A small part of the electromagnetic spectrum 380 ~ 750 nm includes its visible components.

Red light has the lowest frequencies and longest wavelengths, while violet has the highest frequencies and shortest wavelengths. Colors that can be produced by visible light of a narrow band of wavelengths are called pure spectral colors. They can be delineated roughly in wavelength as violet (380-450 nm), blue (450-495 nm), green (495-570 nm), yellow (570-590 nm), orange (590-620 nm), and red (620 to 750 nm).

The divisions between infrared, visible, and ultraviolet are not perfectly distinct, nor are those between the seven rainbow colors. Each color can come in many shades since the spectrum is continuous. The human eye is insensitive to electromagnetic radiation outside this range. Blackbody radiation from the Sun peaks in the visible part of the spectrum but is more intense in the red than in the violet, making the Sun yellowish in appearance.

Visible wavelengths pass through the optical window. The Earth’s atmosphere allows this region of the electromagnetic spectrum to pass through mostly unattenuated, while most UV wavelengths are absorbed by oxygen and ozone in Earth’s atmosphere. The optical window is also called the visible window because it overlaps the human visible response spectrum.

The portion of the EM spectrum used for photosynthesis by organisms is called the photosynthetically active region (PAR). It corresponds to solar radiation between 400 and 700 nm, substantially overlapping with the range of human vision.

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Key Points

• Visible light is the range of EM Wave which wavelength varies between 380 ~ 750 nm.

• The Earth’s atmosphere only allows a region of the electromagnetic spectrum to pass. Visible light coincides with this optical window.

• The portion of the EM spectrum used for photosynthesis also overlaps with the range of human vision.

Key Terms

visible light: the part of the electromagnetic spectrum, between infrared and ultraviolet that is visible to the human eye.

spectral color: a color that is evoked by a single wavelength of light in the visible spectrum, or by a relatively narrow band of wavelengths.

optical window: the optical portion of the electromagnetic spectrum that passes through the atmosphere all the way to the ground.  300 1100 nm, parts of Ultraviolet to Infrared.

photosynthetically active region (PAR): 400 and 700 nm

blackbody radiation: the spectrum of light emitted by any heated object