
#1
Jun906, 08:57 AM

P: 424

# A convex lens of focal length 10 cm forms a real image of an object placed at a distance of 20 cm from it. Midway between the convex lens and the position of the image a thin concave lens is introduced. The image formed now is at a distance of 5 cm away from the earlier position. What is the focal length of the concave lens?
I solved it in the following way: Case1 Let u, v and f be the object distance, image distance and the focal length of the convex lens. Here u = 20 cm, f = + 10 cm 1/v = 1/f – 1/u 1/v= 1/10 – 1/20 v = 20 cm Case 2 Let u, v and f be the object distance, image distance and the focal length of the concave lens. Here u =  10(virtual object), v = + 15 cm 1/f = 1/10 + 1/15 f =  30 cm But the answer given in my book is – 5 cm. 



#2
Jun1106, 02:37 PM

Emeritus
Sci Advisor
PF Gold
P: 11,154

I don't get 5 cm either.
But I think you made a mistake with setting v=15cm in the second part. Would you expect the image for a concave lens to be 5cm closer to the lens or 5cm farther from it (compared to the object)? 



#3
Jun1206, 01:06 PM

P: 424

I assumed it to be away because i felt that concave lens is diverging in nature and so the image would be produced 5 cm away from the lens. What do you say?




#4
Jun1306, 12:13 AM

Emeritus
Sci Advisor
PF Gold
P: 11,154

Focal length of concave lens using convex lens
That sounds right (that the image would be farther away). There must be a mistake with the way I was doing it. I'm too sleepy to think straight now. I'll look at this again when I find some free time. Meanwhile someone else might have something useful to say.



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