How can you calculate the brightness of an image formed by a lens?

The brightness of an image formed by a lens is influenced by the intensity of the light source and the area over which that light is distributed. The intensity refers to the amount of light energy the source emits per unit time. When light passes through a lens, it is refracted and focused, but the overall brightness will ultimately depend on both how much light is entering the lens and how widely that light is spread out in the image.

In practical terms, if the source of light is more intense, the image will generally appear brighter. Similarly, if the area over which that light is spread is larger, the same amount of light would result in a dimmer image since the brightness is distributed over a wider area. Understanding this relationship helps in assessing how effective a lens system will be in producing bright, clear images.