00:03
All right, your homework questions are all about con, it looks like concave mirror and how to use the equations associated with concave mirrors.
00:15
And what do these negative and positive values that we might get mean? so i put up the three equations that we need to solve for the numbers that we need for this question.
00:28
The first one is the primary equation for finding where is an image going to form when we know where the object is and what the focal point of the mirror is.
00:40
So 1 over f equals 1 over d .i plus 1 over d .o.
00:45
So the f stands for focal point.
00:48
D .i is for image distance from the mirror.
00:51
And do is objects distance from the mirror.
00:55
To find the focal point, we use that second.
00:58
Equation, the curvature of the mirror divided by two tells us its focal point.
01:04
And then magnification, capital m, is equal to the negative d .i or image distance divided by do, the object's distance.
01:14
Now tell us how much magnification we have.
01:18
Now, if a magnification is more than one, then it means that the image is larger than the object.
01:24
That would be an example from like a microscope.
01:29
But if the magnification is less than one, then the object is smaller than the original object.
01:36
The image is smaller than the original object.
01:41
A couple of things about what does the negative or positive values mean.
01:45
So for a d .i or image distance, if we get a negative number for our answer when we plug in into this equation, that means that we have a virtual image, forming behind the mirror.
01:59
If it's a positive number for d .i, then we have a real image, and that's forming in front of the mirror.
02:08
With magnification, if we get a negative number for magnification, it means that it's upside down or inverted.
02:15
And if it's positive magnification, then it's upright.
02:20
So let's do the first one as an example, letter a.
02:32
So if i put in that we have a curvature of 30.
02:36
So the focal point is positive 15, since it's a concave mirror, it has a converging rays of light.
02:46
They're all being focused to a point.
02:48
Then we can say that's a converging scenario.
02:51
So that's going to be a positive value for f.
02:54
If this was a convex mirror, then we would use negative 15 for our f value.
03:01
But it's positive here because it's a concave mirror.
03:06
1 over d .i.
03:08
That's what we don't know.
03:09
That's what we're solving for, plus 1 over 60, which was the object's distance from the mirror.
03:20
Well, when we rearrange everything algebraically, we get this.
03:26
We get 115th minus 160th...