Question

How to find focal length of plano convex lens with thickness?

Answer 1

Let us assume we are talking about a positive plano-convex lens, where the 1st surface is convex, and the 2nd surface is plano. Your suspicion is correct - Since R2=∞R2=∞ , the second surface does not contribute any power to the lens, and the efl is determined solely by the first surface. The focal length of lens depends on its construction, but where the image is located also depends on the construction of the lens. At this point, it is important to note which surfaces we are using for a reference point. Since the lens powers is all on the first surface, the thickness of the lens serves only to displace the image. As the lens thickness increases, the image will displace further from the first surface. A plane parallel plate will displace an image by an amount Δz=N−1NΔz=N−1Nwhere NN is the refractive index of the plate (assuming the plate is in air).