Why can people see candles? Why can you see the flame of a candle?

It is the light found by candlelight that enters people's eyes and causes people's vision. Specifically, the eyes change the focal length of the lens by adjusting the curvature (refraction) of the lens (equivalent to the convex lens), so that the candle can obtain an inverted and reduced real image in the retina. The retina is very sensitive to light, where light is received and converted into signals, which are transmitted to the brain through the optic nerve, so candles and candle flames are seen.