Q: What determines the size of the bright spot when you focus sunlight with a lens?
Typically, when you try to focus sunlight with a lens you’ll get something that looks like a comet. In other words, the Sun, and everything else “at infinity”, will be in focus f away from the lens. Now on the face of it, it would seem as though there’s no way to figure out the size of the Sun’s image, h i , since . Solving for the image size, h i , and plugging in what we already know, and d i =f, we get:So, how big is the bright spot when you focus sunlight? A smaller one with legs f and h o and a larger one with legs d i and h o +h i .