Two convex lenses placed in contact form  the image of a distant object at P. If the lens B is moved to the right, the image will

Initially, without gap :  1Fi=1f1+1f2  Finally, with gap : 1Ff=1f1+1f2−df1f2  Comparing,  Ff > Fi Power of the system decreases due to separation between the lenses. So the focal length increases.