Dear Sir:

I am an undergraduate student of pharmacy. I perform gel electrophoresis for DNA. To stain gel bands, I use ethidium bromide and then visualize them using UV light torch. This has switches providing 254 nm and 365 nm. I read that human eye is not sensitive to uv light. If so, then how my friends and I can see the light emitting from the uv lamp of 254nm as well as 365nm and also the light reflecting (or fluorescing?) from the gel bands?

Thanks a lot,

Shirish Sharangpani
Department of Pharmacy
Mahakal Institute of Technology
Ujjain
India

Dear Shirish,
I am a scientist who has done what your doing many times.
First don't look at the light. It is very bad for your eyes! Use safety glasses which are opaque to UV light. Also the ethidium is considered to be a mutagen (wear gloves). Check Wikipedia for more info.
Now to your question, there is a range of wavelengths that are emitted (the technical term is “spectral purity”) by the UV light. So what you “see” is the tiny amount of visible light (So think how bright the light is @ 254 nm! (safety glasses!)). The ethidium absorbs the UV light (there is an absorbance max is near 254 nm) and fluoresces. When ethidium binds (intercalates) into the DNA the fluorescent wavelength shifts to around 500 (emission max) and is much stronger. This results in a bright orange band in the gel where the DNA is.