Double vision is the misalignment of the two images on the retinas of a pair of eyes - often called diplopia by doctors and opticians.  Diplopia occurs when one eye turns away from its normal axis of vision (shown in red on the diagram). This is sometimes called a squint. A PRISM is used to realign the image in the squinting eye.

A sudden onset of double vision must always be investigated immediately by an optician or doctor

Correcting double vision		Types of double vision
Normal eye, the light ray passes correctly along the optic axis Eye with a squint, light ray is not aligned to the optic axis Eye with corrected squint, the prism lens re aligns the light ray to the optic axis	Horizontal diplopia   Vertical diplopia   Mixed diplopia