Predicting Refraction

Have you ever tried running in a pool? It's a lot harder than running outside on the grass, so in result, you move a lot slower. Waves do the same thing!

When waves pass from one medium into another, they change speed which causes them to "bend." This is refraction. You may think that you've never seen this before, but I'm sure you have! Take for example the pool skimmer. When you put it into the water, it appears to bend, when in reality it remains perfectly straight!

https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0HEs7a57C10BMad0bubH4tMNEDPnJt_dBhhOF0VvcEy4KnPzFg9q7m6OK62S4vZPu0e1075uZJD0Bg-0Ub85aGHZghdeKu8B32fd_zWHGFdDzEapUx_65hrEYOmvpqF2hyphenhyphenx15HfVYmALo/s1600/IMG_0090.JPG


Why is this, you ask? The air and the water are two separate mediums with two distinct refractive indices (a number that indicates how fast waves can travel through the medium). Depending on the index of refraction, waves will appear to bend either towards or away from the normal line when they enter a new medium. Pretty neat, don't you think?

If a wave travels from a fast medium (air) to a slow medium (water), it would bend toward the normal line, like the picture below. If a wave travels from a slow medium to a fast medium, it would bend away from the normal line.

Good question! Yes, you can find the angle of refraction by using a formula, thanks to Willebrord Snell - hey, don't laugh at his name! Stop that! He realized that the index of refraction of the first medium times the sine of the angle of incidence is equal to the index of refraction of the second medium times the sine of the angle of refraction.

http://www.math.ubc.ca/~cass/courses/m309-01a/chu/Fundamentals/snell01.gif

We can also find the speed of light will travel through a certain medium by using the formula n=c/v where n is the index of refraction, c is the speed of light, and v is the speed. Refraction is very interesting, and quite predictable, you're right!

There is a point at which no light will exit the medium. Here the refracted light is perpendicular to the normal line. This occurs at the critical angle. If the angle of incidence is bigger than the critical angle what do you think happens? Yes! That's correct! The light is reflected back into the medium. This is known as total internal reflection. It acts just like reflection, which we know all about! Therefore, the angle of incidence must be equal to the angle of reflection. However, in order for this to happen, the light must be traveling from a medium with a high refractive index to one with a lower refractive index. Confused yet? Look at this diagram. I think it will help!

http://micro.magnet.fsu.edu/primer/java/refraction/criticalangle/criticalanglejavafigure1.jpg

Can you calculate the critical angle? What do you think? We already said that refraction is quite predictable! So yes, you can determine the critical angle. All we need is Snell's Law.

You see waves being refracted all the time! Even in movies. For example, have you ever watched Brother Bear? It's good, isn't it? A bit sad though... anyway. You know the seen where Koda tries to teach Kenai to catch fish? Well, Kenai isn't very good at it. He's looking into the water at the fish and trying to reach right down to get them. He must not have realized that the light from the fish is refracted; therefore, the fish isn't exactly where it appears to be. Of course, this isn't only true for animated bears. It happens to people, too!

http://www.daviddarling.info/images/spear_fishing_refraction.jpg