Sunlight

Sunlight is the electromagnetic radiation emitted by the Sun. This energy is often called solar radiation. It is a fundamental force that powers life on Earth. When we talk about sunlight, we usually mean the visible light our eyes see. However, it also includes invisible parts of the spectrum. These include infrared light, which we feel as warmth, and ultraviolet light, which can affect our bodies.

The journey of sunlight begins deep inside the Sun. A particle of light, called a photon, starts at the solar core. Because the core is so dense, the photon constantly hits charged particles. These collisions change the photon's direction many times. This process can take between 10,000 and 170,000 years just to reach the Sun's surface. Once it escapes the surface, the photon travels through space. It takes about 8.3 minutes for that light to reach Earth.

When sunlight hits Earth, it interacts with our atmosphere. The atmosphere scatters and filters the radiation. We call the resulting light daylight when the Sun is above the horizon. If clouds do not block the radiation, we experience it as sunshine. This is a mix of bright light and radiant heat. If clouds do block the light, or if it reflects off objects, the light becomes diffused. Scientists measure the rate of solar energy hitting a specific area as solar irradiance.

Sunlight is composed of different regions based on wavelength. Nearly 98.7% of the radiation striking our atmosphere falls between 200 nm and 4,000 nm. This range includes ultraviolet, visible, and infrared light. Ultraviolet radiation is divided into three types. Ultraviolet C (UVC) has the shortest wavelengths and is mostly absorbed by our atmosphere. Ultraviolet B (UVB) also causes significant absorption and can damage DNA or cause sunburn. Ultraviolet A (UVA) reaches the surface and can cause skin aging or cancer.

The visible and infrared ranges make up the rest of the energy. Visible light spans from 380 to 700 nm and is what we see. Infrared radiation spans from 700 nm to 1,000,000 nm. At the Earth's surface, the energy breakdown is quite specific. About 52 to 55 percent is infrared light. About 42 to 43 percent is visible light. Finally, about 3 to 5 percent is ultraviolet light.

Measuring sunlight requires precise tools and complex math. Researchers use instruments like pyranometers and pyrheliometers. They also use sunshine recorders to track light intensity. To calculate the light reaching the ground, scientists must consider the Earth's elliptic orbit. The Earth's distance from the Sun changes throughout the year. In January, the energy is about 3.3% higher than the average. In July, the energy is about 3.3% lower than the average.

Sunlight plays a vital role in biological and human systems. It is the primary driver of photosynthesis. This is how autotrophic organisms, like plants, convert light into chemical energy. This energy is used to create carbohydrates and fuel their activities.

Sunlight also provides natural lighting for buildings, a process called daylighting. Sunlight is very efficient for illumination. It has a luminous efficacy of about 93 lumens per watt. This is higher than most artificial lights, such as fluorescent or incandescent bulbs. Using sunlight helps light a room while heating it less than many lamps.