Understanding Collimation to Determine Optical Lens Focal Length

Hi, I am Monica, an Optical Engineer here at Edmund Optics. Today, I want to talk about the definition of collimated light and how you can use it to determine the focal length of a lens. Collimated light occurs when light rays are travelling parallel to each other, as opposed to converging to a focus or diverging away from the center. Essentially, you can consider collimated light to be focused at infinity. To collimate a diverging light source with a lens, you can place the lens a distance away from the source, equal to the focal length of the lens. Here, we have a diverging beam of light and a positive lens at a distance equal to the focal length away. As you can see, the light spot stays about the same size at any distance away from the lens. Alternatively, if collimated light enters a lens, it focuses at a distance equal to one focal length. So, if you have a lens with an unknown focal length, you can use collimated light to determine its focal length. We can assume that light is collimated or coming from infinity, if the light source is greater than a distance equal to 10x the focal length of the lens away. An easy way to determine the approximate focal length of a lens is to use the overhead lights in a room, which are a distance much greater than 10x the focal length of common lenses. The distance from the lens to the table when the light is in focus is approximately the focal length of the lens. Another way to measure this in a lab setup is to use collimated light source like this laser. The distance between the lens and the focus spot is equal to the focal length. I hope this answers your questions about collimation and the focal length of a single lens. For more technical information, please see our other videos about focal length, linked in the text below. You can browse more of our technical application notes and videos to learn more key concepts and find answers to common questions on our website.