An ordinary photographic plate or film is abnormally sensitive to the light rays at the violet end of the spectrum and is strongly affected by the ul… - Arthur Hammond

" "

An ordinary photographic plate or film is abnormally sensitive to the light rays at the violet end of the spectrum and is strongly affected by the ultra-violet rays, which are invisible though they are present in sunlight, but it is practically insensitive to red and to the colors at the red end of the spectrum. Therefore, an ordinary plate sees red as black and is affected only very little by orange and yellow, so that those colors appear very dark while, on the other hand, being so sensitive to blue and violet, these colors are made to appear too light. That is why we can use a red light in the darkroom, as the plate is affected, practically, not at all by red light.