# Determine when a light is going into the object [closed]

How can I determine when light is going into the object?

Actually, if there is, what is the simplest way to determine whether light is going into the object or not?

NOTE: I could not find any meaningful title. Sorry.

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## closed as too localized by Qmechanic♦, Manishearth♦, Emilio PisantyDec 10 '12 at 13:14

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@lubosMotl should I use i dot N as a guide ? –  gveaf Nov 4 '12 at 12:38
I can't understand quite correctly of what do you mean by "When light goes?" (it goes if it's switched on) You'd see the object illuminated when light propagates through it. That's a determining way. Do you require Ray tracing or something like that..? Please be a bit clear –  Waffle's Crazy Peanut Nov 4 '12 at 13:08
Are you asking how to tell which is the entry and which is the exit? If so, there is no way to tell because the Physics involved is time reverisble i.e. the behaviour is the same if the light travels from the exit to the entrance. –  John Rennie Nov 4 '12 at 13:22

Light partly penetrates transparent objects and is partly reflected. Depending on the indexes of refraction of the two media. In your case light enters from above ( first medium) it goes through the glass, (second medium) and is also partially reflected back, and goes out to the air ( third medium).

There exists a critical angle with respect to the normal when there is total internal reflection, i.e. light does not go through the glass (your third medium). It follows Snell's law, where n1 and n2 are the indexes of refraction:

$n_1 sin \theta_i = n_2sin\theta_t$

to find the critical angle we set $\theta_t$= 90 degrees.

$\theta_c = \theta_i = arcsin \frac{n_2}{n_1},$

Have a look at the wikipedia articles on refraction and total reflection.

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So ( i dotProduct N ) < 0 ray go into the glass, is it true ? –  gveaf Nov 4 '12 at 14:11
I cannot understand your symbols, but as long as the angle to the normal is not the critical angle part of the light beam will go through when crossing a boundary between two indexes of refraction. –  anna v Nov 4 '12 at 15:05