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What happens to a photon after it hits the retina of my eye?

Herman D’Hondt
Sydney, Australia

Like everything else in life, the photon eventually becomes heat – an increase in entropy.

When the photon strikes one of the light-sensitive cells in the retina, it is absorbed by an opsin molecule. The colour-sensing cone cells use three different photopsins, one for each primary colour.

The rod cells use rhodopsin, which senses only light intensity, not colour. In all cases, the opsin absorbs the photon, which vanishes out of existence. However, its absorption by the opsin transfers the photon’s energy to that molecule. That triggers an electrical signal that is transmitted to the brain by the optical nerves. The opsin itself returns to its original, non-excited state and dumps the released energy as heat.

Although at that point, the photon no longer exists, its effects can be very long-lasting: when the signal reaches the brain, it may be stored as a long-term memory, or it can trigger some action by the body. The photon itself, however, has disappeared long before any of those things happen.

Mike Follows
Sutton Coldfield, West Midlands, UK

The photon gets absorbed and ceases to exist. When looking up at the night sky, it is amazing to think that photons may end their journeys in your eye, having been launched billions of years ago from a star that no longer exists.

Photons are force particles (as opposed to particles of matter) whose sole purpose is to mediate the electromagnetic force. When the photon enters the eye, it can be absorbed by photoreceptors.

These proteins change shape when activated by photons, which ultimately controls the passage of ions through the photoreceptor cell, so that the light energy is transferred as an electrical signal along the optic nerve to the brain.

Pat French
Rockhurst, Shropshire, UK

Whether it stimulates a sensor cell in your retina or completely misses a suitable receptor, the light energy that is the photon dissipates as a tiny amount of heat. It isn’t particularly warm.

A photon reflected from a red rose might have a wavelength of 700 nanometres, giving it an energy equivalent of 1.8 electronvolts, or 2.9 × 10^–19 joules. That is around 10 billion billionths of the energy required to lift a large potato 1 metre high, so not a lot!

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