Mirrored Optics Phenomenon Explained While Swimming

[annatar]

I observed this 'phenomenon' while I was swimming...
Here is the situation:

When I look at my watch (which is submerged), its glass surface acts like a plane mirror at certain angles (e.g. when I rotate my wrist/arm), causing the contents behind the glass become invisible.

Somehow all light coming (it seems) from that angle(s) are all those that are reflected off the surface!
How can this be explained?

 

[mgb_phys]

http://en.wikipedia.org/wiki/Total_internal_reflection

 

[cepheid]

http://en.wikipedia.org/wiki/Total_internal_reflection

Hey, I'm confused. Doesn't TIR only work when going from a material with a higher refractive index to one with a lower refractive index?

In this case, aren't we talking about light going from water into glass?

Don't most glasses have higher refractive indices than water?

http://en.wikipedia.org/wiki/List_of_refractive_indices

 

[mgb_phys]

The TIR probably occurs on the glass-air interface on the back of the watch glass.
Glass is around 1.5-1.8 and water is 1.33 so you wouldn't get very much of an effect anyway.

ps. It was a bit lazy just to post the wiki link, but the OP can come back with more questions if they didn't understand.

 

[cepheid]

oh, so you're saying the light emanating from the watch dial is totally internally reflected *within the watch* and never makes it out. Result...can't see the watch dial?

 

[mgb_phys]

If there is TIR (would depend on the angle) it will be between the back of the glass and the air inside the watch face.
There will be a tiny bit of reflection glass-water on the way back out but small enough you wouldn't notice - water is close to being an ideal AR coating for glass.