THE LENS

The main lens of the instrument would be correctly described as an ‘astronomical aplanatic objective’.  The diameter is 33 cm and 7 meters focal length.   The lens consists of two pieces of special glass each being approximately 3 cm thick, these pieces are correctly known as ‘elements’.  The first element is made of ‘Crown’ glass and the second element is made of ‘Flint’ glass.  The two elements are separated in their cells by a distance of 1 cm.  The lens has been designed specifically for the Plymouth location, with a long focal length in order to give detailed views of the shipping and the buildings of interest in the city.

THE MIRROR

Light is brought into the instrument by a large flat mirror 43 cm diameter and 21/2 cm thickness.  This mirror is made with an accuracy of 100th part of the thickness of a human hair.  The Lens surfaces are also polished to this same accuracy.  The mirror is aluminized with a reflective coating of aluminum, again with a thickness of about 100th the thickness of a human hair.

THE SCREEN

The lens produces a magnified image on a curved screen, so as to give the best possible sharpness in the centre of the screen.  The screen is curved for the same reason that the retina of the human eye is curved.  Lenses naturally tend to form their best images on a curved surface.  Moving the screen up and down rather than moving the lens as in an ordinary camera focuses the instrument.  The average height of the screen has been arranged so that children with special needs can comfortably see the image.

The Camera Obscura can be rotated around the horizon, this movement is called azimuth.  The image can be made to scan up and down to show objects near and far, this movement is called altitude.  The design of our instruments are based on the classic Victorian Camera Obscura but with improved performance in both optical and mechanical details.  The instrument does not ‘zoom’ but objects of special interest can be brought into the centre of the field of view for maximum sharpness and definition.

The majority of Camera Obscuras are sited on a high building.  The Plymouth instrument is unique, being sited underground in the Napoleonic Battery.

ACTIVITIES

The Camera Obscura Experience

The ultimate SCIENCE ‘hands-on’ LIGHT and SOUND experience. Students can visit the only underground Camera Obscura in the world!

The camera has a 1.8 metre diameter viewing table, which gives spectacular 360 degree views over Plymouth Sound, Mount Edgcumbe and the city centre.  Students also have the opportunity to use ‘big’ specialist apparatus situated in the grounds of Seymour House, including an extremely powerful telescope and the gigantic ‘tube tangle’ sound exhibit.  They can also ‘freeze their own shadows’ and even discover the power of their own voices, as they whisper into the whisper dishes! Other smaller, LIGHT apparatus is available inside the eco-building.

It is also possible to include an additional ‘design and make your own mini camera obscura’ workshop.

This session is suitable for KS3 and KS4 and can be used to support Science, Design and Technology.

Aims:

To develop knowledge and understanding of LIGHT and SOUND
To understand how a camera obscura works
To provide a quality, ‘hands-on’ outdoor learning experience

For further details contact Jo McCaren on 01752 608942. 

E-mail: jomccaren.tebp@seymourhouse.org.uk