Please can you explain what a prism is with regards eye sight?

The answer

Both eyes provide information to the visual part of the brain which is normally processed to produce one sharp image. In the case of double vision (diplopia), an individual will have blurred vision or see two separate images. If the vision is otherwise relatively normal and one or both eyes needs positional correction, this can be achieved by the use of prism lenses fitted in glasses. Standard eye glasses usually correct for problems such as near or far sightedness and astigmatism (abnormal curvature of the surface of the eye), but the addition of prism lenses is specifically to correct for diplopia. Prism lenses can be made to suit the degree of diplopia so that it appears that both eyes are working together, resulting in one sharper image.