………………………………………
3.
Earth is located approximately 1.496 × 1011 m from the sun. How long does it take the
light from the sun to reach the Earth? Give your answer in minutes.
(3 marks)
…………………………………………………………………………………………
...………………………………………………………………………………………
…………………………………………………………………………………………
…………………………………………………………………………………………
4.
Light from the sun reach the planet Mercury in only 3 minutes and 13 seconds.
How far from the sun is Mercury located? Give your answer in km.
(3 marks)
…………………………………………………………………………………………
...………………………………………………………………………………………
…………………………………………………………………………………………
…………………………………………………………………………………………
2020 Assignment Speed of Light
Year 9 Physics
Page 1 of 2
5.
While light travels at c = 3.00 × 108 m/s, sound only travels at 340 m/s.
Imagine you are watching fireworks, which are happening 3 km away.
a. How long does it take the light to reach you?
(3 marks)
...……………………………………………………………………………………
……………………………………………………………………………………..
………………………………………………………………………………………
b. How long does it take the sound to reach you?
(3 marks)
...………………………………………………………………………………………
……………………………………………………………………………………....
…………………………………………………………………………………......
c. What does this explain about the fireworks-watching experience?
(1 marks)
...………………………………………………………………………………………
…………………………………………………………………………………………
6.
When light travels through water it can only cover 1.125 × 109 m in 5 seconds.
What is the speed of light in water?
(3 marks)
...……………………………………………………………………………………..
……………………………………………………………………………………....
……………………………………………………………………………………....
………………………………………………………………………………………
7.
In Plexiglas, light has a speed of 1.99 × 108 m/s. How long does it take light to
travel through a block of Plexiglas that is 1 m thick?
(3 marks)
...………………………………………………………………………………………
…………………………………………………………………………………….......
………………………………………………………………………………………..
…………………………………………………………………………………………
2020 Assignment Speed of Light
Year 9 Physics
Page 2 of 2