Posted by **Katie** on Sunday, May 31, 2009 at 4:01pm.

I don't understand snell's law ( nsin(0)=nsin(0) ) and how to use it with refraction.

also how do i apply that to this problem:

'a light ray enters a square piece of plastci (n=1.39) near a corner. as it encounters the adjacent side it gets refracted along the surface (it hits at the critical angle). what was the original angle with which the ray hit the plastic from air?'

i know the answer (75 degrees) but i don't know how to get there.

please help!

- Physics -
**bobpursley**, Sunday, May 31, 2009 at 4:15pm
What does "near a corner" mean? Does that mean the ray is internally reflected once?

- Physics -
**Katie**, Sunday, May 31, 2009 at 4:24pm
i have no idea, i just wrote what the question said. but yes thats what i interpreted it to mean

- Physics -
**bobpursley**, Sunday, May 31, 2009 at 4:40pm
Then start at the end of the ray.

1.39*sinThetainside=1*sin90

theta inside =46 deg.

Now take that angle back to where it was reflected, so it incident angle at the reflection was 46, then trace back to the corner side, so it leaves the surface refracted at 54 deg, no use Snell's law to find the ray angle outside ..

1.39sin54=1*sinTheta

and I think you will get your 75deg. Draw the figure to make certain you understand it.

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