This mirror is a Searchlight mirror constructed during WWII. This concave spherical mirror would have been placed behind a very luminous source of light such as a carbon arc lamp to project a powerful beam of parallel rays. These searchlights were usually constructed so that they could be swiveled around.

An example of a complete searchlight can be seen at the Fort Lytton museum.

This mirror can form two types of image, a real image and a virtual image. An image location is the location from which the light reflected by the mirror appears to come from, so a person looking into the mirror will see a reflection of the actual object.

Virtual images are images that are formed in a location where the light does not actually reach. If you stand very close to this mirror, inside its focal length, you will see a reflection of yourself ‘behind’ the mirror. There is no light coming from behind the mirror, so this is the virtual image.

If you stand further away you will see an inverted image of yourself in front of the mirror. This is the real image and could be caught on a screen inserted between you and the mirror. If you hold out our hand and walk towards the mirror the image will move towards you and reach out to shake your hand. DO IT NOW!

These phenomena can be explained using a technique called ray tracing, which was developed in the 1600s as part of the geometrical and ballistic models of light. In the early 1600s physicists were questioning if light was a wave or a particle. Isaac Newton believed that light was made up of tiny corpuscles that were emitted from hot objects. He suggested that different colors of light were due to the corpuscles having slightly different masses. This is called the ballistic model of light. The corpuscles only travelled in straight lines and their reflection and refraction could be described by simple mathematical rules. These rules included Snell’s Laws of refraction, which we still use to explain lenses today.

This item is part of the UQ Physics Museum ‘2015, International Year of Light’ Tour
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