Why is there no such thing as a plane or harmonic wave? Spherical wave?

Question

My lecturer says there is no such thing as a plane or harmonic, or spherical wave. Why is this?

Ten points!

Answer 1

Your Lecturer sounds like a Building Contractor, not a Physicist!!



I'm not sure about planar, as I was taught they fell into harmonic waves; but to have sound, there has to be harmonic (vibrations), and sherical motion waves are also called concussions, like from explosions, where the air from the explosion's epicenter folds under itself, like ocean waves, in all directions simultaneously.



Examples:



Plane(planar); Take a sheet, and pull it part way from the bed, and holding the edge of the sheet, bounce it up and down. You'll see roll-type waves moving away from you.



Harmonic; Take a handsaw and a violin bow, hold the sawhandle between your knees and the blade tip with one hand, and run the bow accross the straight edge of the sawblade, bending it back and forth and you'll hear various sound pitches.



Spherical; Cup your hands together with your thumbs slightly apart and blow into them, expanding and contracting both hands until you hear a whistling-like sound. The spherical cupping and movements resemble a concussion in reverse.

Answer 2

I'm not sure of the context of your lecture. If it is a sound wave, there are harmonics, and waves are emitted in a spherical fashion - a point source of sound is emitted spherically. It may spread in a cylindrical fashion due to density differences in the medium (i.e. air or water), but it does propagate in all three dimensions.

Harmonics are just higher frequency (or sometimes lower) waves imposed on the base frequency. As an example, a machine rotates at 1800 RPM, with a ball bearing support system - the base frequency is 1800, but there might be a doublet at 3600, and there might be multiples of however many bearings are in each race - i.e. if there are 7 balls in each, there might be harmonics of 7x1800...

Light exhibits similar characteristics, but also has quantum effects- acts like both a wave function and a particle (e.g. a photon) in light, there aren't harmonics per se, but there is typically a mix of different energy photons, such as that generating rainbows from sunlight. There also are absorption gaps in sunlight or most other 'white' light sources...

Answer 3

The fundamental difference is that the spatial function of the amplitude is planar in one case, and spherical in the other case i:e the intensity varies as the inverse square of distance in case of the spherical wave, and is independent of distance traveled in a planar wave. Both of these intensity cases of course hold only in a perfectly lossless medium. If you have one single point source as the light source, then you have a spherical wave, and if you have that light source at an infinite distance away, you have a plane wave. As you can see, both are idealized representations, and extreme ones too. However since they are idealized, a lot of the mathematics in wave optics become simpler if you go along with one of these conditions, so these are pretty frequently made approximations.