Q:

In an experiement with lenses we were asked to put an object infront of a convex lens and then to place a concave lens on the the other side of the convex lens. A real larger image is formed on a screen some distance away. I can’t figure out how this is possible. How does it work? Thanks!

- David

Mesa, Az, USA

- David

Mesa, Az, USA

A:

When you put lenses together, their power actually adds. The power is
the inverse of the focal length. We'll need both of those facts later
on.

Let's start with the convex lens. Convex lenses produce real images. The image is larger than the object as long as the object is between 1 and 2 focal lengths away from the lens.

Now, you have the concave lens next to the convex lens. Since you said a real image is formed, that means the absolute value of the power of the concave lens in smaller than the power of the convex lens. You know that because concave lenses don't make real images. That gives the overall effect of a convex lens with a longer focal length. The longer focal length will allow you to get a larger magnification of the image.

Adam

Let's start with the convex lens. Convex lenses produce real images. The image is larger than the object as long as the object is between 1 and 2 focal lengths away from the lens.

Now, you have the concave lens next to the convex lens. Since you said a real image is formed, that means the absolute value of the power of the concave lens in smaller than the power of the convex lens. You know that because concave lenses don't make real images. That gives the overall effect of a convex lens with a longer focal length. The longer focal length will allow you to get a larger magnification of the image.

Adam

*(published on 10/22/2007)*