Refraction angle physics help

1. May 14, 2006

mmg0789

For each of the following cases, will the light rays be bent toward or away from the normal?

a. ni > nr, where the angle of ni = 20
b. ni < nr, where the angle of ni = 20
c. from air to glass with an angle of incidence of 30
d. from glass to air with an angle of incidence of 30

for a, i put toward. b i think is away (but i'm not sure why it would be) and i am not sure about c and d

2. May 14, 2006

George Jones

Staff Emeritus
Do you know an equation that would help to answer these questions.

Regards,
George

3. May 14, 2006

Hootenanny

Staff Emeritus
You are correct for a and b. Perhaps if you drew a diagram, it would make the answer more apparent. Where are the angles measured from?

As for c and d. Consider snell's law, what happens when a light ray travels through the interface from a lower refractive index (air) to a higher (glass) and visa versa.

~H

4. May 14, 2006

mmg0789

well i think this is a conceptual problem, so no equation is needed. but the equation i have to work with is (ni)(sinXi)=(nr)(sinXr)

--index of refraction of first medium x sine of the angle of incidence = index of refraction of second medium x sine sine of the angle of refraction

5. May 14, 2006

mmg0789

o ok thanks

6. May 14, 2006

George Jones

Staff Emeritus
I find that that looking at a physics problem from different perspectives is often useful. As Hootenanny has indicated, diagrams here are very useful, but I like also to verfiy things with mathematics.

Note that the graph of sinx is increasing for x between 0 and 90, i.e., if sinx2 > sinx1, then x2 is greater than x1.

Now consider a) ni > nr. In order for ni sinXi = nr sinXr to hold, this means that sinXi < Sin Xr, which in turn, from what I wrote above, means that Xi < Xr. Now, a diagram shows what happens with respect to bending towards/ away from the normal.

Regards,
George

7. May 14, 2006

mmg0789

o thank you both