00:01
Okay, so we're doing chapter 34, problem 75.
00:04
So this problem says, what should the index of refraction be of a transparent sphere in order for parallaxical rays from an infinitely distant object shown here, hitting the sphere, be brought to a focus at the vertex of the opposite surface.
00:24
So all these parallel incoming rays get focused on the opposite surface of the sphere.
00:31
Radius r.
00:33
What should this index of refraction be for this to happen? we're going to assume the outside is air, so this is just n equals 1 out here.
00:41
So to set this up, we need to use our equation for the refraction at a spherical surface.
00:47
And so this is given as the end of the first medium over the object distance, plus end of the second medium over the image distance equals the difference in indexes of of refraction over the radius of curvature.
01:08
So in this case, it says the object infinitely far away, so s is approaching infinity.
01:14
So we can neglect this first term.
01:17
What we can also notice is that if this is where the lens starts, s prime here equals the diameter or 2r.
01:26
So s prime equaling 2r as well...