Sunlight

 

Sunlight, also known as solar radiation, refers to the incoming light to the Earth that originated from the Sun. This light represents a portion of the electromagnetic spectrum that includes infrared, visible light, and ultraviolet light. About half of the radiation is in the visible portion of the solar spectrum, with most of the rest in the near-infrared section with a comparatively small amount in the ultraviolet spectrum. Sunlight that is incident upon the Earth's surface has been filtered through the atmosphere, with some of the ultraviolet radiation being absorbed. The radiation that isn't absorbed by the atmosphere can produce a suntan or sunburn.[2]

In addition to providing light for Earth, sunlight also acts as a source of radiant heat, warming the Earth. The contribution of sunlight to the Earth is immense, it is a major component of the greenhouse effect, is one of Earth's major energy flows, as well as being a key factor in photosynthesis. The energy stored in fossil fuels also originates from the Sun, since fossil fuels originate from dead organisms such as algae or plankton.

Although a significant amount of solar energy reaches the Earth as sunlight, some of the incident light is reflected by the atmosphere or dispersed throughout space. Through calculations determining the solar energy to the Earth, it can be concluded that the average value for the amount of energy absorbed by the Earth is approximately 238Wm2

It is fun to note that the light from the Sun takes about 500 seconds (8 minutes and 20 seconds) to reach the Earth.[3] Light travels at the speed of light, which is very large (300 000 km/s), but not infinite.

What are the sources of heat other than the sun?

Examples of sources of heat energy are the  electrical appliances, burning wood, eating food and friction. Some forms of energy can be changed to produce heat energy.

The effects  of sunlight 

Sunlight helps boost a chemical in your brain called serotonin, and that can give you more energy and help keep you calm, positive, and focused. Doctors sometimes treat seasonal affective disorder (SAD) and other types of depression linked to low levels of serotonin with natural or artificial light. Sunlight is essential for human health and well-being.

The health benefits of sunlight include generating the production of vitamin D, supporting bone health, lowering blood pressure, preventing disease, and promoting good mental health.

For some people, reduced exposure to sunlight, which occurs in the winter in the U.S., can lead to the significant mental health challenge of SAD, a form of depression.

SAD is treatable with light therapy, medication, talk therapy, and even vitamin D.