Why Can Pressing Closed Eyes Produce Flashes of Light?

When some people gently press a closed eyelid, they see moving spots, patches or rings of light. No matching light is entering the eye. This experience is commonly called a phosphene. It shows that vision is not a direct copy of incoming light: if enough activity arises anywhere along the visual pathway, the brain may experience the signal as light.

Pressure on the eyeball mechanically changes the state of retinal cells and surrounding tissue, triggering signals that resemble visual input. Because the pressure and retinal response are uneven, the result is usually not a clear object but a flickering, spreading or moving pattern. Electrical stimulation or stimulation of the visual cortex can also produce phosphenes, so the term names a visual phenomenon rather than a single cause.

This is not a reason to rub the eyes as an experiment. Forceful or repeated rubbing can harm the eye surface and aggravate some eye conditions. A brief pattern after mild pressure is not the same as new flashes that appear without pressure. If flashes begin suddenly or recur, especially with a sudden increase in floaters, a shadow in the field of vision or a change in sight, prompt professional eye assessment is appropriate. The central point is simple: seeing light means the visual system produced a corresponding signal, not necessarily that the signal began with photons outside the eye.

https://my.clevelandclinic.org/health/symptoms/24888-phosphenes