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Name: Fiona H.
Status: educator
Age: 30s
Location: N/A
Country: N/A
Date: Thursday, August 22, 2002


Question:
Please describe to me how light and sound waves are transmitted so I can explain these to my year 5 students.


Replies:
Fiona,

First realize that light and sound waves are not related. They transmit differently.

Sound is easiest. Sound is molecules banging into other molecules. A simple example is a slinky stretched across the front of the room. Push on one end back and forth into the slinky, not side-to-side or up-and-down. Each loop pushes on the next loop. The motion started at one end eventually reaches the other end. This is just how sound travels. Vibrations at a speaker bang into molecules, perhaps air molecules. These bang into the next molecules. This continues until air molecules bang into your eardrum.

Light vibrates side-to-side. You can show this with a slinky, too. Light, however, does not make the air vibrate. Light is little packets of electrical energy travelling through space. Faster electrical vibrations make shorter packets. The energy is then more tightly packed. This is why an x-ray is more dangerous than visible light. This is why visible light can cause a sunburn, but radio waves do not hurt you at all. These little packets of energy fly though space until they hit something. This makes the electrons in the object vibrate. This is how an antenna works.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


Hi Fiona,

The light that we see on Earth does not come from the yellow "flames" of the sun. Light comes from the productions of millions, zillions of photons in the core of the sun. This is a byproduct of the fusion going on at the core of the sun. These photons travel through space at ALL wavelengths of the electromagnetic spectrum, which includes visible light, which we refer to as white. The whole EM spectrum includes such things as radio waves, microwaves, infrared, ultraviolet, x-rays and gamma rays.

Weirdly enough, light does not require a substance to travel through, thus can zoom through the vacuum of space.

Sound on the other hand requires a medium, solid, liquid, or gas. The sound waves are compression waves. In other words, they cause the molecules in the substance to bump into one another and this "bump" progresses through the substance. The molecules simply get closer together than farther apart. This is what your ear picks up.

A very cool demo for your students to do:

Take a metal teaspoon and tie it to the middle of a long piece of string. Wrap the string around your fingers and stick them in your ears. (You can just close your little "ear doors" no need to get waxy. Let the spoon dangle and hit the back of the desk. You hear a beautiful chime. The sound travels much better through a solid, (the string and your fingers), than it does through a gas, (the air).

Then you can make all kinds of bio-connections on how ears are put together as well. (Who would have ears most like us? fish or birds? Why? That sort of thing.)

If you would like to know more, check out howstuffworks.lycoszone.com/sun . There are some quick and dirty explanations as well as links to sites with more technical info, (like NASA).

I hope this helps.

Martha Croll


There are two questions: 1. Sound. 2. Light. #1 is easier to understand because it corresponds to our experience. #2. is not so easy to understand.

#1 Sound is the mechanical vibration of matter, back and forth wave away from its "rest" position when the object is subjected to some force. The motion of the matter can either be in the direction of the motion of the wave, or the motion of the matter can be perpendicular to the direction of the sound wave -- or both. The key point is that there is something material moving back and forth. From there your discussion and explanation of the properties of sound waves can get as basic or as advanced as you care to do, and there are any number of texts and resources in the library or on the WEB to aid in your discussion. The key concept is that if there is nothing to move back and forth (vibrate) there is no sound. Sound cannot travel through a vacuum.

#2 Light is a different problem, and one that is less familiar to our experience. What we know about light (actually referred to as "electromagnetic radiation") is the following:

A. It comes in different wavelengths and frequencies.

B. It travels at a constant speed, exactly 299 792 458 meters/sec. It is now considered an "exact" constant along with a few other "fundamental constants" and the value of other physical constants are adjusted accordingly.

C. Nothing "physical" vibrates. The original experimental results of Michaelson and Morley demonstrated that conclusively, and their experiment has been validated numerous times, so that there is no question about it. Light can travel across a vacuum (stars, galaxies, and so on).

D. James Maxwell in the last century "explained" the wave behavior of light by invoking electric and magnetic "fields" (hence the name "electromagnetic" radiation) that oscillate perpendicular to one another and to the direction of propagation of the light (hold up your thumb, index, and middle fingers at right angles). The uncomfortable part is that these "fields" are mathematical constructions. If you accept the premise, and apply a few equations that relate the behavior of these "fields" you can explain ALL OF THE WAVE PROPERTIES of light.

E. Under some circumstances, light behaves like a particle (photons) rather than a wave. The photoelectric effect and other experiments DEMAND this interpretation.

F. Anyone who claims to understand or explain this DUAL nature of light, does not understand the problem. (That is the comment of Richard Feynman, famous physicist, not mine.).

G. There are other properties of light that can only be explained by Einstein's theory of relativity (for example light can be deflected by the force due to gravity of a massive object). This too has been validated numerous times so that its correctness cannot be doubted.

H. The list of PROPERTIES OF LIGHT "ELECTROMAGNETIC RADIATION" is longer and even more mysterious than what I have listed here, but the bottom line is:

MUCH IS KNOWN ABOUT HOW LIGHT BEHAVES -- NO QUESTION ABOUT THAT -- but we do not know what light IS in the conventional sense that the term. So your question of how "light" is transmitted is profound, not simple.

Vince Calder



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