With a class of students, have them each obtain a small plane mirror. Use
one from a make-up compact, or, better yet, a 100 mm 150 mm mirror
that students put inside their lockers. Have the students gather outdoors on
a sunny day and arrange themselves in a semi-circle facing the sun. Place
a target such as a crumpled up sheet of paper out in front of the students
and have them shine their reflection of the sun onto the paper. A thermocouple
style temperature sensor could also be used to investigate the concentrated
energy. The paper will ignite with the collective energy