Photosynthesis requires sunlight. But what is sunlight? To answer this question, we will answer three questions:
The answer to the first question is that sunlight comes from thermonuclear reactions in the sun, which release energy in the form of electromagnetic radiation .
The answer to the second question is that sunlight (and other forms of light) has both wave and particle characteristics and it travels from place to place as both a wave and as discrete particles of energy.
First, let's look at the wave properties. Light is similar to x-rays, microwaves, and radio waves because they all have a periodic fluctuating character (a wave) that travels through space. They differ because of their wavelengths (the peak-to-peak distance between oscillations). Importantly, electromagnetic waves can transfer energy to objects in their paths. Visible light is the portion of the electromagnetic spectrum that has wavelengths detectable by the human eye. These are wavelengths of about 400-700nm.
If light acts as a wave that travels through space, how can it also act as a particle? Light travels as "packets," or quanta of energy known as photons. Each photon has a specific amount of energy. Photons can "bump" into things in their path. These disturbances cause some of the effects of light in the world around us.
This brings us to the third question posed about light. What happens when light reaches another object? We have all seen light reflect off of a mirror, which allows us to see ourselves. We have also avoided wearing a black shirt (instead of a white shirt) in the summer because of the heat it absorbs. These two properties of reflection and absorption can help us to understand how plants use light.
Because light consists of photons of energy, reflection can be thought of as photons bouncing off of a leaf. However sometimes the photons of light are absorbed by the leaf to do work. Pigments are compounds that absorb light. Different pigments absorb different wavelengths of light. If a structure (e.g., a leaf) lacks pigments that absorb light in a given wavelength range, then that wavelength of light will be reflected. In other words, the color perceived is that which is not absorbed.
As shown in the figure above, many colors are absorbed by chlorophyll and carotenoid pigments. Importantly, note that green light is absorbed poorly, which is why chlorophyll-containing leaves appear green to our eyes; green light is not absorbed by green plants, it is reflected.