I'm looking for an explanation on why darker light filters let through less light than lighter filters (secondary/high school level). Although this seems to the layman like the obvious outcome, our science teacher was unable to explain why a darker blue and darker green filter let through less light than a yellow or turquoise filter.


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    $\begingroup$ It's not quite clear to me what you're asking. Are you asking why the Blue filter was darker than the yellow filter? Or you asking what makes a darker filter different than a lighter one? $\endgroup$ – Colin K Oct 24 '12 at 13:21
  • $\begingroup$ Sorry - I mean why does a darker coloured filter (e.g. dark blue) let through less overall light than a light filter (e.g. yellow). If it's relevant, to measure the amount of light we used a solar cell connected to a voltage meter. $\endgroup$ – pighead10 Oct 24 '12 at 13:24

Daylight doesn't contain equal amounts of all colours. The spectrum of daylight peaks around yellow/green, and the amount of energy falls off quite sharply as you move to the blue end of the spectrum. That means even if the width of the blue filter is the same as a green filter, you'll measure less light coming through the blue filter. If you're using artificial light the effect will be even more pronounced as artificial light generally has a lower colour temperature than sunlight.

Incidentally, the response of your solar cell will also vary with the colour of the light, and this may well also be affecting your results. I can't lay my hands on a typical frequency response for a silicon solar cell, but I think the response falls off at the blue end of the spectrum.

  • $\begingroup$ You're right about silicon. Typical silicon detectors have their sensitivity peak in the greenish range. They start to suck really fast in both UV and IR. $\endgroup$ – Colin K Oct 24 '12 at 15:57

Actually, if you also refer to how bright the transmitted light looks and not just artificial detector measurement, the dependency of average retinal response to frequency is more important than distribution of color energy from the Sun (yes, after passing the atmosphere the peak is greenish yellow.) Our eyes' average sensitivity for daylight (rods) also peaks in yellow-green around 555 nm, and are less sensitive to blue and red (which look darker, all else being equal.) Furthermore, "yellow" filters let in a broader band of light (typically red through green) than do red or blue filters, and this makes them look even brighter (such that yellow goggles barely make things look darker.) If they are yellowish from blocking the violet and far blue but let in longer blue and blue-green, they won't even affect night vision much.


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