# Wavefronts and Longitudinal Waves Displacement-Time Graph

Hi,

I am studying waves and I am having a hard time understanding the following concepts:

Wavefronts: I understand the principle. The only thing confusing me is when my teacher was explaining, he drew a light ray and lines perpendicular to that ray and he said those were the crests of the wave. How? I don´t understand that, unless we are looking at the wave from the "side", similar to the view we have of a wave in an oscilloscope, then I understand how those perpendicular lines are the crest and are therefore points in phase.

Displacement Time Graphs of Longitudinal Waves: When I was learning about longitudinal and transverse waves I learned how, in a transverse wave, the oscillations are perpendicular to the direction of energy transfer, yet, the displacement time graph os a longitudinal wave is a sine curve... How doesn't that imply that the oscillations are at 90 degrees to the direction of motion?

Thanks

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PeterO
Homework Helper
Hi,

I am studying waves and I am having a hard time understanding the following concepts:

Wavefronts: I understand the principle. The only thing confusing me is when my teacher was explaining, he drew a light ray and lines perpendicular to that ray and he said those were the crests of the wave. How? I don´t understand that, unless we are looking at the wave from the "side", similar to the view we have of a wave in an oscilloscope, then I understand how those perpendicular lines are the crest and are therefore points in phase.

Displacement Time Graphs of Longitudinal Waves: When I was learning about longitudinal and transverse waves I learned how, in a transverse wave, the oscillations are perpendicular to the direction of energy transfer, yet, the displacement time graph of a longitudinal wave is a sine curve... How doesn't that imply that the oscillations are at 90 degrees to the direction of motion?

Thanks
For the wave fronts:

Suppose you were in a helicopter above the beach.
Children on Boogie boards will show you the direction the waves are going - directly into the beach [clever people on surf boards can actually move along a wave front so I didn't want to include them].
When your teacher drew the ray, he was mapping out the path of those childern.
AT the time, you will have been able to see the wave fronts at right angles to that direction. Indeed when we look at the ocean it is much easier to see the wavefronts than it is to see the direction the waves are going - though we know from experience that it is pretty close to 90 degrees to the wave front.

Longitudinal waves are hard

http://groups.physics.northwestern.edu/vpl/waves/wavetypes.html

This applet demonstrates ignore the green "mixture" wave

Watch a spot on the transverse wave - it just goes up and down

Watch a spot on the longitudinal wave, it just goes left and right.

If you do a position-time graph for each point, they will look identical, but the graph of the transverse wave will be telling you how far up or down the point is displaced, while the graph of the longitudinal wave will be telling you how far left or right the point is displaced.

NOTE: if you were able to freeze that applet [I didn't try] you would notice that the crest of a transverse wave occurs where displacement is a maximum.
A compression in the longitudinal wave actually occurs at a point which has zero displacement - the dots to the left have moved right, the dots to the right have moved left resulting in the crowding at the compression.