Here, the imagery emphasizes and clarifies the message in this phrase, not only because everyone has positive connotations with light and negative with flames, but also because King speaks from a Christian point of view and the light is a reference to Heaven and God, while the flames a reference to Hell and the Devil.
At this time, Christianity was a much bigger part of society, than it is today, so the effect of this imagery was much stronger, than it would be if you read it today, and the contrast between the two images even more striking. This way King reaches a large audience as this imagery is so simple, that everyone, including people, who do not take particular interest in politics, will understand the conflict as well as the ones, who do.
Also, the fact that he uses Christianity as an indirect argument in his speeches is a strategically important move, since it appeals to both the white and the African-American population, but this comes completely natural to King, since he was a dedicated Christian.
Another example of imagery in the speech is when he speaks of America’s debt to the African-American population:
“We’ve come to our nation’s capital to cash a check … America has given the Negro people a bad check, a check which has come back marked “insufficient funds.”” (Line 8 & Line 17-18, page 224)
Here, King compares the situation with what was, back then, an unpleasant, yet quite common scenario: a useless check. This is a quite accurate comparison, as the bad check is a metaphor for the promise of better times that was given from the white population of America to the African-Americans back when the slavery ceased, but when the African-American attempted to cash in the check, it seemed that there was no justice left for them. This, once again, clarifies the situation, since it is explained in simple terms that everyone can relate to.
King continues using imagery throughout the entire speech, often paired in strong contrasts to each other. The overall effect of these imageries is that it relates the situation to something familiar and the message that is send is thereby much easier to understand for the receiver.
Besides the extensive use of imagery, King uses rhetorical devices, like apostrophes, several times. In the beginning of the speech, he addresses the white majority, after which he suddenly shifts audience and speaks to the African-American population:
“But there is something that I must say to my people” (Line 3, page 225)
With these apostrophes, all components of the conflict are addressed, so the each of the receivers gets the feeling that they are directly involved and they are forced to react and form an opinion.
Other rhetorical devices, like anaphors, are used several times. An example could be the following passage:
“I have a dream that my four little children will one day live in a nation where they will not be judged by color of their skin but by the content of their character.
I have a dream today!
I have a dream that one day, down in Alabama, with its vicious racists…” (Line 16-20, page 226)
This is a very strong device. It gives a lot of power to the passage and beside the obvious pathos appeal; it adds ethos appeal as this device along with others, like alliterations, assonances and in general the group of rhetorical devices that manipulates directly with the sound of the words rather than the meaning, makes his language very smooth and convincing.
All in all, it seems that one of Martin Luther King Jr.’s biggest advantages was his fluent language which derived from his natural use of imagery, rhetorical devices and appeal forms and in general his perfect mastering of the art of manipulating with words. His use of his advanced language brought out and explained his message so it could be understand by anyone. These skills made him very convincing and there is no doubt that besides his strong personality and faith, this was one of his strongest forces and without them, he might not had become one of the greatest freedom fighters in modern times.
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[1] Using the version of the “I have a dream” speech from Martin Luther King, Jr.’s Autobiography, first published in 1999