Van Helmont, chloroplasts and the light stage

White light is a mixture of the seven colours of the spectrum: red, orange, yellow, green, blue, indigo and violet. When light strikes an object, some colours are absorbed and others are reflected. An object looks the colour it reflects. Chlorophyll absorbs mainly red light and blue light, and reflects green, so leaves look green. It does not absorb green. This is why plants use red and blue light best. In the light-dependent stage, chlorophyll in the grana absorbs light energy, and part of this energy is used to split water into oxygen and hydrogen. The oxygen is released into the air. A tiny packet of energy is called a quantum. In a simple picture, an electron that gains a quantum of energy jumps up to a higher level and then falls back. We do not need the later steps here. The seven colours are a convention: the rainbow changes smoothly from one colour to the next.