How do you know that light is a form of energy?

Answer 1

If the light has sufficiently high frequency (short wavelength), then it can dislodge electrons from the surface of material upon which it shines, and cause an electric current in the material. (The effect doesn't depend on how bright the light is, only on its wavelength. Explaining this strange effect won Dr. Einstein his first Nobel Prize.)

Nowadays, materials and structures have been developed in which this effect is produced by light of relatively longer wavelength (lower energy). They're used in the photocells that may power your wristwatch and your calculator, and in large assemblies, deliver useful quantities of clean, renewable electric energy.