With people banning incandescent lightbulbs and the days finally approaching appropriate summer length, it seems as good a time as any to be thinking about light. On the human retina, there are two major kinds of photoreceptive cells: cones (which identify colour) and rods (which are only sensitive to the overall brightness of light). Because rods are the more sensitive of the two, people actually see in black and white, when it is properly dark.
Like photographic paper, rods are not sensitive to long wavelengths of visible light, over on the red side of the spectrum. This is because the shorter the wavelength of a particular photon, the more energetic it is. Further to that, only light of a sufficiently narrow wavelength can accomplish certain tasks. For instance, only light of a sufficiently narrow wavelength can excite metals so as to produce the photovoltaic effect used in solar cells. Brian Greene has a rather good explanation of this in his book The Elegant Universe: the first half of which is a highly comprehensible primer on twentieth century physics.
The upside of red light not being able to affect rods is that one can be immersed in red light without losing the ability to see in the dark subsequently. This is why submarines are sometimes illuminated with red light – allowing the crew to see more than would otherwise be possible in the event of a power failure – and one reason I am hoping my replacement headlamp will be especially useful. The last one vanished curiously before the Devon trip, earning me a very nasty knock on the head caused by a thick low beam outside. The lost headlamp served admirably during the 2003 New York City blackout, as well as in a great many places besides, The new one, which has a mode in which it produces only red light, will probably be useful during stays in future hostels. It may also provide some interesting lighting possibilities for future photographs; high contrast red and black compositions can be quite compelling.