If a compact disc (CD) is placed in front of a plane mirror, its image displays different colours
from the ones observed in the real CD. This fact occurs because a CD surface is a diffraction
grating which disperses the incident wavelengths. As the object and its image are seen from
different viewing angles, the observed colours are not the same, so the image cannot be considered
symmetrical to the object. A theoretical discussion on the topic and a simple experimental activity,
adequate to secondary school, are presented.