What is an optical ghost?

Ghost is a phenomenon that a row of weak light spots appear after strong light enters the lens, and it is named ghost.

When light passes through the interface of two transparent objects with different densities, it will be refracted (bent). Specifically, when light enters glass from air and air from glass, it will bend. Ideally, all the light will be refracted, but in fact, about 95% of the light will be refracted, and the other 5% will be reflected back to the first substance.

This is a serious problem for the camera lens, because less light finally reaches the film surface. For a lens with a refractive index of R (the percentage of refracted light divided by 100) and n groups of lenses (so there are 2n reflecting surfaces), only the light with a ratio of L can reach the film surface, and L = r 2n. If r = 0.95, n = 10, and L = 0.36, it is only 36. The rest of the light is scattered, and some of it will produce unpleasant ghosts or glare.