Draw a picture that shows the sun’s rays interfacing with the surface of the lens and bending inward to focus on the leaf, using the following information: The sun is so far away that it, itself, is not part of the picture; instead we just draw the electromagnetic rays coming from it. While the rays are technically diverging out from the sun, by the time they get anywhere near earth and our glass, they can be approximated as parallel. The light rays bend a little inward at the first air-glass interface, and then bend inward a little bit more at the second air-glass interface. The lens actually allows for two bending opportunities in this case where there is air on both sides of it.

Label your diagram with the object-lens distance (s), the image-lens distance (s’), and the focal length of the lens (f). You can leave them in symbol notation because we don’t know any exact distances right now. What is s in this special case where we are imaging the sun onto a leaf? In this special case where the sun is so far away that the light rays are coming in parallel, how is f related to s’ ? (hint: the definition of the focal length is the distance needed to focus incoming parallel rays).

Based on your prior understanding of energy conservation and transfer, explain why you can only burn the leaf when you focus the sun’s rays through a lens that converges the rays. Why can you not burn the leaf with a lens that diverges the rays or when there is no lens at all?