The gas released after the light reaction of photosynthesis is oxygen (O2).

Here's why:

* The light reaction uses water (H2O) as a source of electrons and protons.

* During the light reaction, water molecules are split using energy from sunlight, releasing electrons, protons, and oxygen.

* The oxygen is a byproduct and is released into the atmosphere.

This oxygen is crucial for life on Earth, as it is used by most organisms for respiration.