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Let us look at a few truly amazing "coincidences" which have come to light in our times.

For a little background. The light we see consists of electromagnetic waves which are synchronized oscillations of electric and magnetic fields that propagate at the speed of light.

These waves are similar to sound waves which we hear. Sound waves are generated by vibrating material which creates pressure waves in the air and are detected by your ear drums. A piano for instance has different length strings which produce different frequency sound waves. Higher frequency sound waves have a higher pitch. Likewise, your vocal chords vibrate producing sound waves of different pitch.

Similarly, electromagnetic waves are produced by accelerations of charged particles such as electrons. Analogous to sound, the EM waves can have a wide range of frequencies. For example, radio waves used to broadcast music have wavelengths of several kilometers. Visible light waves have a much smaller wavelength on the order of 1 micron (1 millionth of a meter). X-rays used by doctors have smaller still, and the smallest wavelengths in the cosmos, Gamma Rays, can have wavelengths as small as 10^-20 meters such as those emitted by binary stars.

Thus in the cosmos, we have an enormous range of frequencies in the spectrum of electromagnetic waves spanning a factor of about 10²³, a huge number indeed.

It turns out, all matter emits radiation in proportion to its temperature. The movement of atoms due to heat temperature produces accelerations of particles which emit electromagnetic radiation. The higher the temperature, the higher frequency radiation is emitted.

Thus armies use special infrared cameras to see people at night. For at the temperature of body heat, infrared radiation is emitted which is detected by the cameras. If you heat something hot enough, it will eventually emit radiation in the visible range and appears glowing to the human eye. At roughly 600 degrees celsius, it will glow deep red like a cigarette. At 1,100 degrees, it will appear orange-yellow like a candle flame.

Now, the surface temperature of the sun is about 5800 degrees celsius while its core is around 27 million degrees. In its core, the sun converts about 5 million tons of matter into energy every second through thermonuclear fusion. This is equivalent to about 100 billion one megaton nuclear bombs each and every second. It emits electromagnetic radiation across most of the electromagnetic spectrum.

Although the Sun produces lethal Gamma rays as a result of the nuclear-fusion process, internal absorption and thermalization convert these super-high-energy photons to lower-energy photons before they reach the Sun's surface and are emitted out into space.

Thus it turns out that about 70 percent of all of the sun's radiant energy output is concentrated in a slice of the spectrum from the near infrared (1.5 microns), through visible light to the near ultra-violet (0.3 micron).

This is a mere tiny slice of the electromagnetic spectrum in the cosmos. Something like picking a 1 out of 10²², or about a single drop in the ocean of the spectral range.

This tiny slice is the only useful one for life on our planet as most other electromagnetic radiation is inherently either lethal to life or damaging. Radiation in the gamma ray to x-ray all the way down to ultraviolet is all harmful to life.

This is because the protein molecules essential to life are intricately folded and hinge on the edge of chaos. The internal machinery of the cell needs to be stable enough to function, yet pliable enough to change readily such as in unraveling DNA for reading and copying.

Thus the cell machinery is very delicate. Any break in their bonds can unravel the whole structure rendering it useless. This is why higher energy radiation damages is harmful to life. The photon energy acts like a machine gun on the cell's delicate machinery.

Lower energy EM Radiation such as in the radio region just passes through matter with almost no effect due to its low energy. This is why you can get radio reception inside the walls of your house, while visible light cannot pass through walls. So too higher up, until one gets to infrared frequency.

Only when the frequency reaches the threshold of infrared and light region does it interact gently enough with matter to be useful to life. Due to quantum mechanical rules, a threshold energy must be reached before the radiation has enough energy to interact with matter such as accelerating and raising an electron to a higher orbital causing interesting things to happen.

Likewise, the near infrared radiation has just the right threshold energy to interact with atoms causing them to vibrate, thus imparting energy in the form of heat. Without heat from the sun, we would not be happy campers.

It is thus highly significant that the sun's radiant energy is compacted mostly in the tiny band of the infrared-visible spectrum useful to life and almost none in those regions harmful to life.

Now, all this radiation from the sun would be useless if the atmosphere reflected it out to outer space or just absorbed it before it could reach the surface. Fortunately, the combination of gases in our atmosphere transmits 80% of the sun's radiation in the visible and near infrared. If air were more opaque to light like clouds, it would obviously impair our ability to see things.

Furthermore, the earth's atmosphere also conveniently filters out virtually all of the electromagnetic radiation outside the band useful to life. Thus, almost no radiation in the gamma, x-ray, ultraviolet, far-infrared and microwave radiation reaches the surface of the earth.

Next, another filter in the form of water comes into play. Water is opaque to nearly all electromagnetic wavelengths except light in the visible spectrum. i.e. water is transparent only to visible light. All the rest except for radio waves gets blocked out by water. Even far ultraviolet and infrared radiation only penetrate a fraction of a millimeter below the surface of water. The only region of the spectrum which penetrates water is the small band of light useful for photosynthesis and visible light.

This is obviously significant to life on Earth. Not only for ocean life, but also to land plants, as the sunlight inevitably needs to penetrate a thin layer of water before it can reach the machinery of the cell for photosynthesis.

Likewise, the low penetration of infrared radiation in water causes the upper layer of the oceans to absorb the sun's heat. This aids to transfer the sun's heat to the air and winds and stabilizes the temperature of the atmosphere. It also prevents freezing of the upper layer of water which is significant as ice floats.

To summarize:
* The sun's radiant output is almost entirely in the spectrum useful for life (the near infrared and visible light region) and almost none in the spectrum harmful to life.

* The infrared region is just the right frequency to transmit heat through electromagnetic radiation.

* The visible light region is just the right frequency for photosynthesis and is weak enough not to be harmful to molecular machinery of the cell.

* The atmosphere passes through only this same narrow range of spectrum.

* Water is transparent to this same narrow range and blocks the spectrum outside this range.

There is much more to talk about here. For example, the spectrum of visible light is also perfectly fit for the high resolution camera type eye of the design and dimension of creatures of our size. We could go on and on in this fascinating topic.

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