Chapter VI review and final summary
Chapter VI studied light. Sources are natural (Sun, stars) and artificial (lamps); the Moon is an illuminated body. Light travels in straight lines in a homogeneous medium, so shadow and penumbra form; in a solar eclipse the Moon is between the Sun and Earth, in a lunar eclipse the Earth is between the Sun and Moon (saros — 6585 days). The speed of light in a vacuum is about 300 000 km/s, in water about 225 000, in glass about 200 000 km/s. The law of reflection is α = β; refraction is a change of direction at the boundary of media. In a plane mirror the image is virtual, upright, the same size and at the same distance behind; if the mirror turns by α the reflected ray turns by 2α. Spherical mirror: 1/d + 1/f = 1/F. Lens: D = 1 : F (dioptres); convex — converging, concave — diverging. White light splits into 7 colours in a prism (Newton, 1666); a rainbow is dispersion in drops. Beruni and Ibn Sina explained light, vision and eclipses on the basis of observation. Safety: never look straight at the Sun, a laser, or a beam focused by a lens.
“Light relay”: teams take turns writing a phenomenon (reflection, refraction, spectrum, shadow, eclipse) on the board and giving one experiment or example; the other team corrects mistakes.