# Race between light rays

1. Mar 17, 2014

### quawa99

Imagine two light rays parallel to each other , one of them is traveling in vaccum and the other one in water. We place a detector at the same distance from the source and fire the two rays inside their respective media.who will win the race? Or is it possible to have a tie in the competition?

2. Mar 17, 2014

### phinds

Can you Google "speed of light in vacuum" and "speed of light in water" ???

3. Mar 17, 2014

### quawa99

Oh sorry I am a really dumb kid who can make a new thread but can't Google something.
I did do it but wasn't satisfied by the results, some places they said that the light ray travels with the absolute velocity but with a phase shift and so I wanted to know if that could result in actual delay in traversing a geometric path.

4. Mar 17, 2014

### dauto

Light speed in water is about 75% of vacuum light speed.

5. Mar 17, 2014

### Staff: Mentor

Since we get a lot of that here, it is helpful to provide that information upfront. We can't provide the appropriate help if we don't know where you are.

6. Mar 18, 2014

### sophiecentaur

Phase lag, at a given frequency, and time delay are effectively the same thing. In the case of light frequencies, there could be a pretty massive phase shift over a long path. It is quite possible to detect two 'racing' pulses when they arrive at different times, even over a few metres of 'race track. 3 X10^8 m/s is a very 'finite' quantity.

Lenses all work on the principle that the phase / time shift through the thick parts of the glass is greater than the air+glass path through the thin parts.

I could suggest that you try to hone your searching skills. What you read may not immediately strike you as useful but is worth while making the effort to make sense of it - or use another link. It is all too easy to ask questions and all too easy for them to be mis-interpreted. PF is much more useful for sorting out individual points than as an education system. Books are incredibly good as a source of total learning.