Figure 5.1 shows the atmospheric opacity for various wavelengths of electromagnetic wave with wavelength ranging from 100 nm to 1 mm. There are two windows where the atmosphere is transparent to light—one is at the visible light region and the other is centered around \(10\,\mu \) m. The sun’s surface temperature is about \(5,800\) °K. Black body at \(5,800\) °K has the strongest radiation in the visible light region. Solar radiation is significantly weaker at \(10\,\mu \) m wavelength. Hence, solar radiation is strongest on the Earth’s surface at the visible light region. There is no surprise that creatures on Earth develop eyesight to see visible light. The visible light that we refer to are the wavelength visible to human. Birds are able to see part of the ultraviolet (UV) spectrum and are tetrachromats; they see four colors—UV, blue, green, and red. The birds’ world is more colorful than human’s! Mosquitoes, bullfrog, and some snake species are able to see infrared. Goldfish’s and zebrafish’s vision extends from the infrared spectrum all the way into the ultraviolet spectrum; they are tetrachromats.