But to drive 25 feet out of the crossing in 6 seconds, you only have to go 2.3 miles per hour. I’m pretty sure I could crawl that fast on my hands and knees, providing I was - ah - properly motivated. My car, with its stupid automatic transmission, will go way faster than that if I only press the brake.
I don’t want to derail the thread on sound, but could you elaborate on this, or point me to a source that might explain it? I thought the OP was pretty intriguing, but this aside really piqued my interest.
Okay, “derail” just struck me as I was proofreading, but I’m gonna stand by it.
Yes. The wayside horn I linked to is designed to replace use of the train horn through town. There is also a bell that goes off before the gates come down. first they start blinking, then the bell, then the gates come down.
Also, we don’t have gates on both sides of the road in the vast vast majority of crossings in the US. Some crossings (out in the country) have nothing but markings and it is up to the driver to see and avoid trains.
We don’t need to be training the other Dopers to make bad puns, but never mind that.
About light going through substances that are transparent - if light, to you, is electromagnetic radiation through space, a ripple in the ether, a propagating change in an electric field, you’d be surprised to know that what happens in a transparent substance is that the electrons in the substance are behaving somewhat like a gas on their own and are mechaniclly vibrating. It’s this vibration, coupled mostly between neighboring electrons through their local electrical fields, that propagates through the substance. To the extent that there are vibrating massive bodies in a system with mechanical elasticity, it’s more like sound. In a gas, the pressure, the mechanical elasticity, comes through enormous numbers of collisions between gas molecules (in air the mean distance a molecule travels between collisions is 7e-8 meters). In a solid, sound going through the solid pushes on atoms that are bound (chemically or physically) through the electromagnetic force, which makes sound going through a solid very similar to light going through a transparent substance.
This is why materials have an index of refraction, which is the ratio by which propagation speed is decreased. The index of refraction depends on how much substance there is per volume because the disturbance has to be passed from electron to electron (kind of). It also depends on the elasticity of the electron gas relative to the solids (the system of nuclei) it pervades, and since this property also dictates how easy it is to polarize the solid with an electric field, we know it as dielectric constant. In fact you can calculate the index of refraction of a substance pretty well if all you know is the dielectric constant and the density (IIRC it’s proportional to both, but not confident on this detail).
This is also why substances can polarize light according to their structure. If you take a clear polymer and stretch it hot so the molecules are oriented, the index of refraction of the polymer is different in the long direction than across it, so this causes birefringence.
When light enters a solid there is a change in impedance. Some of the light reflects, in proportion to the change. The reflection depends on electron vibration near the surface of the solid. The vector component of polarization that is in the plane of the incoming, transmitting, and reflecting light beams reaches a minimum of zero when the angle of the vibration of the transmitting beam is such that its reflection would be polarized along the length of the reflecting beam, like a pressure wave in the ether rather than a shear wave. Pressure waves don’t work in the eather. A light beam can’t have its electric field vibrations pushing and pulling along the length of the beam. So, at this angle, Brewster’s angle, there’s no light in the reflected beam with polarization in this plane, only polarization in the orthagonal one, and this is a way to make a high quality polarizer.