The sun is probably the one thing we see most often throughout our lives. Whenever we raise our sight to the sky during the day, we can see its dazzling light. If someone were to come up and ask "What good is the sun? We would probably reply without even a thought that the sun gives us light and heat. That answer, although a bit superficial, would be correct.
But does the sun just "happen" to radiate light and heat for us? Is it accidental and unplanned? Or is the sun specially designed for us? Could this great ball of fire in the sky be a gigantic "lamp" that was created so as to meet our exact needs?
Recent research indicates that the answer to the last two questions is "yes". "Yes" because in sunlight there is a design that inspires amazement
The Right Wavelength
Both light and heat are different manifestations of electromagnetic radiation. In all its manifestations, electromagnetic radiation moves through space in waves similar to those created when a stone is thrown into a lake. And just as the ripples created by the stone may have different heights and the distances between them may vary, electromagnetic radiation also has different wavelengths.
The analogy shouldn't be taken too far however because there are huge differences in the wavelengths of electromagnetic radiation. Some are several kilometers long while others are shorter than a billionth of a centimeter and the other wavelengths are to be found in a smooth, unbroken spectrum everywhere in between. To make things easier, scientists divide this spectrum up according to wavelength and they assign different names to different parts of it. The radiation with the shortest wavelength (one-trillionth of a centimeter) for example is called "gamma rays": these rays pack tremendous amounts of energy. The longest wavelengths are called "radio waves": they can be several kilometers long but carry very little energy. (One result of this is that radio waves are quite harmless to us while exposure to gamma rays can be fatal.) Light is a form of electromagnetic radiation that lies between these two extremes.
The first thing to be noticed about the electromagnetic spectrum is how broad it is: the longest wavelength is 10 25 times the size of the shortest one. Written out in full, 10 25 looks like this:
A number that big is pretty meaningless by itself. Let's make a few comparisons.
For example, in 4 billion years (the estimated age of the earth) there are about 10 17 seconds. If you wanted to count from 1 to 10 25 and did so at the rate of one number a second nonstop, day and night, it would take you 100 million times longer than the age of the earth! If we were to build a pile of 10 25 playing cards, we would end up with a stack stretching halfway across the observable universe.
This is the vast spectrum over which the different wavelengths of the universe's electromagnetic energy extend. Now the curious thing about this is that the electromagnetic energy radiated by our sun is restricted to a very, very narrow section of this spectrum. 70% of the sun's radiation has wavelengths between 0.3 and 1.50 microns and within that narrow band there are three types of light: visible light, near-infrared light, and ultraviolet light.
Three kinds of light might seem quite enough but all three combined make up an almost insignificant section of the total spectrum. Remember our 10 25 playing cards extending halfway across the universe? Compared with the total, the width of the band of light radiated by the sun corresponds to just one of those cards!
Why should sunlight be limited to such a narrow range?
The answer to that question is crucial because the only radiation that is capable of supporting life on earth is the kind that has wavelengths falling within this narrow range.
In Energy and the Atmosphere, the British physicist Ian Campbell addresses this question and says "That the radiation from the sun (and from many sequence stars) should be concentrated into a minuscule band of the electromagnetic spectrum which provides precisely the radiation required to maintain life on earth is very remarkable." According to Campbell , this situation is "staggering".(1)
Let us now examine this "staggering design of light" more closely.
From Ultraviolet to Infrared
We said that there was a range of 1:10 25 in the sizes of the longest and shortest electromagnetic wavelengths. We also said that the amount of energy that was carried depended upon the wavelength: shorter wavelengths pack more energy than longer ones. Another difference has to do with how radiation at different wavelengths interacts with matter.
The shortest forms of radiation are called (in increasing order of wavelength) "gamma rays", "X-rays", and "ultraviolet light". They have the ability to split atoms because they are so highly energized. All three can cause molecules-especially organic molecules-to break up. In effect, they tear matter apart at the atomic or molecular level.
Radiation with wavelengths longer than visible light begins at infrared and extends up to radio waves. Its impact upon matter is less serious because the energy it conveys is not as great.
The "impact upon matter" that we spoke of has to do with chemical reactions. A significant number of chemical reactions can take place only if energy is added to the reaction. The energy required to start a chemical reaction is called its "energy threshold". If the energy is less than this threshold, the reaction will never start and if it is more, it is of no good: in either case, the energy will have been wasted.
In the whole electromagnetic spectrum, there is just one little band that has the energy to cross this threshold exactly. Its wavelengths range between 0.70 microns and 0.40 microns and if you'd like to see it, you can: just raise your head and look around-it's called "visible light". This radiation causes chemical reactions to take place in your eyes and that is why you are able to see.
The radiation known as "visible light" makes up 41% of sunlight even though it occupies less than 1/10 25 of the whole electromagnetic spectrum. In his famous article "Life and Light", which appeared in Scientific American, the renowned physicist George Wald considered this matter and wrote "the radiation that is useful in prompting orderly chemical reactions comprises the great bulk of that of our sun."(2) That the sun should radiate light so exactly right for life is indeed an extraordinary example of design.
Is the rest of the light the sun radiates good for anything?
When we look at this part of the light we see that a large part of solar radiation falling outside the range of visible light is in the section of the spectrum called "near infrared". This begins where visible light ends and again occupies a very small part of the total spectrum-less than 1/10 25 . (3)
Is infrared light good for anything? Yes, but this time it's no use to look around because you can't see it with the naked eye. However you can easily feel it: the warmth you feel on your face when you look up on a bright sunny summer or spring day is caused by infrared radiation coming from the sun.
The sun's infrared radiation is what carries the thermal energy that keeps Earth warm. It too is as essential for life as visible light is. And the fascinating thing is that our sun was apparently created just to serve for these two purposes, because these two kinds of light make up the greatest part of sunlight.
And the third part of sunlight? Is that of any benefit?
You can bet on it. This is "near ultraviolet light" and it makes up the smallest fraction of sunlight. Like all ultraviolet light, it is highly energized and it can cause damage to living cells. The sun's ultraviolet light however is the "least harmful" kind since it is closest to visible light. Although overexposure to solar ultraviolet light has been shown to cause cancer and cellular mutations, it has one vital benefit: the ultraviolet light concentrated in such a miniscule band (4) is needed for the synthesis of vitamin D in humans and other vertebrates. (Vitamin D is necessary for the formation and nourishment of bone: without it, bones become soft or malformed, a disease called rickets that occurs in people deprived of sunlight for great lengths of time.)
In other words, all the radiation emitted by the sun is essential to life: none of it is wasted. The amazing thing is that all this radiation is limited to a 1/10 25 interval of the whole electromagnetic spectrum yet it is sufficient to keep us warm, see, and allow all the chemical reactions necessary for life to take place.
Even if all the other conditions necessary for life and mentioned elsewhere in this book existed, if the light radiated by the sun fell into any other part of the electromagnetic spectrum, there could be no life on Earth. It is certainly impossible to explain the fulfillment of this condition having a probability of 1 in 10 25 with a logic of coincidence.
And if all this were not enough, light does something else: it keeps us fed, too!