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September 21, 2014

September 21, 2014

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

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 4:15pmWhat does "near a corner" mean? Does that mean the ray is internally reflected once?

- Physics -
**Katie**, Sunday, May 31, 2009 at 4:24pmi 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 4:40pmThen 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.

- Physics -
**bobpursley**, Sunday, May 31, 2009 at 4:41pmoops, typo it is 44 deg, not 54 deg. The sum has to be 90 between 46 and 44.

- Physics -

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