Solar Cells and Camera Sensors

The Idea

In explaining the photoelectric effect, Einstein showed that one quantum of light gives its energy to one electron. If the quantum carries enough energy, the electron is freed. That one-to-one exchange is how light becomes an electrical signal.

Light Detectors

Some devices use the effect exactly as Einstein described it, with electrons knocked out of a surface into a vacuum.

• Photomultiplier tubes can register a single photon, and are used in medical scanners and physics experiments.
• Night-vision goggles convert faint light into electrons, multiply them, and turn them back into a visible image.

Camera Sensors

The sensor in a phone camera works on a close relative of the same effect. Photons striking silicon free electrons inside the material, and the freed charge in each tiny pixel is counted. The charge-coupled device, invented in 1969 by Willard Boyle and George Smith, earned its inventors a share of the 2009 Nobel Prize, and the Nobel committee described it as making use of the photoelectric effect.

Solar Cells

A solar cell is the same process run for power. Sunlight frees electrons in a semiconductor, and the structure of the cell drives them around a circuit.

Here some care is needed. The photovoltaic effect was first observed by Edmond Becquerel in 1839, forty years before Einstein was born. The first practical silicon solar cell was demonstrated at Bell Laboratories in 1954, converting about 6 percent of the sunlight that hit it into electricity. Solar power now supplies nearly 9 percent of the world's electricity, and its share is rising quickly.

How Direct Is the Link?

It varies by device.

• For photomultipliers and night vision: direct. They use the effect he explained.
• For camera sensors: strong. The same principle, inside a solid.
• For solar cells: foundational, not inventive. Einstein did not discover or predict the photovoltaic effect. What his work supplied was the explanation of why it happens, and that understanding is what made it possible to engineer.