What must be the angle of incidence for total internal reflection to occur? Let medium 1 be the medium with the higher index of refraction. As θ1 increases, θ2 also increases, albeit at a faster rate. When θ2 reaches 90 degrees, there is total internal reflection, and there is no transmission of light. The corresponding angle of incidence, θ1, is the critical angle where total internal reflection occurs. Let the critical angle be θc. Substituting into Snell’s law:

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If astronomical works be searched through and through, there will not be found a single instance of a bold, unhesitating, or manly ,statement respecting a proof of the Earth's " rotundity." Proctor speaks of "proofs which serve to show ... that the Earth is not flat," and says that man "finds reason to think that the Earth is not flat," and speaks of certain matters being "explained by supposing" that the Earth is a, globe; and says that people have "assured themselves that it is a globe;" but he says, also, that there is a " most complete proof that the Earth is a globe:" just as though anything in the world could possibly be wanted but a proof - a proof that proves and settles the whole question. This, however, all the money in the United States Treasury would not buy; and, unless the astronomers are all so rich that they don't want the cash, it is a sterling proof that the Earth is not a globe.
It is in evidence that, if a projectile be fired from a rapidly moving body in an opposite direction to that in which the body is going, it will fall short of the distance at which it would reach the ground if fired in the direction of motion. Now, since the Earth is said to move at the rate of nineteen miles in, a second of time, "from west to east," it would make all the difference imaginable if the gun were fired in an opposite direction. But, as, in practice, there is not the slightest difference, whichever way the thing may be done, we have a forcible overthrow of all fancies relative to the motion of the Earth, and a striking proof that the Earth is not a globe.
Astronomers tell us that, in consequence of the Earth's "rotundity," the perpendicular walls of buildings are, nowhere, parallel, and that even the walls of houses on opposite sides of a street are not! But, since all observation fails to find any evidence of this want of parallelism which theory demands, the idea must be renounced as being absurd and in opposition to all well-known facts. This is a proof that the Earth is not a globe.