detect <the> blue

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The speaker explains why the sky isn't purple but blue to our eyes:
Well, this is where being human comes into play. Our human eyes perceive the middle of the spectrum most sharply which is why blue is way easier to see than violet. So even though violet light is being scattered around like crazy, we just don’t detect it as well as we detect the blue.
World's Most Asked Questions: Why is the Sky Blue?, YouTube video

Could you explain to me why she use the article with the last word?
  • entangledbank

    Senior Member
    English - South-East England
    Having mentioned violet light (in the sky), she has established a specific referent - light scattered in the sky, not just light in general - so she can now refer to the blue light (which we know is also in the sky). She could also have just said 'blue', generically. We see blue better than violet, and we see (the) blue light in the sky better than the violet (light).
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