Science Buddies: "Ask an Expert"

i want the name of a lens or a lens arrangement way so that parallel rays are always focused on the same point, no matter from which direction they come

Hi Sumit,

Just curious - is your project about determining the focal points of different lenses? If so, a great reference page can be found here: http://www.physicsclassroom.com/class/refrn/u14l5da.cfm.

yesiamsumit wrote:i want the name of a lens or a lens arrangement way so that parallel rays are always focused on the same point, no matter from which direction they come

Any convex lens will focus parallel rays to a point in its focal plane. But parallel rays coming from different directions will end up at different points. You can see this from the fact that rays hitting the center of the lens are not deflected; they proceed along a straight line to the point at which all the other, non-central, rays will be focussed. I am not an expert on exotic optical systems, so I cannot be sure that there is some, non-obvious way to get plane waves from all directions coherently summed at one spot. If you can tolerate getting the rays from different directions focussed close to each other rather than exactly coincident, then you could use a very short focus lens, such as used in a microscope, to make the physical size of the image of the input aperture very small. For example you might get rays from different directions focussed on one pixel of the CCD light detector of a digital camera. The required optical system would be like a microscope objective but used in reverse to make a big image (at "infinity") tiny rather than making a tiny object look huge.

If you can provide us with more details of your project, we may be able to provide more help.

Best wishes.