Except that this problem doesn’t specify distance between horseman, so I think it’s a bit bogus — no need to resolve an individual person to be able to tell that they’re there. And for hair color, if you make assumptions about the clothes being worn, you could perhaps infer color of hair, even if the hair isn’t resolvable (a person being a “single pixel” would have a different hue depending).
So, a typical pupil is around 2 mm in diameter in bright conditions. With the Rayleigh limit that results in an angular resolution of 1.22 * 60010^-9 m / 210^-3 m = 3.66*10^-4 rad
At a distance of 5 x 3 mi = 15 mi = 24.1 km this corresponds to a point to point distance of
tan(a/2) = (d/2)/l
d = tan(a/2) * l * 2 = tan(3.66*10^-4) * 24100 * 2 = 8.8 m
So in conclusion, with regular, human-like eyes he could discern points that are at least 8.8 m apart in the best case scenario. Discerning hair color from the color of the clothes would need a much higher resolution, and the horsemen are probably not 10 m apart from each other either. And again, this is a theoretical limit, real-world resolution would probably be significantly lower.
That’s part of the “make appropriate estimates” bit. You can just pick any reasonable number for the angular resolution Legolas needs and answer the question using that. Provided you do the method right, you’ll get the marks.
The only way this would make sense is if the horsemen are all riding next to each other, which would allow him to estimate the count based on the average width of one riding horseman. As soon as one is even partially in front of another, the 105 number breaks.
Except that this problem doesn’t specify distance between horseman, so I think it’s a bit bogus — no need to resolve an individual person to be able to tell that they’re there. And for hair color, if you make assumptions about the clothes being worn, you could perhaps infer color of hair, even if the hair isn’t resolvable (a person being a “single pixel” would have a different hue depending).
So, a typical pupil is around 2 mm in diameter in bright conditions. With the Rayleigh limit that results in an angular resolution of 1.22 * 60010^-9 m / 210^-3 m = 3.66*10^-4 rad
At a distance of 5 x 3 mi = 15 mi = 24.1 km this corresponds to a point to point distance of
tan(a/2) = (d/2)/l
d = tan(a/2) * l * 2 = tan(3.66*10^-4) * 24100 * 2 = 8.8 m
So in conclusion, with regular, human-like eyes he could discern points that are at least 8.8 m apart in the best case scenario. Discerning hair color from the color of the clothes would need a much higher resolution, and the horsemen are probably not 10 m apart from each other either. And again, this is a theoretical limit, real-world resolution would probably be significantly lower.
which is why legolas has huge anime eyes
That’s part of the “make appropriate estimates” bit. You can just pick any reasonable number for the angular resolution Legolas needs and answer the question using that. Provided you do the method right, you’ll get the marks.
The only way this would make sense is if the horsemen are all riding next to each other, which would allow him to estimate the count based on the average width of one riding horseman. As soon as one is even partially in front of another, the 105 number breaks.