There are other areas of the mouth where lighting is difficult, even with overhead dentists' lights. In these instances, the mouth mirror is used to reflect light onto those surfaces. This is especially useful if the mirror is simultaneously being used for indirect vision of an obscure area.
Additionally, the mouth mirror is used to retract tissues, such as the tongue or cheeks, to gain better visualization of the teeth.
Dentist's mirrors are also commonly used by engineers to allow vision in tight spaces and around corners in equipment. They are a common tool in optics and laser labs as well.