Lasers and their types
An excited atom emits a photon on its own (spontaneously), and such photons have random phases and directions. Einstein showed that if a photon of energy hν equal to the transition energy passes an excited atom, it can force the atom down to the lower state: the emitted photon has the same frequency, phase, direction and polarisation. If there are more excited atoms than ground-state ones (population inversion; the atoms must have a long-lived metastable state and an outside source, the «pump», must keep exciting them), the photons multiply like an avalanche. In a resonator between mirrors the light passes back and forth, is amplified and partly leaves on one side. In the first ruby laser chromium ions are excited by a flash of light and give ≈ 694 nm red light. Laser light is coherent, nearly monochromatic, highly directional (small divergence) and can be very intense. Types: solid-state, gas, semiconductor, fibre and others; uses include measurement, communication (optical fibre), welding and cutting, medicine and bar-code reading. Safety: laser light, even from a weak pointer, can damage the eye; never aim it at people, animals or mirror-like surfaces.
Do not use a laser pointer without the teacher's permission; never aim it at eyes, people, animals, mirrors or glass. Exercise: in your notebook write four properties of laser light and one use for each (for example coherence, directionality).