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One demonstration I would do for my physics classes on a clear-sky day was to show the dangerous effects which can be produced by concentrating solar energy.  I used a concave glass mirror which was commonly obtainable from the science apparatus companies, about  16 inch diameter with about a 12 inch focal length.  When I was handling it on a sunny day I was always VERY careful to keep it facing away from the sun until the moment of the demonstration activity.  The demonstration involved activity involved the mirror and a board about 2 inches wide and 2 feet long.  Then, holding the mirror facing the sun (with all the students BEHIND it) and with my free hand holding one end of the board I would bring the other end of the board close to the center of the mirror and bring it not-too-slowly to the mirror's focal point.  At the point where the board actually reached the bright image of the sun for about 1 or 2 seconds, that place on the board would immediately char and break into open flame.  The solar energy collected by the 16-inch diameter circle was being concentrated into an image of the sun which was about a half inch in diameter, about a thousandfold concentration.

In the follow-up discussion the the students would vividly appreciate the danger to the eye of looking directly at the sun, so that the eye's lens would focus the sun's image on the retina.  Or what would be worse, using a telescope with an objective lens much larger in area than the eye's pupil to gather even more energy.

For further exploration, I had also used a Stirling engine whose heat supply came from the sun as concentrated by a shiny metal concave reflector about 20 inches in diameter and a focal length of about 8 inches.  This also came from a science apparatus supply company about 20 years ago.  It was not always reliable and was more for demonstrating a concept than for illustrating a viable energy application.

Albert J Read
Director Emeritus
Science Discovery Center of Oneonta (NY)
and retired faculty member, SUNY-Oneonta

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