G GmbH H G A convex (focusing) lens of focal length f is mounted on an optical b

Question

G GmbH H G
A convex (focusing) lens of focal length f is mounted on an optical bench. On one side of the lens, a light bulb is glowing. On the other side, its image is in perfect focus on a screen which has been placed a certain distance away from the lens. The physical height of the image is exactly TWICE the height of the actual light bulb. How far away is the actual bulb (object) from the center of the lens, as a multiple of the focal length f? A DC light bulb uses 18 Watts of power when it is subjected to an EMF of 3.0 Volts. What is the bulb's resistance?

Explanation / Answer

10) Real images created by convex lens are always inverted.

hi / ho = -di / do = -2

=> di = 2do

Thin lens formula:

1/do + 1/di = 1/f

=> 1/do + 1/2do = 1/f

=> do = 1.5f

11) P = V2/R

=> R = V2/P = 32/18 = 0.5

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