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When light becomes trapped in the cable, it can be represented by rays (left picture) or by standing waves (right picture). In case of standing waves, different incident angles are establishing different standing wave patterns, which propagate in superposition with each other (right picture demonstrates superposition of modes 01e, 31o and 31e).
For graded-index fibers, mode energy distribution pattern is spread evenly, such that oscillation amplitude is the same at all cross-sectional area.