| From: Malcolm Oliver | 11/04/99
13:07:35 |
| Subject: Ripple Tanks and Light | post id:
6345 |
| Small waves in tanks are
frequently used as models for light waves. We can use ripples to
successfully demonstrate reflection, refraction, dispersion, etc.
1. How far can the information obtained from ripples be relied upon to explain light phenomenon? 2. At the beach, waves run agrouond and break. Do light waves "break?" 3. Small waves moving across a sand bar can run into a channel of deepeer water. This channel then generates a wave running in the opposite direction. Do light waves reverse from space time channels in a similar way? 4. Two extended linear waves approaching each other at a small angle meet. The point at which they cross moves more quickly along than either of the creating waves. There is quite a "rooster tail" thrown up due to constructive interference. What light phenomenon does this represent? 5. When a pebble is dropped into a pond, the water is accelerated away from the pebble which displaces it and forms waves. Why doesn't a super nova explosion accelerate light in a similar way? | |
| From: James Richmond (Avatar) | 11/04/99
16:43:51 |
| Subject: re: Ripple Tanks and Light | post id:
6364 |
| Good questions,
Malcolm! 1. How far can the information obtained from ripples be relied upon to explain light phenomenon? The information from ripple tank experiments can tell us a lot about the wave nature of light, by analogy. Light also has particle-like properties. Obviously, the ripple tank waves are not much help in describing particle-like behaviour.. 2. At the beach, waves run agrouond and break. Do light waves "break?" I'll come back to this, below. 3. Small waves moving across a sand bar can run into a channel of deepeer water. This channel then generates a wave running in the opposite direction. Do light waves reverse from space time channels in a similar way? The analogous situation to water waves running into a sand bar is light travelling in one medium running into one with a lower refractive index. For example, light travelling in glass hits the edge of the glass and continues on through the air, perhaps entering another piece of glass a short distance away. The glass is the sand bar; the air is the channel. Just as when a water wave hits a channel, a reflected wave is produced (good observation, by the way), so too is some of the light reflected as the light passes from glass to air. 4. Two extended linear waves approaching each other at a small angle meet. The point at which they cross moves more quickly along than either of the creating waves. There is quite a "rooster tail" thrown up due to constructive interference. What light phenomenon does this represent? Exactly the same thing can be seen with light. If two coherent light beams are intersected at a small angle, a bright region will be seen where they intersect - this is analogous to the "rooster tail" you're talking about, and arises in the same way. If you change the angle, the intersection point can move faster than the speed of light. 5. When a pebble is dropped into a pond, the water is accelerated away from the pebble which displaces it and forms waves. Why doesn't a super nova explosion accelerate light in a similar way? A true wave involves no net movement of the particles in the medium carrying it. For example, when a water wave passes a certain point in the water, the water molecules at that point rise vertically upwards, then return to their original position. Only the wave moves along, not the water molecules. However, dropping a pebble in a pond does cause net movement of the water (which has to get out of the way to make room for the pebble). The pushing of the water out of the way of the pebble, and its subsequent movement back as the pebble sinks, provides energy to start a wave. But once the wave is started, the water stops accelerating in the direction away from the pebble, and we have only the normal up-down movement of molecules. The point here is that the water isn't the wave - it is just the medium which carries the wave. The wave is really a pulse of energy in the medium. This is where the analogy starts to break down for light. Light waves need no medium to carry them. So, while the water isn't the wave, the light, in a sense, is. A supernova explosion moves matter around, and some of the matter emits light. But the light is produced in the explosion - it is not already sitting there waiting to be accelerated. A similar explanation applies to question 2, above. When a water wave breaks, it is no longer really a wave, but a particle disturbance. The wave energy is converted to a physical disturbance of the medium (the water). But, as I said above, light needs no medium. JR | |
| From: The Kernel | 12/04/99
2:01:40 |
| Subject: re: Ripple Tanks and Light | post id:
6396 |
| 1. The use of the ripple tank as
a simulation of the properties of light is very limited because the
velocity of light is constant at 3.0 E8 and water ripples (waves) have a
variable velocity. 2. Waves at the beach break because of two reasons. a) as the waves slow-down as they reach shallows, they convert horizontal kinetic energy to vertical kinetic energy which increaces amplitude, hence the wave breaks, as the velocity at the top of the wave is higher than at the centre of the wave, b) as the waves reach the shallows they are pushed upwards by a reaction with the ocean floor. Extrapolate from this the constnat velovity of light, hence light waves do not break. 3. Light waves can be reversed (that is what a mirror does), the waves of different wavelengths also may form a "beet" as exibited in the sandbar channel (this is why we have an RGB CRT). 4. I believe you are refering to a two-slot interference pattern by which when light waves converge at a point in-phase to form an anti-node (bright region) and when the light waves converge at a point out of phase to form a node (dark region). Once again the velocity of light remands constant. 5. Light can not be accelerated! there is no acceleration in light! Mater ejected from a super nova explosion will exibit a shift in wavelength due to velocity called a red/blue shift, which can be used to measure the velocity and hence + and - acceleration of the explosion and given the spectra, brightness and size of the object, mass and energy output deduced. | |