This means that for many simple problems it is sufficient to concentate on the light rays , the lines that describe in a simple geometric way the path of light propagation.
(the lines; describe; the path of light propagation) (This; means; that for many simple problems it is sufficient to concentate on the light rays) Context(This means,List([0, 10))):(it; is; sufficient to concentate on the light rays)
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the lines This it
describe means is
the path of light propagation
that for many simple problems it is sufficient to concentate on the light rays
sufficient to concentate on the light rays