Why Does Water Make a Perfect Mirror?

[sollinton]

First off, let me apologize if this is in the wrong section, but I was not entirely sure what branch the answer would fall under.

Okay, my question is this (and its probably pretty obvious): If water is clear, what makes it an almost perfect mirror when you look across a still lake?

My thoughts were perhaps the substances in the water affecting the change, or perhaps surface tension and light played a roll...but I'm really just stabbing in the dark here. Any help wpuld be greatly appreciated.

Thank you in advance.

 

[Mapes]

It's not only water. All transparent materials reflect a bit of light at the air-material interface. But more interestingly, as you've noticed, the amount of reflection increases as you change your view from straight-on to a glancing angle. This effect is quantified by the Fresnel Equations, a part of optics theory that deals with the propagation of light from one medium to another.

 

[sollinton]

Alright, now I'm not a pro with physics or anything (I'm only a senior in high school), but would I be right in generally comparing your answer with skipping a stone? As in, if I throw the stone at a more glancing angle, it will more readily "bounce" off the surface of the water than it would if I simply dropped it straight in (of course, I am comparing the stone to light)?

 

[atyy]

It is the smoothness of the surface that ensures the image isn't distorted when the light is reflected back to you.

However, the mirror-like properties depend on lighting conditions. A lake which is very transparent is less mirror-like, since you see less of your reflection, and more of what is under the lake.

It is the same with a window pane. It always reflects your image back to you. But in the day, there's a lot more light coming through from the other side, so you notice what's outside instead of your reflection. At night, the outside is dark, your reflection is more noticeable, and the window is more like a mirror.

 

[atyy]

Alright, now I'm not a pro with physics or anything (I'm only a senior in high school), but would I be right in generally comparing your answer with skipping a stone? As in, if I throw the stone at a more glancing angle, it will more readily "bounce" off the surface of the water than it would if I simply dropped it straight in (of course, I am comparing the stone to light)?

It has to do not just with the "stone" from outside skipping back to you. It also has to do with "stones" from under the water being able to get to your eyes.

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

From there you can follow the links to the Fresnel equations that Mapes mentions.

 

[Mapes]

Alright, now I'm not a pro with physics or anything (I'm only a senior in high school), but would I be right in generally comparing your answer with skipping a stone? As in, if I throw the stone at a more glancing angle, it will more readily "bounce" off the surface of the water than it would if I simply dropped it straight in (of course, I am comparing the stone to light)?

The analogy works, but it's superficial; that is, it doesn't give any deeper insight about the physics. The skipping rock is a dynamics problem that depends on the geometry of the rock. The behavior of light is caused by the necessary continuity of the electric and magnetic fields on both sides of the air-water interface, and the photon has no geometry.

 

[Andy Resnick]

Alright, now I'm not a pro with physics or anything (I'm only a senior in high school), but would I be right in generally comparing your answer with skipping a stone? As in, if I throw the stone at a more glancing angle, it will more readily "bounce" off the surface of the water than it would if I simply dropped it straight in (of course, I am comparing the stone to light)?

Absolutely not- those are totally different phenomena. Skipping stones is a hydrodynamics problem (and a complex one, at that). Reflection of light is an electrodynamics problem. They have nothing in common- and making analogies between the two is likely to lead to confusion rather than enlightenment.

 

[atyy]

The analogy works, but it's superficial; that is, it doesn't give any deeper insight about the physics.

They have nothing in common- and making analogies between the two is likely to lead to confusion rather than enlightenment.

Yeah, I tried playing with it and got it backwards. The skipping stone moves in air and is "totally" reflected at the boundary of the denser medium. The light moves in the water and is "totally" reflected at the boundary of the less dense medium... if that made no sense, it isn't supposed to.