The Sun is the source of nearly all energy that reaches Earth, delivered through electromagnetic (EM) radiation that travels in waves. The EM spectrum is the entire range of this radiation, classified by wavelength or frequency, extending from gamma rays to radio waves. The Sun emits energy across the full extent of this enormous range. While the human eye can only perceive a tiny slice of this output—visible light—the Sun continuously bombards Earth with energy in every other part of the spectrum as well.
The Physics of Solar Emission
The Sun’s emission profile is modeled by blackbody radiation, which describes the continuous spectrum of light emitted by an object based on its temperature. Nuclear fusion reactions in the Sun’s core power this emission, but the distribution of wavelengths is determined by the temperature of its visible surface, the photosphere. The photosphere’s average temperature of approximately 5,778 Kelvin dictates the shape of the emitted radiation curve.
This temperature causes the Sun’s radiation intensity to peak at a specific wavelength, defined by Wien’s displacement law. For the Sun’s temperature, the peak emission wavelength is around 500 nanometers (nm), falling in the greenish-yellow region of the visible light spectrum. Although the Sun approximates a perfect blackbody, its actual spectrum contains dark absorption lines caused by elements in its outer layers absorbing energy. This temperature determines that the majority of the Sun’s energy output is concentrated in a narrow band including the visible, ultraviolet, and infrared regions.
The Dominant Energy Components
The majority of the Sun’s total energy output is concentrated in three adjacent parts of the electromagnetic spectrum: infrared, visible light, and ultraviolet (UV) radiation. Collectively, these three bands account for roughly 98% of the total solar irradiance reaching Earth’s atmosphere. The energy distribution is not equal, with the longest wavelengths carrying slightly more power overall.
Infrared (IR) radiation, with wavelengths longer than visible light, comprises the largest portion of the total energy, contributing about 49% to 55% of the Sun’s output. This radiation is primarily responsible for the sensation of heat we feel from sunlight. Although IR photons carry less individual energy than visible or UV light, their abundance makes them the largest energy component.
Visible light, the narrow band our eyes are sensitive to, represents a significant portion of the output, ranging between 42% and 47% of the total energy. This portion of the spectrum drives photosynthesis in plants and allows us to see, corresponding almost perfectly with the Sun’s peak blackbody emission. The peak intensity near 500 nm provides the necessary energy for biological processes to thrive on Earth’s surface.
Ultraviolet (UV) radiation has shorter wavelengths and higher energy than visible light, making up the smallest but most energetic fraction of this dominant trio (3% to 8% of the total solar energy). This band is categorized into UV-A, UV-B, and UV-C. UV-A has the longest wavelength and is least energetic, while UV-C has the shortest wavelength and is most energetic. The high energy of UV photons causes chemical changes, such as sunburn and the fading of materials, demonstrating their biological significance.
The Extreme Ends of the Spectrum
Beyond the dominant components, the Sun also emits energy at the extreme ends of the spectrum, including the highest-energy X-rays and Gamma rays, and the lowest-energy Radio waves and Microwaves. These regions account for less than 1% of the Sun’s total energy output but provide unique insights into solar activity.
High-energy X-rays and Gamma rays are not generated by the steady blackbody emission of the photosphere. Instead, they originate from the Sun’s superheated outer atmosphere, the corona, and from transient, explosive events. Solar flares, which are sudden releases of magnetic energy, accelerate particles to produce intense bursts of hard X-rays and Gamma rays. Soft X-ray emission comes from the extremely hot plasma of the corona, which can reach millions of degrees Kelvin.
On the opposite end, the Sun emits Radio waves and Microwaves, which are the longest wavelengths and carry the lowest energy per photon. These emissions are often associated with solar flares and other disturbances in the Sun’s atmosphere, though a faint, continuous radio emission comes from the quiet Sun. While these waves contribute negligible amounts to the total solar energy, they are important for studying solar plasma dynamics and can interfere with Earth-based communication systems.
How Earth’s Atmosphere Filters the Light
The electromagnetic spectrum measured at the top of Earth’s atmosphere is significantly different from the spectrum that reaches the ground. The atmosphere acts as a selective filter, attenuating, scattering, and absorbing different wavelengths to create a protective shield for life. This filtering process is highly dependent on the wavelength of the incoming radiation.
The highest-energy radiation, including all X-rays and Gamma rays, is almost completely absorbed high in the atmosphere by molecules of nitrogen and oxygen. This prevents the most damaging radiation from reaching biological organisms. Similarly, nearly all UV-C radiation and most UV-B radiation is absorbed by stratospheric ozone and molecular oxygen.
Visible light is the most efficiently transmitted part of the spectrum, passing through the atmosphere with minimal absorption. Some visible light is scattered by air molecules, which makes the sky appear blue, but the majority reaches the surface. Infrared radiation is selectively absorbed by atmospheric gases like water vapor and carbon dioxide, creating distinct absorption bands that block certain IR wavelengths. Radio waves, like visible light, mostly pass through the atmosphere, creating an “atmospheric window” used in communication and astronomy.