A group of 8 suspects; 3 wearing red shirts, 2 wearing green shirts and 3 wearing blue shirts. Eye-witness claims to have seen 3 people in the scene, the shirt of one of them appeared red, while the rest appeared black. The lighting at the scene was by a monochromatic red light source.

Now, the crazy executioner decides he's going to pick one of those suspects at random for early judgement. Based on the witness's testimony, can you give the executioner any advice which will increase his odds of picking a culprit?


Monochromatic light is light which is of only 1 uniform frequency or wavelength; so it can't be split any further. For example, sodium street lamps emit monochromatic yellow-ish light, and so it only reflects off yellow or partially yellow objects, making them the only things to be seen colored.

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    $\begingroup$ I think I have the answer but can't figure out the spoiler tags on mobile. Please hold. $\endgroup$ May 14, 2016 at 11:39
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    $\begingroup$ Kill the red shirts. Everybody knows red shirts die first. $\endgroup$
    – dr_
    May 15, 2016 at 13:38
  • $\begingroup$ It's possible for an object that appears red in daylight to absorb a particular wavelength of red light, or for an object that appears to be some other color to still reflect some particular wavelength of red light. When the guard took the prisoner's shirts to the crime scene, one appeared very bright, two appeared very dark, and the rest were sorta medium. So the guard was able to identify the culprits with 100% certainty, assuming the eye-witnesses were correct. $\endgroup$
    – supercat
    Jun 6, 2016 at 22:17

1 Answer 1


I would advise

Send one of the people not wearing a red shirt. There is a 2/5 chance that they are one of the suspects, rather than the 1/3 chance of one of the red shirts. The monochromatic light will make the non red shirts look black, therefore indicating that two non red shirt were at the scene.


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