Unable to understand waves I am unable to understand what actually is a wave. A google search gives the definition -In physics a wave can be thought of as a disturbance or oscillation that travels through space-time, accompanied by a transfer of
energy. My questions are -

*

*What is meant here, by the word disturbance ?


*How is energy being transferred when a wave propagates ?
3.What do we mean when we draw the sin curve to represent a wave? For example, light is an electromagnetic wave (and also like a particle but I don’t think it matters in this question) right ? Then why doesn’t it travel like the sin curve and instead travels in a straight line ?
 A: *

*Think about an array of dominos. The first knock the second, which in turn knocks the third, and so on...

In this metaphor, the disturbance is the tilting of the dominos (the energy they temporarily acquire and release in the successive collisions).


*In the metaphor above, energy travels as the successive domino fall. Only one domino at the time is in motion (possesses kinetic energy).


*If instead of dominos on the floor, you picture an array of balls joined by springs (ball, spring, ball, spring, etc.), then if you wiggle one of the balls it acquires kinetic energy (vibrational) which it will communicate progressively to its nearest neighbours and so on, recursively, hence again propagating energy.
This is exactly what happens when you pluck the string of a guitar: the entire string starts vibrating and the energy propagates along the string. Basically a guitar string is nothing else than a sum of little spring arranged along a line.
If the string is elastic and the force $f$ the string develops under the applied external stress $x$ (plucking) is a linear response:
$ f = k.x $ ($k$: a constant specific to the string)
Then is is easy to demonstrate that the evolution of the displacement of the string over time is of the form:
$ x = A . sin(t) $ (simplified version)
The sin(t) function you are referring to is the motion of the string (what a movie made with a high speed camera would reveal about a vibrating string).
The same thing happens when you throw a rock in a pond, the initial shock propagates in the shape of concentric waves (powered by the initial collision in which the rock is slowed down by the surface of the pond, whereby its transfers energy to said surface, which in turn propagates outwards in the shape of successive ringlets).


*Think about the guitar string example above: The (deformation) wave propagates along a line, but each point along the line oscillates in time as $sin(t)$.

A: *

*In the reference of waves, disturbance is a change in a certain quantity pertaining to that wave. Like in the sound waves, the disturbance is the pressure or density variation at a point. In the case of a string wave, the disturbance is in the amplitude of a point on the string at different times.


*A wave carries energy with it thus, a wave can transfer energy. Consider our string wave, you can feel its energy if you block its path by your hand. Any point on the string has potential energy due to the displacement of the particle from its mean position and kinetic energy due to its motion.
Light and sound also carry energy with them, in fact they are considered forms of energy. In light, energy is carried by its electric and magnetic fields while the case of sound is very similar to that of a string.


*We don't draw the sine curve to represent the path of a wave (which I think from your question is your doubt). Waves are of many types. One of them is the sine wave which is also the simplest one. The sine wave shows the variation of disturbance in the quantity under focus. The disturbance can be of any type.

Like in the example of light, the electric and magnetic fields variate as a sine function. But this does not mean that light has to travel like a sine curve. It travels in a straight line in the form of rays.
A: The sinusoidal shape of a wave on water or on a string represents a spatial displacement- the actual movement of the surface of the water or the position of the string. `The sinusoidal representation of an electromagnetic wave doesn't represent a spatial displacement at all, so you must not think of it as being directly comparable to the water wave in that sense.
The sine function associated with an electromagnetic wave represents the degree to which the value of the electromagnetic field varies at every point. It is nothing to do with spatial displacement.
If you are comfortable with the idea of an electric field then you should be able to imagine that in steady state the field exists with a certain strength at every point in space. In a very simple classical case the strength of the field might be the same at every point over a region. When light passes through the region, the strength of the electric field is disturbed. The degree by which the field strength is disturbed at any given point is a periodic function of time and space. When it is drawn as a sine wave along an x-axis, say, the y-axis doesn't represent a spatial dimension- it represents the change in the field strength at each value of x. So if a ray of light is moving along a path represented by the x-axis- ie in a straight line- the y-axis shows by how much the ray has made the strength of the electric field change at every point.
