# Homework Help: Help Now Please

1. Jan 22, 2005

### lotsoluv4ya

What happens to a sound wave and a light wave tht are trying to pass through a piece of plastic. I think that the light wave slows down and may eventually stop and thre sound wave will get faster. Is this for which of the following reasons:

A. Sound travels faster because light cannot move through many materials.
B. All types of waves travel at the same speed in the same medium.
C. Sound travels faster in the air than in solids.
D. Sound travels faster in denser materials than it does in air.

2. Jan 22, 2005

### HallsofIvy

a) makes no sense at all. What happens to light doesn't say anything about sound.

b) is just wrong! light and sound certainly don't travel at the same speed in air.

I will just tell you that the answer is one of either c or d. You should have seen examples of the speed of sound in different materials. What IS the speed of sound in air? What is the speed of sound in iron?

3. Jan 22, 2005

### Andrew Mason

That is an interesting approach. If photons were like bullets, you would be right. But photons can travel only at one speed: the speed of light. They are absorbed by matter and stopped but they don't slow down before they are stopped! That is what makes light so fascinating.

When light strikes a wall of matter, the atoms in the wall will begin to absorb some of the photons. If the atoms are in a stable state after absorbing the photons they just hang onto them and the photons all get absorbed after a very short distance into the wall. But if the atoms' electrical energy state is not stable after absorbing the photon, they will re-emit the photons. So the photons appear to pass through the wall by the atoms relaying photons from one to the other (that is what glass does, or some plastics). This 'relay' has the effect of increasing the time to pass through (or reducing the average speed of the photons) but in fact the photons are either captured or always travelling at one speed: c

Sound, on the other hand, is a mechanical motion of the medium itself rather than 'something' passing through. Sound is very, very slow compared to light.

As I say, a better way to think of a captured photon is that the photon ceases to exist and its energy is converted into electrical potential of an atom. Light is never 'stopped' - it either travels at c, or it ceases to be. Who said science isn't poetry?

Not true at all. Sound always travels much more slowly.
Not true. It travels over 4 times faster in steel, for example, than in water and in water sound travels 4 times faster than in air.
This is often true, but it is not because of density. It is despite increased density. Sound travels faster in warm air than cold air. Cold air is more dense. Sound travels more slowly as the density of a medium increases.

AM

Last edited: Jan 23, 2005