A welder wears special glasses to …

Welders wear special goggles with glass windows to protect the eyes from Ultra Violet rays also known as UV rays. Goggles provide them a degree of eye protection from heat, optical radiation and sparks produced by the welding. Welding produces UV and IR radiations which cannot be seen by naked eye and can produce eye injury If special goggles are not wear. UV blocking protective welding goggles are considered primary protection, with this eyes are still protected even when the face shield or helmet is lifted up. Ultraviolet rays UV rays have frequency range 400nm to 0.6nm. Where nm denotes Nanometers

A welder wears special glasses to protect his eyes mostly from the harmful effects of the ultraviolet rays produced during welding.

Welding is the process of joining two substances (usually metals) by heating them enough to melt them and then allowing them to solidify.
Ultraviolet rays are electromagnetic waves that have wavelength less than 400nm ( that is less than the wavelength of violet light).
The special glasses of welders absorbs these ultraviolet rays and protect the eyes of welders from the harmful effects of ultraviolet rays.