# Optics in water

1. Oct 25, 2008

### shirel

Hey

I've wanted to know why does the image of an object, which was put in water, seems to be larger than the real object, for a person, who sees it from outside the water.

Thanks

2. Oct 25, 2008

### Danger

Welcome to PF, Shirel.
Optics is an extremely complicated field, but the basic answer deals with the difference of the refractive properties of air vs water. The interface where the two meet acts as a lens.
That's all that I've got, but someone more involved with the subject will be along shortly.

3. Oct 25, 2008

### shirel

Thank you very much :)
I'll be glad if someone could give me more specific answer. I know that the refractive index of water is great than the refractive index of air, but I still didn't understand why does it occurs.

4. Oct 25, 2008

### phyzmatix

The index of refraction for a medium is given by

$$n_{medium}=\frac{c}{v_{medium}}$$

So the whole situation is related to the fact that the speed of the light waves changes as the waves move between media with different indices of refraction (and the subsequent effects this has on the light waves).

In short: As the light reflected from the object moves from a medium with one index of refraction to that of another, it "bends" at the interface between the two media, either creating the appearance of "shrinking" the image or "blowing it up" depending on the situation.

As to the "why" this happens, you might want to have a look at Snell's law and Hugyens' Principle.

Does that help?

5. Oct 25, 2008

### shirel

hey thank you too, actually I already know Snell's law. I only learned geometrical optics, and therefore I'm not familiar with Huygens–Fresnel principle. However, now I understand it, thank you very much!

6. Oct 25, 2008

### phyzmatix

Ha! My good deed for the day!

7. Oct 27, 2008

### Andy Resnick

That is only true sometimes- when there is a curved interface between the water and air (i.e. the sides of a glass). When this happens, the interface acts as a lens with optical power, thus there is magnification.