When Form 8655 Dependents 2024

Ultraviolet (UV) is electromagnetic radiation with a wavelength from 10 nanometers to 400 nanometers, shorter than that of visible light but longer than x-rays. UV radiation is present in sunlight, constituting about 10% of the total light output of the Sun. It is also produced by electric arcs and specialized lights such as mercury vapor lamps, tanning lamps, and black lights. Although long wavelength ultraviolet is not considered an ionizing radiation because its photons lack the energy to ionize atoms, it can cause chemical reactions and causes many substances to glow or fluoresce. Consequently, the chemical and biological effects of UV are greater than simple heating effects, and many practical applications of UV radiation are derived from its interactions with organic molecules. Suntan and sunburn are familiar effects of overexposure of the skin to UV, along with a higher risk of skin cancer. Living things on dry land would be severely damaged by ultraviolet radiation from the Sun if most of it were not filtered out by the Earth's atmosphere. More energetic, shorter wavelength extreme UV below 121 nanometers ionizes air so strongly that it is absorbed before it reaches the ground. Ultraviolet is also responsible for the formation of bone-strengthening vitamin D in most land vertebrates, including humans. Specifically, UVB in the UV spectrum has both beneficial and harmful effects on human health. Ultraviolet rays are invisible to all humans, although insects, birds, and some mammals can see near UV. The lens of the human eye blocks most radiation in the wavelength range of 300 to 400 nanometers. Shorter wavelengths are blocked by the cornea. Humans lack color receptor adaptations for ultraviolet rays. Nevertheless, the photoreceptors of the retina are sensitive to near UV, and people lacking a lens, a condition known as aphasia, perceive near UV as whitish blue or...