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A sentence from TOEFL reading: " The best signal depends not only on its brightness but also on how well it contrasts with the background against which it must be seen."

My question is, I can not understand the rear part of the sentence:"it contrasts with the background against which it must be seen." Specifically, what confuse me is "against which" and model verb " must". Please help me out. Thankyou!

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  • It sounds like a comparison. E.g., 'Your suggestions should be viewed against similar suggestions made by others as of now."
    – Ram Pillai
    Mar 29, 2020 at 6:09
  • Some background noise (metaphorical usage) is inevitable. No matter how bright a signal is, if there is insufficient contrast with the background, it will not 'stand out' clearly. eg taking a photograph of a firework display with the sun behind it.
    – Edwin Ashworth
    Apr 28, 2020 at 14:12

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A signal is seen against a background. For instance, an aircraft's navigation lights are seen against the background of the sky. And this background is "the background against which the signal must be seen".

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  • The OP's question is." Specifically, what confuse me is "against which" and model verb " must". Your answer doesn't answer the OP's question.
    – Ram Pillai
    Mar 29, 2020 at 14:33
  • Just add that the word which substitutes for the antecedent background.
    – Yosef Baskin
    Apr 28, 2020 at 14:12

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