Posted by Sarah on Wednesday, June 4, 2008 at 9:48pm.
I used, 1/f = 1/di +1/do
1/di = (1/20) - (-1/60)
= 15cm
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.
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