On a cloudless day, the sun emits 663 billion trillion joules (1.7 quadrillion megawatts) of visible light from its radiating surface.
Of that amount, only 0.8% (21 billion trillion joules) becomes visible energy that reaches Earth’s surface and is absorbed by vegetation or our bodies to maintain life.
The other 99.2% (661 billion trillion joules) never makes it to Earth’s surface except as scattered sunlight in space or infrared radiation in the atmosphere, which is too concentrated for human usable light.
This is because almost all of the sun’s visible light travels in straight lines in what is known as a “ray path” (beam) toward Earth. The ray path of the light is only blocked by particles large enough to scatter it.
These particles are too few and far between in Earth’s atmosphere to block the sun’s invisible but concentrated infrared radiation, but they intercept about half of the visible light rays on their way toward earth.
Here are some points discussed about what happens to most of the visible light headed toward earth-
1. Scattered Sunlight.
As mentioned above, the sun’s visible light has no trouble reaching the Earth because it travels in a straight line. The only objects that can scatter it are huge particles, and they are too few and far between to block infrared radiation.
Besides, the concentration of infrared radiation is too high for sunlight to be scattered in any significant amount toward Earth by those few large particles.
2. Colored Sky or Blue Sky.
On a clear day, the sky appears blue because blue light scatters more easily than other colors of visible light red, orange, yellow or green. Blue is scattered more than seven times more than red. And red is scattered nearly four times more than green.
When sunlight approaches the Earth, it becomes refracted (bent) by the atmosphere, keeping some of the sun’s light from being absorbed by the Earth.
Most of this light will be scattered back toward space in a cone-shaped beam, because particles in the atmosphere are much smaller and more plentiful than those big enough to block infrared energy. They could scatter any color of visible light except blue, which is only about 4% of the sun’s total energy.
3. Blue Sky or White Sky?
The sky on a clear day is white, not blue. The color of the light that shines on us is not directly related to the color of the sky. The sun’s light is made up of a mixture of all colors, and it can be white or any other color when it is reflected by an object.
The only way to see what color the light is before it strikes an object would be to use a prism. Reflected sunlight appears colored because of the particles in Earth’s atmosphere. They bend or scatter some colors more than others, allowing only certain colors to reach Earth’s surface, depending on their size and quantity.
4. Sun Rays and Temperature.
The most obvious difference between sunlight (white light) and the sun’s infrared radiation (prominences, or blue light) is that when sunlight reaches Earth, it warms things. But the sun’s rays are not all the same.
Some of them are warmer than others, with shorter wavelengths in the middle of the visible spectrum.
Movies show how sunlight coming from the bottom of a swimming pool will warm things that are still cool to the touch. It also gets hotter at greater distances from the pool. The temperature increase depends on how much solar radiation strikes objects near and farther from where you stand.
The same is true of the Earth. Sunlight gives off energy, which warms objects on the Earth’s surface.
That’s why a polyethylene terephthalate (PET) plastic bottle feels warm on a spring day, when sunlight that has been absorbed by the bottle gives off more heat than it absorbs in winter.
5. Sun Rays and Atmospheric Gases.
Scientists say visible light is all around us, but only a small portion of it gets to Earth’s surface because most of it doesn’t penetrate our atmosphere very well. Some of it does reach our surface and is absorbed by the atmosphere.
Inside the atmosphere, however, invisible light from the sun’s infrared range is scattered by air molecules. So those portions of the spectrum can’t make it back to Earth’s surface, but they are still reflected by the lower atmosphere and can reach space.