# How long does it take light to travel

1. Oct 5, 2015

### ducmod

1. The problem statement, all variables and given/known data
Hello!
This is an elementary problem, but I somehow can't compute it.

How long does it take light to travel in:
(a) 1.0 ft (report answer in nanoseconds)

(b) 4.5 billion km, the average separation between
the sun and Neptune (report answer in hours
and minutes)

Here is what I have tried:
(a) I assume 1.0ft means 1 foot. Light travels 3 x 10^8 meters / second, or 186 000 miles per second (information from a chemistry textbook). 1 meter = 3.2808 feet, hence
9.8424 x 10^8 feet in 1 second
1 foot in x seconds
hence it takes 1 / (9.8424 x 10^8) = 0.10168 x 10^(-8) seconds
To transfer to nanoseconds, I need to divide by 10^(-9), and I get 0.10168 x 10^(-8) / 10^(-9), hence
I get 1 nanosecond as the answer. Correct?

(b) 3 x 10^8 meters / second, hence 0.003 x 10^8 km/ second,
4.5 x 10^8 km / 0.003 x 10^8 km = 1500 seconds
There are 3600 seconds in one hour, hence 3600 / 1500 = 2.4 hours.
This answer is wrong, it should be 4 hours 10 minutes

Thank you!

2. Oct 5, 2015

### Staff: Mentor

Yep, 1 ft/ns, strange, but quite useful sometimes.

First mistake.

Second mistake.

3. Oct 5, 2015

### ducmod

Thank you, but what are my mistakes?

4. Oct 5, 2015

### Staff: Mentor

Come on, I told you were they are, do some effort and rethink your numbers and what you did with them. You will learn much more if you find what is wrong on your own than if I spoon feed you what the problem is.

5. Oct 5, 2015

### Staff: Mentor

Sigh, there goes my pedagogy.

6. Oct 5, 2015

### SteamKing

Staff Emeritus
I deleted my post Borek. I didn't see your Post #4.