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3. Surfaces - EM radiation, reflection and absorption of thermal radiation (infrared) experiments

Although objects are constantly absorbing and emitting radiation, not all the radiation is absorbed because some of it is reflected away and absorbed elsewhere.

The nature of the surface of any materials affects the relative amounts of radiation absorbed or emitted.

See experiments further down in this section to investigate this phenomenon.

Heat transfer by electromagnetic radiation is usually via the infrared part of the spectrum (thermal radiation).

 

Dark, matt surfaces are the best absorbers and best emitters of infrared radiation

eg rough black surfaces. Black matt surfaces are the nearest thing to a black body radiator and emitter.

Applications of maximising absorption of infrared radiation

Solar panels for hot water comprise of pipes carrying water to be heated, set in a black matt surface to efficiently absorb the infrared radiation from the Sun.

Applications of maximising emission of infrared radiation

Hot water radiators should have a matt surface, preferably black, but rarely so - they don't look very attractive!

They maximise radiation of infrared into the room.

The pipes at the back of a refrigerator should be matt black to maximise thermal energy transfer by infrared red thermal radiation from the heat pump to the surrounding air/wall.

The heat pump is a means of transferring thermal energy from the inside of the refrigerator to the outside.

 

Light, shiny surfaces are poorest absorbers and poorest emitters of infrared radiation

eg white gloss paint, shiny metal surfaces.

Applications of maximising reflection of infrared radiation

Light, shiny surfaces are good reflectors of infrared radiation, this maybe to keep heat in to keep things warm or to minimise heat radiation in to keep things cool eg the silvered surfaces of the walls of a vacuum flask.

Specialised firefighter suits have shiny surfaces to reflect infrared radiation when going to high temperature environments.

Applications of minimising emission of infrared radiation

The shiny surface of 'silver' teapots reduces heat loss by infrared emission, slowing down the cooling effect of the surrounding cool air.

 

Simple experiment to demonstrate the effect of surface on the rate of emission or absorption of infrared radiation

(a) Simple experiment to compare the absorption of infrared by two different surfaces

Two identical metal plates (same metal in area and thickness - fair test) are set up, equidistant from a radiant heater - a good source of thermal radiation (infrared).

The metal plate on the left can be coated with a matt black material and the other on the right, with a shiny metal surface or the metal surface painted in gloss (shiny) white paint.

At the 'shaded' side of the plate you fix on a brass weight with a drop of molten wax, which holds it in position on cooling.

The radiant heater is a powerful source of thermal radiation (infrared) and is absorbed by the surface of the metal plates.

When the plate is hot enough, the wax melts, and brass weight slides down!

You can time how long this takes with different surfaces for the same metal and maybe different metals.

You should observe the blackened surface plate heats up much faster than the shiny metallic/white surface, as indicated by the shorter time needed for the brass weight to fall.

 

(b) Using boiling tubes of hot water with covered with different surfaces to compare their emission of thermal radiation (infrared)

You set up four identical pyrex glass boiling tubes in a test tube rack.

Each is covered by wrapping around the boiling tubes the same area of paper of different textures.

(Two factors to keep constant for a fair test)

1.  black matt paper,  2. black paper with gloss surface,  3. white matt paper,  4. white gloss paper

You can try other materials too, such as aluminium foil.

Each is filled with the same volume of boiling hot water and lightly seal with an insulating rubber bung.

(Third factor to keep constant for a 'fair test', so only the external surface is varied)

Allow a minute for the boiling tube and coating to warm up and the, at regular time 1 minute intervals, temporarily remove the bung and measure the temperature, replacing the bung each time.

A graph of the results (temperature versus time) shows you the cooling curves (idealised below):

From these you can measure the initial negative temperature gradients as the boiling tubes of water cool down.

The black matt surface boiling tubes cools the fastest - steepest downward temperature gradient - best emitting surface

The gloss white paper should cool the slowest - the lowest downward temperature gradient - the poorest emitting surface

This fits in with the described pattern of behaviour described above.

Doing cooling curve graphs is a better data analysis than just one set of readings.

The rate of cooling should be in the surface order

matt black  >  shiny black  > white matt  > shiny white

 

(c) Using the Leslie cube - multisided box can with different surfaces to compare their rates of infrared emission

The Leslie cube is a hollow aluminium or steel metal can with four different surface coatings on the four vertical sides.

e.g. matt and gloss black paint, matt and gloss white paint, or other surfaces like shiny and dull metal surfaces.

The cube needs to be made of a good conductor so the surfaces heat up rapidly.

An infrared (thermal radiation) detector is positioned in line with the middle of a surface of the cube and connected to some kind of meter or data logger.

The Leslie cube is filled with boiling hot water - take care!

Being a cube shape ensures the same surface area is emitting radiation - fair test factor.

The can is given a few  minutes to warm up all the surfaces - all will come to the same temperature (fair test).

The thermal radiation reading is taken for all four faces of the Leslie Cube, making sure there is an equal distance between the Leslie cube and detector - must be kept absolutely constant to make it a 'fair test'.

The radiation spreads out and intensity decreases by a factor of 1 / distance squared (inverse square law).

The higher the 'meter' reading, the greater the intensity of radiation emission and the more efficient the surface in heat transfer by emitting thermal radiation (infrared).

The value of the infrared should be in the surface order

matt black paint >  shiny black paint > white matt/shiny paint  > shiny metal

 

For more on heat transfer see Introduction to heat transfer - conduction, convection and radiation revision notes

INDEX physics notes: Absorption and emission of EM radiation by materials