Solar radiation is a universal term for the electromagnetic radiation emitted by the sun. By definition, the total frequency spectrum of electromagnetic radiation liberated by the Sun is known as Sunlight. Sunlight is filtered through the Earth's atmosphere, and solar radiation is clear as daylight when the Sun is on top of the horizon, in Earth. If the direct solar radiation is not obstructed by clouds, it is felt as sunshine which is a mixture of both glowing light and radiant heat. Solar radiation is very much valued, due to its effect on living beings and the viability of its application for many practical purposes. It is an everlasting source of natural energy that, in conjunction with other forms of renewable energy, has a great prospective for a broad range of applications since it is plentiful and easy to get. Solar radiation is quickly attracting people as an increment to the nonrenewable sources of energy, which only have restricted resources.
The electromagnetic radiations produced by the sun exhibits a broad range of wavelengths which can be separated into two chief areas:
Luckily, the extremely harmful ionizing radiation does not break through the earth's atmosphere. Solar radiation is generally classified into different regions or bands on the wavelength basis. They are: Ultraviolet: 0.20 - 0.39 µm (microns), Visible: 0.39 - 0.78 µm, Near-Infrared: 0.78 - 4.00 µm, Infrared: 4.00 - 100.00 µm.
The sun hits the surface at various angles ranging from 0º (just above the horizon) to 90º (directly overhead), as the Earth is round in shape. The Earth's surface obtains all the potential energy, when the sun's rays are perpendicular. When the sun’s rays are further slanted, they pass through the atmosphere to a longer distance, becoming more scattered and dispersed. As the Earth is round, the cold Polar Regions do not get high sun, and due to the slanted axis of rotation, these regions receive no sun all throughout the year. The Earth rotates around the sun in an elliptical orbit and is closer to the sun during half of the year. The Earth's surface gets more solar energy when the sun is closer the Earth. The Earth is closer to the sun when it's summer in the southern hemisphere and winter in the northern hemisphere. The 23.5 degree tilt in the Earth's axis of rotation is a very important factor in finding out the amount of sunlight striking the Earth at a specific place.
The factors that affect the amount of solar radiation are:
Solar energy is created at the core of the sun when hydrogen atoms are merged into helium by nuclear fusion. The core takes up an area from the sun’s center to about a quarter of the star’s radius. At the core, gravity pulls all of the mass of the sun interior and produces strong pressure. This pressure is much more adequate to force the fusion of atomic masses. For each second of the solar nuclear fusion process, 700 million tons of hydrogen is converted into the heavier atom helium. Since its formation 4.5 billion years ago, the sun has used up about half of the hydrogen found in its core. The solar nuclear process also produces enormous heat that makes the atoms to release photons. Temperatures at the core are about 15 million degrees Kelvin (15 million degrees C or 27 million degrees F). Each photon that is created travels about one micrometer before being absorbed by an adjacent gas molecule. The journey from the sun’s surface to the Earth takes about 8 minutes. The radiative surface of the sun, or photosphere, has an average temperature of about 5,800 Kelvin. A good number of the electromagnetic radiation released from the sun's surface sits in the visible band positioned at 500 nm (1 nm = 10-9 meters), though the sun also emits considerable energy in the ultraviolet and infrared bands, and small amounts of energy in the radio, microwave, X-ray and gamma ray bands. The total quantity of energy emitted from the sun's surface is just about 63,000,000 Watts per square meter.
The solar constant is the amount of energy received at the top of the Earth's atmosphere on a surface oriented perpendicular to the Sun’s rays (at the mean distance of the Earth from the Sun). The generally accepted solar constant of 1368 W/m2 is a satellite measured yearly average.