# Two Questions -> Speed of Light

1. Jan 24, 2005

### Kawrae

I worked through these and thought I had the right answer but apparently not... and honestly I have no idea why. I think I did everything right?

1. The wavelength of red helium-neon laser light in air is 632.8 nm. What is its frequency?

>> Well I know that v=lambda*f and that v=c/n. So I solved for v using v=c/n, where c=2.9996e8 and n=1.000293 (refraction of light in air). Solved for v and got 299872137.5. Then I solved for f using f=v/lambda and plugged in f=(299872137.5)/(6.328e11) <-- changed wavelength to m and got an answer of 4.739e-4.
How can this be wrong? I went back and tried the equations again, only this time I just used the wavelength as 632.8nm instead of converting. What am I doing wrong???

2. In an experiment to measure the speed of light using the apparatus of Fizeau, the distance between light source and mirror was 11.47 km and the wheel had 720 notches. The experimentally determined value of c was 2.996e8 m/s. Calculate the minimum angular speed of the wheel for this experiment to 4 significant figures. (Assume that the return ray is unblocked by the tooth).

>> I used two formulas: t=2d/c and w=angle/t. I used d as 11470 m, c as 2.996e8 m/s, and the angle as 1/1440 (doubled the 720 notches because there would be an additional 720 spaces). Plugged it all in and got 9.0655 rev/s, converted to rad/s and got 56.99. I think Im messing up with the angle Im supposed to be using... should it not be 1/1440?

2. Jan 24, 2005

### Andrew Mason

632.8 nm = 632.8e-9 m = (6.328e2)e-9=6.328e-7 m, not 6.328e11. The answer should be 4.74e14 Hz.