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Tell me please why the was used in one sentence and omitted in the other! It is from Crash Course Astronomy.(The sentence is at around the first minute and thirty secondth.)

Hydrogen glows most strongly in the red.

Kate was dressed in black that day.

I am confused why in the first sentence the is used and in the other left out. Both sentences were said by native english speakers. What is the difference?

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In the sentences before the one in question, the speaker is referring to how nebulae are classified based on the composition of gas particles. This is achieved through astronomical spectroscopy which uses the visible light spectrum to determine which particles are emitting light. By glowing "in the red" part of the light spectrum, they can infer that hydrogen likely makes up some of the nebula.

Use of the definite article here is due to the implied clause referring to the visible light spectrum. It could be editorialized to include the extra clause, e.g. "hydrogen glows most strongly in the red [part of the light spectrum]." An alternative way to say it would be that it simply "glows red."

The second sentence does not use the definite article because it refers to an ambiguous article of clothing, as opposed to something unique, e.g. the visual light spectrum as used in the first sentence. It could be written as "Kate was dressed in black clothes that day" to illustrate that "black" describes the property of an arbitrary article of clothing rather than a specific one.

  • This is an excellent answer. I would only add that the former is much less common and only occurs in specialized contexts. Other instances of "in the X" include "the engine is in the red" (meaning that the engine is running too hit or too fast, and therefore the needle of the engine gauge is in the red area, indicating danger) or "this company is in the black" (an idiom referring to black ink in an accounting sheet which means that the company is profitable). – Tom Church Feb 23 '18 at 4:59

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