Question

Why do ultraviolet bulbs have an outer envelope made of quartz instead of glass?

Answer 1

Ultraviolet bulbs have an outer envelope made of quartz instead of ordinary glass because quartz is completely transparent to ultraviolet radiation, whereas ordinary glass completely absorbs it.

Quartz allows the ultraviolet rays to pass through freely into the surroundings without any obstruction. On the other hand, if ordinary glass is used, its structural impurities cause it to absorb the UV rays completely, which would prevent the bulb from emitting any useful ultraviolet radiation.