# Light and Sound Waves

1. Jan 26, 2007

### americanforest

A strange question that I recently thought of. Both sound and light are waves so why is it that I can hear through walls and around corners but can't "see" through walls and around corners?

2. Jan 26, 2007

### Staff: Mentor

Waves on the ocean are waves too, so why don't you get wet when listening to the radio?

Sound waves and light really don't have much of anything to do with each other. Light really isn't even a wave anyway - it just sometimes acts kinda like a wave.

3. Jan 26, 2007

### americanforest

Ok, let's forget about the fact that they are both waves. Can you just explain why I can hear through walls and around corners and why I can't see through walls and around corners? Does it have anything to do with frequency?

4. Jan 26, 2007

### ranger

Yes it has to do with frequency and interaction with matter. Why do you think you are able to receive radio signals in your home, but not able to see through your wall?

5. Jan 26, 2007

### americanforest

Ok then, I can generalize my questions and ask why do low frequency waves pass through matter and high frequency waves are not able to?

6. Jan 26, 2007

### pivoxa15

You are thinking low freq = sound. high freq = light. But you must first understand that light and sound are fundalmentally two different waves. LIght is an electromagnetic wave and sound is a longitudanal wave due to longitudanal oscillations of particles in the medium in which sound exists hence a mechanical wave.

However if you are thinking all electromagnetic. i.e why you can pass an x ray through your body but not ordinary colour wave than its because higher freqeuncy => higher energy due to E=hf where h is plank's constant. Your body does not absorb EM waves above a certain frequency so x rays gets passed.

Last edited: Jan 26, 2007
7. Jan 26, 2007

### Gokul43201

Staff Emeritus
http://www.physicsclassroom.com/Class/sound/U11L3d.html

Read the whole thing, and for the answer to your question, scroll down to the section on diffraction.

In this case, the difference between the behaviors of light and sound is explained through the underlying similarity in their wave nature.

8. Jan 26, 2007

### americanforest

Fascinating article, thanks Gokul. The stuff about elephants and bats was particularly interesting.

Here's another sound related question : If I am in the desert (or any barren, flat, place with air to propagate through) and my friend is standing miles and miles away with some kind of GPS system so I can look directly at him, can I shout and have him hear me, assuming there is nothing in the way. As far as I can tell, the problem with getting sound waves a certain distance is one of avoiding refraction and reflection.

9. Jan 26, 2007

### ranger

The further you are from the source of the sound, the longer are its wave lengths with a lower frequency. Do you know how the frequency of the wave affects our ability to hear?

10. Jan 26, 2007

### Gokul43201

Staff Emeritus
What you are describing here is the frequency dependent attenuation due to absorption losses. This is a relatively weak effect compared to the geometric power reduction with distance (inverse square for a point source, linear inverse for a line source, etc.).

The important point here is that the sound intensity level decreases with distance from the source. When the intensity level falls below the minimum detection threshold for a human ear, the sound will no longer be heard by the average human being.

11. Jan 26, 2007

### americanforest

I think I understand what you're saying, and I think it helped me understand something else, this inverse square rule that seems to pop up all over the place. Looking at this image I realized that the energy(or whatever quantity) of the sound in this case starts our propagating in a small circle whose radius gets bigger and bigger, eventually the same energy (??) has to fill a much bigger circle's area and so is obviously weaker at any given point on the level curve. Since the area of a circle is ~r^2 this helps me understand all the ~r^2 rules popping up in Electricity, Gravity, who field strength decreases the same way.

This leads to another question which is not really related at all to this conversation: do all fields propagate out in circles like this, thus explaining the inverse square dependency (at least for electricity and gravity, the two I have studied so far)?

12. Jan 26, 2007

### Staff: Mentor

You didn't respond to this, americanforest, so I want to make sure you saw it:
Light and sound are completely different things. The reasons why radio waves pass through walls is different from why sound waves are transmitted through walls (read that wording carefully!).

Sound waves are pressure waves and when you hear sound transmitted through a wall, it is literally because the sound makes the wall shake. Radio waves pass through walls with no interaction because of the frequency of the wave and material in the wall simply not being right for interaction. I it depends on the material, though - since light/radio is electromagnetic, it is generally reflected or absorbed (but not transmitted) by metal.

13. Jan 26, 2007

### Staff: Mentor

Neither light nor sound necessarily propagate in circles (and they aren't fields themselves). A laser or even a good flashlight does not and sound from a trumpet (for example) is also highly directional, as, of course, is sound transmitted through a rod.

But anything that does disperse radially from a point in a spherical pattern will decrease in intensity in an inverse square proportion. Note that a ripple on a pond is moving 2 dimensionally outward in a circle, so it follows an inverse linear proportion, not a square one.

14. Jan 26, 2007

### ranger

This is true for a gravitational field and electric field. You can say the field is radially symmetric.

Last edited: Jan 26, 2007
15. Jan 27, 2007

### Gokul43201

Staff Emeritus
This is close, but not correct. Actually, the energy from a point source radiates outwards so that all the energy emitted during an interval $\delta t$ is, at some subsequent time t, spread over a spherical shell of radius r=ct (where c is the speed of propagation of the energy, assuming all the energy propagates at the same speed) and thickness $\delta r=c \delta t$. The volume of this spherical shell is given by the product of the thickness and the surface area, $\delta V = 4 \pi r^2 \delta r$. Energy conservation then leads us to the result that $I(r) \cdot 4 \pi r^2\delta r = constant$. Thus, the origin of the inverse square power dissipation from a point source is due to the surface area of a sphere being proportional to $r^2$.

If you have an ideal line source (instead of a point source), the energy emitted at some instant is subsequently found on the surface of a cylinder coaxial with the source. The surface area of the cylinder is porportional to the radius, and hence the power obeys a 1/r law (eg: sound power levels near a busy highway, light intensity near a tubelight). If the source were planar, the energy intensity would ideally be a constant, at all distances from the source (eg: constant E-field between the electrodes of a parallel plate capacitor).

Last edited: Jan 27, 2007
16. Jan 27, 2007

### Gokul43201

Staff Emeritus
Russ, even though light and sound are dissimilar in many respects, they are also similar in some. And particularly for the question raised in the OP, it is specifically the similarity arising from their wave nature that explains the two cases. In other words, the reason sound bends around walls while light doesn't is not because sound and light are different, but because they are similar when it comes to the property of interest here (i.e., diffraction). What is true, however, is that the fact that there are certain similarities does not mean we should be able to extrapolate all behaviors from one to the other. Any extrapolation must necessarily be based on an underlying principle that is common to both phenomena.

Last edited: Jan 27, 2007
17. Jan 27, 2007

### americanforest

Is there any intuitive example for why short wavelengths cannot difract around things as well as large wavelengths, and if not, is there an equation that expresses this?

18. Feb 7, 2007

waves

We are doing that too... I do not really understand it that well though. Can someone help me with this though?

A person lying on an air mattress in the ocean rises and falls through one complete cycle every 5.5 seconds. The crests of the wave causing the motion are 19.0 m apart. Determine (a) the frequency and (b) the speed of the wave.

19. Feb 7, 2007