Question

What do you understand by the invisible spectrum ?

Answer 1

The portion of the spectrum either below the red (infrared) or above the violet (ultraviolet), which is invisible to the eye, the waves being too long or too short to affect the retina. The invisible spectrum includes rays less than 3900 Å in length (ultraviolet, roentgen or x, gamma, and cosmic rays) and those exceeding 7700 Å in length (infrared, high-frequency oscillations used in short- and long-wave diathermy, radio, hertzian, and very long waves). These range in length from 7700Å to 5,000,000 m.