Question

plane faces of two identical Plano convex lens and is equal to 1.5 each having focal length 40 cm are placed against each other each other to form a usual convex lens then what is its focal length of lens so formed

Answer 1

The plane faces of two identical Plano convex lenses each having focal length of 40cm ,are placed against each other to form a usual convex lens.The distance from this lens at which an object must be placed to obtain a real inverted image with magnification_1 is 
1) 80cm 2)40cm 3) 20cm 4) 160cm

Answer 2

20  cm

Solution: The formula for focal length of two thin lenses combination is

1/F=1/f1+1/f2

1/F=1/40+1/40

    =2/40

     =1/20

Hence F=20 cm