Amplitude is the maximum value reached by a wave.

In case of the light wave how the bigger amplitude gives more brightness/intensity ?

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    $\begingroup$ A larger amplitude means that the rate of energy transfer is increased. $\endgroup$ – Farcher May 26 '18 at 21:35
  • $\begingroup$ @Farcher . i am not understanding also how amplitude increased the rate of energy transfer. can u please explain this ? $\endgroup$ – Alex May 26 '18 at 21:41
  • $\begingroup$ A travelling wave transports energy (and momentum). $\endgroup$ – Farcher May 26 '18 at 21:46

From a classical point of view, intensity is proportional to $|A|^2$, so more amplitude implies more energy, and hence more brightness.

  • $\begingroup$ ok . so how amplitude amplitude increases the intensity ? $\endgroup$ – Alex May 26 '18 at 21:21
  • $\begingroup$ What part don't you understand? If the amplitude $A$ increases, its squared modulus also does. $\endgroup$ – FGSUZ May 26 '18 at 22:15
  • $\begingroup$ what meant by the amplitude of an electromagnetic wave ? i can imagine about the amplitude of a water wave. sound wave but how can i imagine the amplitude of an electromagnetic wave and how can i imagine that increase in amplitude of wave will cause more brightness ? $\endgroup$ – Alex May 27 '18 at 20:43
  • $\begingroup$ Oh so that's the problem, then it is a very different question man. You should ask a separate question asking "how EM waves are". I can't answer this in the comment, maybe you can search images of radio waves or stuff. To sum it up, an electric field fills the whole space with vectors, you already know this. Well, a wave just means that those vectors, associated to every point in space, vary with time. The amplitude is the maximum modulus of those vectors. $\endgroup$ – FGSUZ May 27 '18 at 22:43

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