# New focal point of converging light through a concave lens

Suppose I pass rays of already converging light through a concave lens. The rays are equally centered on the lens such that the original focal point is through the lens and centered on the lens axis.

My understanding is that the lens will bend the incoming rays, such that they all focus on a new focal point closer to the lens. Is there a formula I can use to determine the location of the new focal point, given the focal point of the lens and the original focal point?

## 1 Answer

When you're calculating the effect of several lenses combined the trick is to start with the first lens and calculate the image position, then use that image as the object for the second lens and so on. In this case your setup looks like: You start with just the convex lens so $u$ is the distance of the object and $v$ the distance to the image. Calculate $v$ using the lens equation:

$$\frac{1}{u} + \frac{1}{v} = \frac{1}{f}$$

Now consider the second, concave, lens. Take the image from the first lens as the object, so the distance to the object is $u'$. Note that $u'$ will be a negative number because the convention is that to the left of the lens is positive and to the right of the lens is negative. Us the lens equation again (rememer $f$ is negative for concave lenses) with $u'$ as the object and you can calculate the position of the final image $v'$.

• Ahhh, fooey: give the OP a set of Jones matrices :-) – Carl Witthoft May 15 '14 at 15:52
• Jones matrices or ray transfer matrices? In either case I think it might be a matrix too far :-) – John Rennie May 15 '14 at 15:56
• Thank you for the response. However, I don't know the origin of these rays, all I know is that they are centered on the lens and are focused at a particular location. – user74223 May 16 '14 at 19:07