In optics, a ghost image is a faint image caused by reflections at uncoated or antireflection-coated surfaces. A prime example is the faint reflection from the uncoated front surface of a rear-surface mirror. Using a front-surface mirror eliminates the ghost image (but may cause other problems).

In spectroscopy, a ghost image is a false image of a spectral line caused by irregularities and imperfections in the ruling of a diffraction grating.