00:01
So we're going to have a concave spherical mirror with an electric light bulb, which i'll just kind of draw like that.
00:11
And we're given a few pieces of information.
00:14
We are given that it is placed 15 centimeters from the mirror, and it forms an image that is 8 .5 centimeters away.
00:26
So from this, if we use the mirror equation, we can actually determine what the focal length of this mirror.
00:35
Is, right? so we're going to plug in 1 over 15 plus 1 over 8 .5 is 1 over f, right? and so you can plug that in to a calculator again.
00:47
Don't forget to invert at the end.
00:50
And if you do that, you find that the focal length of this mirror is 5 .43 centimeters.
00:57
And so now we're asked to move the light bulb.
01:01
So we're going to move it back where it's 25 centimeters away, right? and and we know the focal length is the same because it's the same mirror.
01:14
And we want to find the image, right? so we're going to use the same mirror equation.
01:26
And so we're going to have 1 over 25 plus 1 over q is 1 over 5 .43.
01:36
Right? and so 1 over q is 1 over the focal length minus...