Is it possible that, for the past 600 years, famous artists have been secretly using mirrors and lenses to help them to paint realistic pictures? That is the belief of David Hockney and others, who claim they can see artefacts in the paintings which are the result of optical projection. The Hockney proposition is contentious, but gives an excellent opportunity for a meeting of science and art in the classroom right across the age range.

In the case of one artist, Johannes Vermeer, there is general agreement that he did, indeed, use optical tools in his painting.

For many years people have been puzzled by the photograph-like quality of his paintings. In the background of many of his works are wall maps and paintings by other artists which are reproduced in minute detail. The clincher, though, is that many of his paintings contain features which appear to be out of focus.

A plausible explanation is that he was copying an image that had been produced by a lens.

Phillip Steadman has made an exhaustive study of the works of Vermeer. He believes that the Dutch master used a camera obscura to help him in his work. This consists of a darkened room, or chamber, with a lens inserted in one wall so that an image of the view outside is projected on to the opposite wall within the chamber. The artist then traces or paints over the projected image to give a picture of photographic quality.

David Hockney has taken the idea further, and suggests that many artists from the 15th century onwards have been using optical methods in their work. During the 15th century, there was a huge transformation in the appearance of art, from flat unrealistic images, to modern photograph-like images. Hockney believes this was because of the discovery and use of optics by artists.

He says the evidence is in the paintings. Often, objects on a table at different distances from the observer appear unnaturally tilted relative to the surface they are on, and some patterned tablecloths appear strangely distorted.

The vanishing point is the term used in perspective to describe the point where parallel lines in a picture appear to meet. The objects in a painting should all have the same one. However, paintings often contain several rectangular objects, such as books, which have different vanishing points.

Some patterned tablecloths display different vanishing points at the front and back of the pattern. These problems would arise if optical systems were being used.

Lenses have a limited depth-of-focus, therefore the artist would have to refocus in order to paint the near and far parts of the scene. However, refocusing changes the magnification, which would lead to distortion of the overall image. This is possibly the reason for the unnatural appearance of many paintings.

Chris Holt is a freelance science writer