Asked by: Otto Mayo (11 years old)
When looking up at a blue sky, many people see tiny dots in their field of vision that follow squiggly lines. These move in sync with our pulse, briefly accelerating with every heartbeat, and they usually disappear after about a second.
The dots are actually white blood cells moving along the fine blood vessels (capillaries) in front of the retina at the back of the eye. This experience is called the ‘blue field entoptic phenomenon’ because it’s especially noticeable when looking into bright blue light, such as a cloudless sky.
Whereas red blood cells absorb blue light, white blood cells do not, so they let the light through to the retina. Light-sensitive cells in the retina read this as a signal of increased brightness, causing us to see the white blood cells as spots.