Heat and light are both forms of energy. The sun supplies most of both on earth, and almost every warm or bright thing you meet traces back to it, including the flame of a stove and the glow of a bulb.
Three ways heat travels
| Way | What happens | Example |
|---|---|---|
| Conduction | heat passes through a material by direct contact | a metal spoon warming in soup |
| Convection | warm fluid rises and cooler fluid sinks, carrying heat with it | warm air rising above a heater |
| Radiation | heat travels as rays and needs no material at all | sunlight warming your face |
Radiation is the one worth dwelling on. Conduction and convection both need something to travel through, which is why a vacuum flask works: the gap around the liquid has no air, so two of the three routes are blocked. Radiation is the only way the sun's heat crosses the empty space between it and the earth.
Some materials pass heat better
Metals are good conductors of heat, which is why a pan is metal and its handle is not. Wood, plastic, air and cloth are poor conductors, called insulators. A woollen sweater does not make heat. It slows the heat you already produce from leaving.
Dark surfaces warm faster
Dark surfaces absorb more light energy, so they heat up faster in sunlight. Light and shiny surfaces reflect more, so they stay cooler. A black car left in the sun becomes much hotter inside than a white one, and both become hotter than the air around them.
What light does when it meets an object
- Reflection. The light bounces off. A mirror reflects almost all of it in one direction, which is why you see an image.
- Absorption. The light is taken in and usually becomes heat.
- Transmission. The light passes through, as with glass and clear water.
Materials are described as transparent when almost all light passes through, translucent when some passes but the image is blurred, and opaque when none passes. Frosted bathroom glass is translucent on purpose.
Shadows
Light travels in straight lines, so an opaque object blocks it and leaves a shadow shaped like the object. Moving the object closer to the light makes the shadow larger, because it blocks a wider spread of the rays. Two light sources make two shadows.
Common mistakes
Thinking cold moves into things
Cold is not a substance and does not travel. Heat moves, always from warmer to cooler. An ice cube does not send cold into a drink, it takes heat out of it.
Thinking a sweater makes heat
It insulates. Wrapping an ice cube in a sweater keeps it frozen longer, which surprises students until they realise insulation slows heat travelling in either direction.
Check yourself
A metal spoon in hot soup becomes hot at the handle. Which way did the heat travel?
Conduction, through the metal by direct contact.
Why does the sun warm the earth even though space is empty?
Because radiation needs no material to travel through. Conduction and convection cannot cross a vacuum.
Would a black shirt or a white shirt be cooler on a sunny day?
The white one, because a light surface reflects more light instead of absorbing it as heat.
Is frosted glass transparent, translucent or opaque?
Translucent. Light passes through, but not in a straight enough path to form a clear image.
See also
Sources
- Heat transfer and the behaviour of light as taught in United States elementary physical science. Examples on this page are our own.