Photosynthesis: A Historical perspective
Photosynthesis
Photosynthesis is a series of chemical reactions that occur in plants and other autotrophs, which use energy from sunlight to convert carbon dioxide and water into carbohydrates. Plants need carbohydrates for energy ( respiration) and growth, and animals get carbohydrates by eating plants, so photosynthesis is essential to both the plant and animal kingdoms. Literally "synthesis" means building and "photo" means light, so plants in "photosynthesis" are building carbohydrate molecules using light energy.
History
One of the first scientific studies of plant growth was conducted by the Belgian scientist Johannes Baptista Van Helmont. He was born into a wealthy family in 1579. However in his twenties he gave his personal wealth to his sister, and although he was a physician he refused to accept money from his patients, believing it was wrong to profit from the sick ( He would have been thrown out of the American Medical Association for ideas like that!). Luckily he was not left penniless because within four years of this he marrried, and his wife owned several estates that provided them with a comfortable living.
He studied plant growth by weighing a small sapling, some soil and a pot, then he planted the tree in the pot filled with the soil. He watered the tree regularly, and after several years removed the tree, and reweighed the tree, pot and soil. The pot was the same weight as before, the soil had decreased in weight by a small amount ( due to minerals absorbed by the plant ) and the tree had increased in weight by a large amount. Van Helmont concluded that plants only need water to grow. We now know that plants also need carbon dioxide from the air, but in 1600 no-one had any idea of what air consisted of, let alone any way to measure the weight of gases. Van Helmont wrote several medical books, but in the last 20 years of his life he published very little, mainly due to the Spanish Inquisition condemning him for heresy ( he was under house arrest for some time, but was eventually acquitted two years before his death in 1644).
It took almost 150 years before the next big step was taken, by the English pastor and scientist Joseph Priestley. With a name like that, how could he avoid being associated with the church? Priestley was born in 1733, and had his interest in science, and particularly electricity, sparked by a meeting with Benjamin Franklin in 1766.
Priestley published "The History of Electricity" the next year ( it was a pretty short book ), and soon after he discovered the gas which was later identified as carbon dioxide ( remember, the one that plants need). He lived next to a brewery ( as you can tell from this cartoon which was published in 1790), 
which produced large amounts of CO2 gas. Priestley noticed that this gas extinguished the fire in burning wood, and he later found a way of producing CO2, and dissolving it in water, inventing the first carbonated drink.
He also discovered that candles put in a closed container soon went out, but if a living plant was put in the container, the candle could be relit. He was therefore the first person to show that plants produce oxygen gas ( another reason that animals depend on photoynsthesis for their survival). He was later awarded a medal for this discovery, and the citation stated that:
"We are assured that no vegetable grows in vain, but cleanses and purifies our atmosphere".
The next step in understanding photosynthesis was taken by the Dutchman Jan Ingenhousz, who was born in 1730. Like Van Helmont he was trained as a physician, and was one of the first to promote Edward Jenner's new smallpox vaccine, which he successfully used on the Austrian royal family. Later he worked mainly in London, and in 1779 published a book "Experiments upon Vegetables" in which he stated that light was necessary for photosynthesis ( he found that in the dark, plants "injure" the air by releasing carbon dioxide). Ingenhousz also found that only the green parts of the leaf were photosynthetic. Interestingly, in 1766 (the same year that Joseph Priestley met the electric personality of Benjamin Franklin) Ingenhousz invented equipment for producing static electricity. At that time electricity was apparently a hot topic.
Plants are green because they contain the chemical chlorophyll. Different colors of light are different wavelengths. Red light has a long wavelength ( roughly 700 nm ) compared to blue light ( 400 nm ). White light ( like sunlight ) is a mixture of all different wavelengths. The color of an object depends on the wavelength of light which reflects off the object: a red shirt reflects red light. Wavelengths that are not reflected by plants are absorbed, and used for photosynthesis. This diagram shows the wavelengths of light that are absorbed by chlorophyll :
This shows that chlorophyll absorbs red and blue light, and reflects green light. So plants use both red and blue light for photosynthesis, but do not use green light.
If you grow plants under artificial light, you can either use white light ( which contains both red and blue), or you could use red and blue lights. Plants that are lit with just green light will die, because they cannot use that wavelength for photosynthesis.