Scientists use laser range-finding to measure the distance to the moon with great accuracy. A brief laser pulse is fired at the
moon, then the time interval is measured until the "echo" is seen by a telescope. A laser beam spreads out as it travels because it diffracts through a circular exit as it leaves the laser. In order for the reflected light to be bright enough to detect, the laser spot on the moon must be no more than 1.0 km in diameter. Staying within this diameter is accomplished by using a special large-diameter laser. If λ=532 nm, what is the minimum diameter of the circular opening from which the laser beam emerges? The earth-moon distance is 384,000 km.
Even though it's called free speech, it really isn't free. You can't say certain things in public or on social media because people are too sensitive these days to handle the truth about something. You can, however, say what you want in a book or a protest with like-minded people because you really can't get in trouble for supporting whatever you want.
