A few weeks ago Doug Natelson had a post about handy numbers to know … if you’re doing some kind of physics involving liquid nitrogen or liquid helium, or anything else in which a condensed-matter/nanoscale physicists might be involved. (I assume this is in addition to knowing basic constants)
But I do atomic physics. A few favorite things that help me out if I’m away from a calculator or reference book, with some additions from my colleagues. I probably knew more of these, once upon a time.
— The speed of light can be written as 30 GHz-cm. Thus a 30 GHz signal has a wavelength of 1 cm. 1 Ghz means 30 cm.
— 1 nanosecond is 1 foot (light travel in a vacuum)
— a 1 eV photon is 1240 nm
— Room temperature is 1/40 eV (kT)
— Planck’s constant is 0.4 amu-micron-meters/second (useful for deBroglie wavelength calculations)
— There are about $latexpi$ x 10^7 seconds in a year
General reference:
– \(sqrt{g} = pi \) (to about 0.3%) Handy for Pendulum problems — just cancel or combine the two values.
30 Ghz – cm is really obvious, really useful and I feel really dumb for missing it.
incidentally, the first two say the same thing. 30 cm is one foot. 1 Ghz period is 1 nanosecond.
Phil Plait at Bad Astronomer recently noted that the number of inches in a mile is roughly the same as the number of AU (earth to moon distances) in a light year. That’s kind of neat.
i use 1 eV photon is 1234 nm, cuz it’s a little easier to remember.