Phrase 'A shadow of tough black bristles' - What does 'shadow' suggest here?

[Haihao]

Hi,

Could you help me understand the passage below better?

[Mister Micawber]

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A heavy beard, even when shaven, creates a darker impression, like a shadow, compared to the surrounding skin. Men with fast-growing beards often develop 'five o'clock shadow', as their whiskers have grown considerably since they shaved that morning.
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Native English teacher at Mister Micawber's

[Haihao]

Thank you so very much again, Mr Micawber!