00:01
The given question have a concave mirror, a concave mirror, which and its focal length is given as 20 centimeters.
00:12
Since it is a concave mirror, we have to take the focal length as minus 20 according to the sign convention.
00:19
Height of the object ho is given as 4 centimeters and the image distance is given as 30 centimeters and because it is, it will be a real image, so we have to consider it as negative according to sign convention.
00:34
Now using the mirror formula, we have 1 over f is equals to 1 by v plus 1 by u.
00:41
So let us substitute the values.
00:43
So f is 20 minus 20, so minus 1 by 20 is equals to minus 1 by 30 plus 1 by u.
00:52
So we will have 1 by u is equals to 1 by 30 minus 1 by 20.
00:59
When we solve this further, we get the value of u as minus 60 centimeters...