Core Physics Tutorial 1 - Infra Red Radiation

Radiation passes heat on as an electromagnetic wave called infra-red radiation.  Infra-red is often called heat radiation or thermal radiation.  All the heat from the Sun reaches us as electromagnetic radiation.  (We will look at the electromagnetic spectrum in another topic.)   You can feel the infra-red from the Sun.  If you concentrate the rays of the Sun using a magnifying glass, you can set fire to paper.

Thermal radiation behaves just like light:

• It travels in straight lines;

• Its speed is 300 million m/s in a vacuum;

• It can be reflected and refracted.

Thermal radiation heats objects up by making the molecules move faster.

Our eyes cannot see infra red, but a digital camera can.  Here is a picture of a hot plate that appears much brighter than it actually is because of the infra red radiation.

All objects can absorb or emit radiation.  Infra red cameras pick up radiation emitted by hot objects. This allows wildlife cameramen to film animals at night.  The animals cannot see infra-red, so are not disturbed by bright infra-red lights.  The images are in black and white; colours cannot be seen.

Burglars are detected by security cameras because they give off infra-red.  They cannot prevent that.  A burglar giving off no heat is dead; dead burglars are not a threat.

Picture by Juffy, Wikimedia Commons

The hotter the object, the more the radiation it emits.  A heat sensitive camera can show hot spots.  The cat's nose in the picture below is the hottest part.  It is quite warm in the cat's ears as well.

You can also use a heat sensitive camera to show where heat is escaping from a house.

 Question 1 What other parts of the cat's head are warm?

Dark (especially black) surfaces absorb infra-red well.  Dark coloured cars get hot in the sunshine.

White and silver surfaces reflect radiation.  You may have done simple experiments like this with your teacher.

This experiment is called Leslie's Cube.  Hot water is poured into the cube, and heat passes from the sides by radiation, which is picked up by a detector called a thermopile.  The thermopile is connected to a galvanometer, a very sensitive ammeter which can detect tiny currents.  The more infra red that is given out, the further the galvanometer moves across.  We see the following:

 Surface How far the galvanometer moves (cm) White 1 Silver 2 Gloss black 6 Matt black 9

Another experiment you will have seen is this one where infra red is shone onto a silver surface and a matt black surface at the same time.

 Question 2 Explain which cork slip moves first.

Some uses for Infra Red

TV remote controllers use an infra-red LED.  You can see it flashing if you shine it into a digital camera.  Old remote controllers used ultrasound, which had the unfortunate effect of sending the cat up the curtains when you changed channels.

The shape of an object affects the rate at which it absorbs or emits heat.  The heat-sink in an electronic circuit is:

• made of metal so that it conducts well;

• fin-shaped so that the area for cooling by convection is increased;

• painted black so that heat can pass by radiation.

Dull black surfaces absorb more radiation than white shiny surfaces. White surfaces reflect radiation. Some important uses:

• Cooling fins on heat exchangers are best coloured matt black to emit more thermal radiation.

• Houses in hot countries are often pained white to reflect thermal radiation.

• A fell-runner suffering from hypothermia (has got too cold) is wrapped up in a space blanket which is made of highly reflective foil.

Thermal radiation is an important factor in global warming. The ice on the poles reflects a lot of heat while the dark blue ocean absorb a lot of heat. Loss of the ice caps will make global warming more serious.

 Question 3 Explain the following: a)     solar panels are matt black               d) cookers are shiny b)    caravans are light-coloured               e) the insides of vacuum flasks are shiny c)     fridges are white                            f) ovens are black inside