A flashlight produces light, but it’s messy light: a jumble of photons going in slightly different directions, at slightly different wavelengths, out of sync with each other. That’s why a flashlight beam spreads out so quickly - it’s not really one beam, it’s a chaotic crowd.
A laser is the opposite. Every photon in a laser beam is going in the exact same direction, at the exact same wavelength (color), with their wave crests perfectly aligned. The word “LASER” started as an acronym: Light Amplification by Stimulated Emission of Radiation. By forcing photons to all match each other, lasers produce beams that stay narrow and focused over huge distances.
The Apollo missions left mirrors on the Moon specifically so that scientists on Earth could bounce laser beams off them. The laser travels 240,000 miles to the Moon, hits the mirror, and bounces back to Earth - and we can still detect it on return. We use that data to measure the Moon’s distance to within millimeters, and to verify Einstein’s theory of gravity. Just a really, really good flashlight, sent into space.
