posted by Sarah on .
A lamp 10 cm high is placed 60cm in front of a diverging lens of focal length 20 cm. By means of a scale ray digram, locate the image and determine the height.
I'm not too sure how i would find the distance of the image, as it is a diverging lens
I used, 1/f = 1/di +1/do
1/di = (1/20) - (-1/60)
but shouldn't the answer be -15 and not 15?
The image would be virtual with a diverging lens. Use -20 cm for the focal length
-1/20 = 1/di + 1/do = 1/di + 1/60
1/di = -0.0500 - 0.0167 = -0.0667
di = -15 cm
The image height is 15/60 * 10 = 2.5 cm
We can't draw the ray diagram for you. it isn't hard to do. Parallel rays from the top and bottom of the lamp, after intersection, will intersect at the focal point in front of the lens.