Jaeick Woun, of St Andrews College, asks :-

Is there a difference between heat and infrared? How is it we can feel an electric heater but not the infrared TV changer?

John Campbell, a physicist who uses infrared radiation to probe the vibrations of impurity atoms in crystals, responded.

Infrared means `below' the red ie electromagnetic waves of lower energy than red light.

All objects radiate electromagnetic waves depending on the temperature of the object. If it is heated hot enough (say 900C) the upper edge of the broad band of waves it radiates is in the red end of the spectrum, ie it becomes red hot. If heated hotter still, say to the temperature of the Sun (6000C) or an incandescent light bulb, it emits higher colours than red so looks white. An incandescent light bulb is very inefficient at turning electricity into light. Only ten percent of the electrical energy is emitted as visible light. Ninety percent is wasted as heat, ie it radiates away as infrared radiation.

As an example, a one-bar electric heater glows red hot. It radiates about one kilowatt of power of which only a few percent can be seen by the human eye. The large amount of power radiated as infrared radiation, over 900 watts, means we can `feel' the radiant heat because it heats our skin. The radiated power is spread over a wide range of frequencies so there is very little energy per small frequency range.

In contrast, an infrared TV changer uses a very efficient electronic diode to emit a very small amount of power (the small battery lasts a long time). However it is all concentrated at or near one infrared frequency so that the power emitted in that one small frequency range is far higher than that for a red-hot radiator. The infrared detector can easily 'see' this emitting diode but there is not enough total power to noticeably heat our skin. Hence we dont feel it.