Properties of light
Light is a form of energy that lets us see.
- It travels in straight lines. That is why a torch beam is straight and why you cannot see round a corner.
- It is stopped by opaque things. The dark patch behind them is a shadow. A closer object to the lamp gives a bigger shadow.
- It reflects. A mirror or shiny surface sends light back. The angle in equals the angle out.
- It carries energy. Solar panels turn sunlight into electricity, and plants use it to make food.
Uses of light
Mirrors in cars and bathrooms, periscopes, torches and solar cells all use these properties. Pinhole cameras use straight-line travel. Dark glasses and curtains use the way light is stopped or absorbed. Shadow lengths on a sundial tell the time.
Properties of heat
Heat is energy that moves because of a difference in temperature. Temperature tells how hot or cold something is (measured in °C or K).
- Heat always flows from the hotter thing to the colder thing, until both have the same temperature.
- Conduction: heat moves through a solid, such as a spoon in hot tea.
- Convection: heat moves with a flowing liquid or gas, such as warm air rising from a heater.
- Radiation: heat travels as rays, even through empty space, like warmth from the Sun.
- Dark, rough surfaces take in and give out radiation better. White and shiny surfaces reflect more of it.
Uses of heat
Cooking pots use metal because metals conduct heat well, with plastic handles because plastic does not. Thermos flasks stop all three kinds of heat transfer. Solar cookers use shiny mirrors to reflect sunlight on to a dark pot. Blankets trap air, which conducts heat badly, so your body heat stays in.
Try it
Place two cups of the same cool water in the sun: one wrapped in black paper, one in white paper. After 30 minutes feel both, or use a thermometer. Predict first which will be warmer, then check. Next, put a metal spoon and a wooden spoon in hot water for one minute and touch the other ends.
Key formulas and definitions
- Heat flows from hot to cold
- Angle of incidence = angle of reflection
- Ways of heat transfer: conduction, convection, radiation
- Dark surface: absorbs more; shiny surface: reflects more
Worked examples
1. A ray hits a mirror at 35 degrees to the normal. At what angle does it leave?
Angle in equals angle out, so it leaves at 35 degrees on the other side of the normal.
2. Why does a metal spoon in hot soup get hot at the other end, but a wooden spoon stays cool?
Metal conducts heat well, so heat travels along the spoon. Wood conducts badly, so little heat reaches the end.
3. A black cube is at 50 degrees C and a white cube at 30 degrees C after the same time in the sun. Which absorbed more sunlight and how do you know?
The black cube, because it became hotter. Dark surfaces take in more of the light energy.
Common mistakes
- Thinking cold flows into a hot thing. It is heat that flows out of the hot thing.
- Mixing up heat and temperature. Temperature says how hot; heat is the energy that moves.
- Thinking shadows are things. A shadow is the place light cannot reach.
- Measuring the mirror angle from the mirror surface instead of from the normal line.