Every Shadow Has Two Parts
- Light travels ONLY in straight lines. It cannot bend round an object
- UMBRA: the dark centre, where the light is blocked completely
- PENUMBRA: the pale ring, where the light is only partly blocked
- A point source would give a razor edge. Real lamps have width
Presenter notes: Begin with the safety rule, before anything else and before anybody is excited, because this is the one lesson in the book where a student could genuinely be hurt by what they learn. Nobody ever looks at a solar eclipse with the naked eye, or through sunglasses, smoked glass, film, a CD, a phone screen, water, or above all binoculars. Say it plainly and say why: the back of the eye feels no pain, so the damage is silent and is noticed hours later. Then open the lesson properly with the question left hanging last week. Twice every month the sun, earth and moon come into a line, at purnima and at aunsi. So why is there not an eclipse twice a month? Tell them the answer is the first slide but one.