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Double image: Sophie Duncan sees the point of having two eyes

Why do we need two eyes? To help your students understand the importance of having stereoscopic vision, get them to try this simple experiment. Draw a dot on a piece of paper and mount it on the wall. Stand 60cm away from the wall, and cover one eye. Now use a pencil to touch the dot on the wall. Try again, this time covering the other eye. When they try it using both eyes, your students will see how much easier it becomes.

In 1838 Charles Wheatstone invented the stereoscope, a device that could be used to see in 3D. Later devices made use of a pair of stereoscopic images that were viewed through two magnifying lenses.

It is possible to take stereoscopic pictures using a normal camera, although it takes a lot of practice. In order to take a stereoscopic pair of images, choose a static scene with a prominent feature. Take a picture of the view with the feature in the centre of the image. To take the second image, move the camera horizontally by about 7cm - the distance between your eyes. Now take the second picture, ensuring that the feature is in the centre of the image again. These two images form a stereoscopic pair. Have them processed as small prints.

Although it is possible to view these images with the naked eye, it is easier to make a stereoscopic viewer. Make a wooden frame to hold a pair of magnifying lenses mounted 30cm away from the two images in such a way that the viewer can look through them easily. Place a dividing panel at least 10cm long between the lenses to ensure that each eye is looking at each image separately. When the images are viewed in this way your brain interprets them as one three-dimensional picture.

Sophie Duncan is project manager for science at the BBC www.bbc.co.ukscience

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