# if the velocity of light in the medium is assumed to be 2.10^-8 m/s, how much time...

#### sofianets

Joined Nov 24, 2012
8
if the velocity of light in the medium is assumed to be 2.10^-8 m/s, how much time does it take the light to travel through the distance of 60 cm?

#### Wendy

Joined Mar 24, 2008
23,460
First off, the speed of light is ≈3.00X10E8 m/s, I'm not sure where you got that number (X10E-8?).

60cm = 0.06m

So 3.00X10E8 m/s ÷ 0.06 m = 5e9 / s

Invert it. S / 5e9 = 2e-10 S or 200 nanoseconds.

#### bertus

Joined Apr 5, 2008
22,290
Hello Bill,

I think you made a mistake with the conversion of cm to m.
60 cm = 0.6 m
So the answer is 3 X 10^8 / 0.6 = 5 X 10^8
wich gives 2 X 10^-9 = 2 nano seconds.

Bertus

#### Wendy

Joined Mar 24, 2008
23,460
Good enough.

I am a lot familiar with inches / feet / yards / and miles. c = 186e3 miles/second, good enough for antenna work.

#### WBahn

Joined Mar 31, 2012
30,248
First off, the speed of light is ≈3.00X10E8 m/s, I'm not sure where you got that number (X10E-8?).

60cm = 0.06m

So 3.00X10E8 m/s ÷ 0.06 m = 5e9 / s

Invert it. S / 5e9 = 2e-10 S or 200 nanoseconds.
The speed of light IN A VACUUM is ≈3E8 m/s. The OP specifically said that he was talking about "in a medium".

My guess is that he meant 2.1E8m/s, and not 2.1E-8 m/s. If I'm correct, then he is talking about a medium with a refractive index of about 1.4, which is very common. In fact, this is pretty much the index of refraction of the human eye lens. Most glasses are between 1.4 and 1.5.

The speed of propogation of an electromagnetic signal in many transmission lines is about 2E8 m/s.

#### Wendy

Joined Mar 24, 2008
23,460
In designing antennas we are taught to use 90% of c. I'm not too clear on refractive index, but frequency does enter into this. Different colors of light move at different speeds in various mediums, which is how a prism works. It was also a problem in early fiber optic systems, which was solved by ultra pure LASERs.