You have two lenses at your disposal one with a focal lenght F1=35.5 cm , the ot

Question

You have two lenses at your disposal one with a focal lenght F1=35.5 cm , the other with a focal lenght f2=-35.5 Question: If you want to produce an image of the bulb that is enlarged by a factor of 2.40, how far from the wall should the lens be placed? di= m You have two lenses at your disposal one with a focal lenght F1=35.5 cm , the other with a focal lenght f2=-35.5 Question: If you want to produce an image of the bulb that is enlarged by a factor of 2.40, how far from the wall should the lens be placed? di= m Question: If you want to produce an image of the bulb that is enlarged by a factor of 2.40, how far from the wall should the lens be placed? di= m

Explanation / Answer

I guess you want to see an enlarged real image on the wall. This means an inverted image thus negative magnification (-2.4). First, you have to use the converging (f = +35.5) lens. From the thin-lens formula (ref.), with R1=object distance and R2=image distance,
1/F = 1/R1+1/R2; M = -R2/R1 ==> R1 = -R2/M
Substituting, 1/F = 1/R2-1/(R2/M) = (1-M)/R2 ==> R2 = F*(1-M) = 1.207 m
If you really meant positive magnification, it's a virtual image and a wall seems irrelevant. In that case, using M=2.4 and the same equations, you will see the image at R2 = -0.497 m.

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