It is caused by light passing straight through the emulsion and being reflected back by the film base on the light sensitive layer. This records slightly out of register with the original image.
Halogens are a group of chemical elements.
In photography, three of these, bromine, chlorine and iodine are used with silver to produce light sensitive material. Half-frame is a negative format of 18 x 24mm. Images are recorded on a vertical axis on standard 35mm film, thus giving 72 half-frame images on film designed for 36 exposures. Some early cameras produced negatives of this size. Half-silvered mirror is a glass sheet evenly coated with a substance which transmits part of the light incident on it and reflects the remainder.