# Water waves - is this reflection, or refraction, or both?

1. Jun 19, 2012

### Steve143

http://img42.imageshack.us/img42/9712/refractionk.jpg [Broken]

Now my understanding is that water slows down when it goes over a shallow area because the ground under the shallow water is interfering with the wave. When this happens the wave becomes more transverse which I guess is due to reflection as the wave is hitting the ground under it, perhaps the bottom part of the wave is being reflected and the rest of it is not.

When the slide is at an angle why is it refraction and not reflection since the wave is actually hitting it the glass slide?

It becomes abit more confusing when you take into account that the top of the wave is travelling through air (the more transverse the wave is the more this happens), and the rest of it is travelling through water, which might result in refraction too, but the image suggests the slide is the cause of the refraction.

Last edited by a moderator: May 6, 2017
2. Jun 19, 2012

### olivermsun

From the picture, I'm kind of getting that the glass slide is placed horizontally (parallel to the ground) and so the depth in that area is less than in the tank as a whole. Having the slide at an angle just bends the phase lines (and hence the nominal ray path). Is that what you see?

How do you get that?

3. Jun 19, 2012

### Steve143

The way i'm interpreting it like this:

when the slide is flat on the bottom of the tank the wavelength is reduced. (as far as I know this is because the wave touches the bottom and is probably reflected off it)

When the slide is at an angle the wave is refracted.

I think of sea waves, the parts of the wave that are higher than the rest of the water are moving through air

Last edited: Jun 20, 2012
4. Jun 20, 2012

### olivermsun

I think you may be misreading the description:

"If a small glass is plate is placed in the centre of the ripple tank, the depth of the water is reduced."

The glass plate is parallel to the bottom but not flat on the bottom (or else why would it matter?). This makes the water above the glass plate effectively shallower.

It's because the phase speed of shallow water waves decreases as the water depth gets smaller.

Figure 11.16 is a top down view. From the description:

"If the boundary between the shallow water and the deep water is at an angle to the direction in which the waves are moving, the direction of the waves changes."

The bending of the wave direction is just like the bending of light rays as they pass through an pair of interfaces or a lens or whatever.

As it happens, the water particles are hardly moving at all. But this has nothing at all to do with the wave refraction effect being described.

5. Jun 20, 2012

### sophiecentaur

Not sure where this is first mentioned but it is not correct. Surface waves in water involve both transverse and longitudinal oscillation of the water. The motion is, in fact, circular in deep water (just watch a cork floating on a wave, from the side) and the radius of the circle is maximum on the surface and zero at the bottom.
The motion of the particles throughout the water is a combination of longitudinal and transverse waves and the boundary conditions at the bottom can be regarded as producing a reflection on the bottom so that the reflected and direct waves will interfere (diffract) to produce a non circular motion (peaky) and a net wave speed that is less as the water gets shallower.