The miracle year of 1905
As a 26-year-old clerk at the patent office in Bern, he published four papers in a single year that changed physics. He explained the photoelectric effect and the motion of tiny particles in a liquid, and put forward special relativity along with the relationship between mass and energy, E = mc².
Light in packets
Einstein showed that light arrives in small packets of energy that we now call photons. When a photon strikes a metal or semiconductor, it can knock an electron loose. This is the principle behind the image sensors in cameras and in security cameras alike: every point on the sensor gathers photons and turns them into an electrical signal. He won the Nobel Prize precisely for explaining the photoelectric effect, not for relativity.
Relativity in your pocket
Clocks on GPS satellites run differently from clocks on Earth because of weaker gravity and fast motion, by about 38 microseconds a day. Without a correction based on Einstein's equations, a navigation fix would drift by kilometres every day.
The laser and the internet
In 1917 he described the stimulated emission of light. Forty-three years later this gave rise to the laser, and lasers today send data down optical fibres between continents. The formula E = mc² in turn explains why a nuclear power plant gets a huge amount of energy from a small amount of fuel.