What are ghost images in optical terms?

Ghost images refer to the phenomenon of dull duplicate images that can occur within optical systems due to internal reflections within the lens elements. When light enters a lens, some of it can reflect off the surfaces of the lens elements instead of passing straight through. These reflections can create additional images of the subject being viewed, causing a softer, less distinct image to appear alongside the primary image.

This phenomenon is particularly noticeable in lenses with multiple elements, where the internal design can lead to multiple reflections. Ghost images can detract from the overall clarity and contrast of the main image, making it essential for photographers and optical engineers to understand and mitigate this effect when designing lenses or choosing the right equipment for specific conditions.

The other choices describe other optical phenomena or effects that do not accurately represent what ghost images are. For instance, reflections from the lens surface (choice A) would create sharper images and are not internal reflections. Lens distortion related to color separation (choice C) pertains to chromatic aberration, while external lighting conditions affecting image appearance (choice D) do not fall under the technical definition of ghost images as understood in optics.