It is an age old question. Ok, its not as often asked as ‘why is the sky blue?’ but it is a question worthy of a decent answer.
Glass Prism Rainbow
by iWanders – on flickr
To find the answer we have to go back to school. Remember when, in science class, you were told light is made up of different colour light combined. If you have a prism to hand you can split white light in to red and yellow and pink and green. Orange and purple and blue. I can sing a rainbow… (sorry got side tracked there). The range of colours you can see (red through to blue) are referred to as the ‘visible spectrum’ or visible range of colours. There are other colours (such as infrared and ultraviolet) but i won’t cover these here.
Now we need to change subject to biology, don’t worry I will try and keep it simple. Many plants, trees and algae absorb the suns rays to produce their own food, it is a remarkable process and is called photosynthesis. The key ingredient for photosynthesis to work is a chemical called chlorophyll, a type of pigment. When a tree photosynthesises (the same applies to anything which photosynthesises) it absorbs carbon dioxide from the air which is combined with water in a chemical reaction which produces sugar for the tree. This chemical reaction requires energy to take place and it is chlorophyll that provides the energy needed for the reaction.
Now, back to the subject of light. When light hits an object some of the light is absorbed and some light is reflected. Some items absorb some parts of the visible spectrum better than others. In other words an item, such as my red candle, is able to absorb green and blue light but unable to absorb red light which is reflected. Because the red light is reflected it appears red in colour.Leaves are good at absorbing red and blue light and not very good at absorbing green light which is reflected, making the leaf appear green in colour. You now know why they appear green but this is only part the story.
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Many objects are the colour they are by coincidence or by man. My candle is red because it was made in a factory. Dyes and colouring agents were used to make it red but why are leaves green? We know why they appear green but we also know trees and plants need energy from light to photosynthesise. Surely it would make sense to absorb all the light, including green light) and reflect nothing (and be black in colour)?
We are starting to get a bit more technical here but please bear with me.
Remember we briefly mentioned how the light spectrum extends past the visible colours into infrared and ultraviolet? Well, chlorophyll works best with red and blue light in addition to infrared and ultraviolet light. Chlorophyll is useless at using the energy within green light which is why it is reflected. Chlorophyll is not alone within a leaf, it is joined by other pigments called carotenoids. There are over 600 know carotenoids, such as lutein (appears yellow in colour), and each carotenoid absorbs a different colour(s) of light. The presence of Lutein, and other carotenoids, is not obvious because there is so much chlorophyll but the carotenoids do change the colour of a leaf which is why we see leaves in different shades of green.
So, leaves are green because they are stuffed full of chlorophyll, a chemical which is great at absorbing red a blue light but useless at absorbing green light. Plants do contain other chemicals which do absorb green but as there is so much chlorophyll most of the green light is reflected.