The simple answer is, UV400 protection is a type of lens coating that helps block out harmful ultraviolet rays from the sun. These rays can cause damage to the eyes over time, so it’s important to have a pair of sunglasses that can protect against them.

UV light is a type of electromagnetic radiation with very short wavelengths. UV light is found in the electromagnetic spectrum in between X-rays and visible light. The electromagnetic spectrum is a range of wavelengths of electromagnetic radiation.

It is divided into different categories based on the wavelength.
The categories include:
Gamma rays. X-rays. UV rays, Visible light, Infrared rays, Microwaves Radio waves

UV light is often divided into three categories: UVC UVB UVA
UVC is the shortest wavelength, the most harmful, and completely filtered by the atmosphere.
UVB causes sunburn and is partially filtered by the atmosphere.
UVA is the longest wavelength and penetrates the atmosphere, but is not as harmful as UVB and UVC.
UV rays have shorter wavelengths than visible light.

UV rays are divided into three categories:
UVC (100-280 nm)
UVB (280-315 nm) 
UVA (315-400 nm)

All of Tomahawks Sunglasses are 99-100 percent UV400 protected. UV protection is build into the lens and will last the lifetime of the glasses.

UV damage can cause a number of problems for your eyes, including: -Cataracts -Macular degeneration -Pterygium -Photokeratitis

Photokeratitis is an inflammation of the cornea that is caused by exposure to ultraviolet (UV) rays. The condition is also known as "sunburn of the eye." Symptoms of photokeratitis include pain, redness, and tearing. The eyes may also be sensitive to light. Photokeratitis usually heals within a few days.