Why Does Total Internal Refraction Occur at a Specific Angle?

[JimmyRay]

Ok so now our class is onto snell's law...

We did a few experiments where light refracts when it is passing into a new medium...

In an experiment today we were trying to find the total internal refraction, and my friend shined the light at an angle and we observed a spectrum... ? but it was only there at one certain angle, why did this occur?

 

[ceptimus]

Different frequencies of light are refracted at slightly different angles by the glass (or water or whatever). That is why a prism works for creating a spectrum. Normally when you are calculating the angles of incidence and so on, you ignore this, and assume that all the different frequencies of light are refracted the same amount.

The different refraction indicies at different wavelengths is a problem when making lenses for telescopes, cameras and so on. If you think about it, a triangular prism is much the same shape as the edge of a thick lens. The effect causes a smearing of colours known as chromatic aberration in these optical instruments. Luckily this can be mostly cured by making compound lenses out of two or more different kinds of glass.

 

[JimmyRay]

So you're saying that different frequencies of light were refracting to cause a spectrum? But we used one light source, the lights were off and just one ray box shot a light ray at the glass...one source, one frequency?

 

[ceptimus]

If you used a laser, or a really good filter so that only a single colour of light was in the beam, then you wouldn't get a spectrum.

White light, which I guess is what you used, is made of all the different colours mixed together. The refractive medium spreads these out into a spectrum, as different colours are refracted by different amounts.

 

[hypermorphism]

What color was it ?
White light contains all frequencies of light, while other colors will contain other frequencies, unless you have a pure frequency, ie. a laser.

 

[JimmyRay]

Ohhhhhhhhhh I see... we used a orange light, lol you know those..orange ones.... it wasnt white... it wasnt a laser for sure...

Hmm so I am still not sure HOW it causes a spectrum but let me try to understand with what you guys have said...

Ok so since it's not a pure light source and it is composed of different frequencies of light, when it's shot at the glass it refracts but not as a whole? all of those colours the light source were made of break up? and they arnt concentrated anymore? this causes the spectrum?

 

[hypermorphism]

Ok so since it's not a pure light source and it is composed of different frequencies of light, when it's shot at the glass it refracts but not as a whole? all of those colours the light source were made of break up? and they arnt concentrated anymore? this causes the spectrum?

Yep. The orange light beam is actually made up of many different frequencies of light, each of which has a slightly different index of refraction in the medium, and thus they refract at slightly different angles when crossing interfaces, making them distinct.
A more detailed explanation can be found here

 

[JimmyRay]

oooooooooo thanks