Why we have Einstein to thank for our phone cameras

The Raspberry Pi team has said that the affordable computer they build for schools and electronics enthusiasts hates being photographed—at least not when photographed with a large xenon flash bulb.

"We all enjoyed it," recalls Raspberry Pi founder Eben Upton. They had noticed that a chip in the computer was sensitive to the photoelectric effect. This effect occurs when light triggers the emission of electrons and, therefore, an electric current. It's a kind of reverse "light switch." Upton and his colleagues hadn't anticipated this problem. It was discovered by a Raspberry Pi 2 user shortly after the device went on sale in early 2015.

In later versions of the computer, the problem chip was given a black coating thick enough to absorb incoming light.

According to content prepared by Nobel Prize Outreach and the BBC, Albert Einstein described the photoelectric effect in a groundbreaking paper published more than 100 years ago, in 1905, while he was still a doctoral student. This paper earned him the Nobel Prize in Physics in 1921.

The photoelectric effect has shaped all sorts of technology, from burglar alarms to solar panels to the cameras in smartphones.

To better understand, let's consider the question that puzzled Einstein in 1905: What makes light?

At that time, many scientists claimed that light existed purely as a wave.

Max Planck had already proposed the theory of "quanta," which held that radiation, including light, was composed of discrete packets of energy, but this theory was highly controversial.

THERE WERE UNEXPLAINED EVENTS

Scientists, including Heinrich Hertz, had also demonstrated different versions of the photoelectric effect by creating tiny sparks with light or by electrically charging pieces of gold leaf, causing them to repel each other.

"There were some weird, unexplained phenomena where light could produce electricity and it blew people's minds; it didn't make any sense," says Steve Gimbel of Gettysburg College in the US.

The strangest thing was that the intensity of the light did not affect the energy of the electrons produced, but the frequency or color of the light did.

It was mind-boggling. More light must mean more energy, right?

Einstein realized that if light consisted of discrete packets or particles (later called photons) with wave-like properties, the energy of these particles could explain it.

EXCITED ELECTRONS

"When a single photon hits an electron, the electron becomes excited," explains Paul Davies of the University of York.

As long as the photon falls with sufficient energy, the photoelectric effect occurs and the electron is released from the substance.

The energy of a photon is directly related to the color of visible light. For example, photons in blue light have more energy than those in red light.

For this reason, Hertz found in one of his experiments that particularly energetic ultraviolet light would produce stronger sparks.

Einstein's work, particularly his theory of relativity, was so divisive that some members of the Nobel physics committee hesitated to award him the prize. When they did, they chose to award it for his work on the photoelectric effect.

Scientists have long debated whether this is the best option. There's little doubt that harnessing the photoelectric effect is changing the way our world works, as so many technologies rely on it.

For example, motion sensors in burglar alarm systems emit a beam of infrared light.

When this beam is interrupted by an intruder, the light reaching the sensor changes, changing the electrical current, which sets off the alarm.

Finish lines at the Olympic Games use photoelectric cells to detect exactly when runners have crossed.

This technology allows ships to detect fog and cars to detect rain and automatically activate fog horns and wipers.

Another popular application of the photoelectric effect is in camera sensors, the light-sensitive part of digital cameras that captures images.

Nearly all of them use CMOS technology, which was fine-tuned by NASA in the 1990s for use in space and later installed in billions of smartphones.

MOON SHINE

Eric Fossum, one of the engineers involved in this project, is currently working on image sensors that are sensitive to the smallest amount of light imaginable—a single photon.

These "photon counters," already in use in laboratories, could revolutionize digital imaging by improving image quality in medical tomography scanners, for example, while exposing patients to less radiation.

"With this new technology, we will be able to see almost in the dark," says Fossum, who works at Dartmouth College.

Since Einstein wrote his theory of the photoelectric effect in 1905, many fun things have been discovered about this subject.

In the 1960s, the first lunar landers noticed something odd on the lunar horizon: a strange glow, almost like a slowly fading sunset.

The Moon doesn't have an atmosphere like Earth's, and it's light scattered by particles in our atmosphere that creates sunrises and sunsets as the planet spins on its axis.

WHERE DID THIS MOONSHINE COME FROM?

It turns out that light from the Sun hits dust on the Moon's surface and gives it a positive electric charge through the photoelectric effect.

These tiny dust particles thus repel each other and periodically hover above the lunar surface. As they do so, they catch the light of the newly setting Sun and create that magical glow.