Physics 55.4 Optics- Multiple Lenses (3 of 12) The Double Lens: The Equivalent Focal Length

In this video I will derive the combine equations for the focal length of 2 convex lenses. In other words, the equivalent focal length of 2 lenses touching each other. Each lens may have focal lengths of their own.

Next video in this series can be seen at:

[grantkobe]

Dear Sir,
The first s2' formulate have miswrite, in the denominator S2 should be S1.
It got right back after third formulate.
And Thanks your good work . I learned a lot from it.