A 4.6 cm long insect is perpendicular to the optical axis of a thin lens; the bu

Question

A 4.6 cm long insect is perpendicular to the optical axis of a thin lens; the bug is 6.7 cm away from the lens. The lens forms an inverted image of the insect which is 1.1 cm long on a screen behind the lens. (Use three significant figures.)

a. How far behind the lens is the screen? cm

b. What is the lens' focal length (pay attention to sign)? cm

c. If the insect were instead only 0.22 cm from the lens, what would the image distance be (pay attention to sign)? cm

d. Could the new image be projected on a screen?

I understand the problem and have the diagram all drawn out but I'm not sure about how to find the answers.

Explanation / Answer

hi/ho = 1.1/ 4.6 = 0.239 = s'/s

a) s' = 0.239*s = 1.601 cm

b) 1/s + 1/s' = 1/f

1/6.7 + 1/1.601 = 1/f

f = 1.29 cm

c) 1/s + 1/s' = 1/f

1/0.22 + 1/s' = 1/1.29

s' = -0.265 cm

d) s' = negative it is a virtual image

so image cannot be projected

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