Light phenomena
The Sun, a lamp and a flame are light sources; the Moon and a book give no light of their own but reflect the light that falls on them, and so we see them. In a uniform medium light travels in straight lines, so an opaque body casts a shadow. The speed of light in vacuum is about 300 000 km/s. The law of reflection: the angle of incidence equals the angle of reflection. In a plane mirror the image is the same size as the object, appears as far behind the mirror as the object is in front, and is reversed left to right. When light passes from one medium (air) to another (water) it changes direction – refraction. That is why a pencil in water looks broken and a pool looks shallower, only about 3/4 of its real depth. Sunlight gives the energy for photosynthesis. Safety: never look directly at the Sun, a laser beam or a mirror’s reflection of the Sun – it can damage the eyes.
Coin experiment: tape a coin to the bottom of a bowl. Look from a position where the coin just disappears behind the rim and ask a friend to pour in water steadily: the coin reappears (refraction). Reflection: use a small mirror to direct a desk lamp’s beam onto a wall and compare the angles of incidence and reflection with a ruler and protractor. Safety: never aim a mirror at the Sun or at eyes; use an unbreakable mirror; do not spill water.