00:01
Question number 74 is working backwards to figure out the focal length.
00:07
So we're going to be using this equation a lot, and i'm going to try not to write it over and over again.
00:13
But we'll see if i hold it my word.
00:16
So what we want to do is set di, the image distance, as sort of our starting point for the original image distance, and then some new di plus 10 centimeters for the new one that has been shifted.
00:37
And then we'll use this equation for both cases and then eliminate the focal length and solve for image distance.
00:47
And then we can start again at this point and solve for the focal length.
00:53
So basically we're going to be setting the two equations equal to each other and we'll get something like this.
01:03
1 over the o from 1, the first case, plus 1 over the i.
01:10
Like i said, this is going to be our starting point.
01:13
Air quotes, can't see that, but they're there.
01:17
And set that equal to 1 over the focal length, but since that doesn't change, it's immutable...