Question

Two convex lenses of same focal length are fixed in a PVC pipe at a distance double to their focal length.what happens if a boy sees the moon with the arrangement

Answer 1

Here two convex lens of same focal length are fixed in a PVC pipe at a distance double to their focal length.Then following process will happen:
1.A parallel beam of light rays will converge on focal point of the lens after refraction.
2. So image of moon appear as a point size on focus.
3. This point size images formed at focus will act as object for second lens.
4. Then light rays through focal point will be parallel to principal axis after refraction.
5. so when a boys sees through other end of PVC pipe he gets enlarged image of moon as rays are parallel after reflection.
Hence boy can see enlarged image of moon through other end of PVC pipe.

Answer 2

combination of two lenses-
1/F=1/f1  +  1/f2   -   D/(f1*f2)
1/F=1/f  +  1/f   -  2f/(f^2)
1/F=2/f   -  2/f
1/F=0
F=infinite
combined focal length of two lenses = infinity
1/F=1/v  -  1/u
0=1/v  -  1/u
v=u
m=-v/u=I/O
m=-1
m is magnificaton