2.3.5P2. Thermal Physics

Investigating Thermal Radiation — Combined Science 0653 - Physics Revision Notes & Topical Questions

5 min read 7 parts Lesson 14 of 14 Unit quiz

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Part 1 of 5The idea being tested

Emission of infrared

Emission of infrared Every object emits infrared radiation, and the hotter it is the more it emits.
Surfaces differ in how well they emit: dull black surfaces are the best emitters, while shiny white and silver surfaces are the worst emitters.

Absorption is the mirror image

Absorption is the mirror image A surface that is a good emitter is also a good absorber of infrared,
- so dull black surfaces absorb best and shiny silver surfaces reflect most of it.
The same investigation idea can be turned round by placing identical objects at an equal distance from a heater and measuring how fast each one warms up.

Aim

Aim To find out how the colour and finish of a surface affect the amount of infrared radiation it gives out as it cools.

Type	Variable	How it is handled
Independent	Colour and finish of the flask surface	Four flasks painted black, dull grey, white and shiny silver
Dependent	Temperature of the water, measured at regular intervals	Thermometer in each flask, read to 1 °C
Control	Size, shape and material of the flasks	Use identical flasks that differ only in their outer surface
Control	Volume and starting temperature of the water	Measure the same volume of freshly boiled water into each flask
Control	Time interval and room conditions	Same stopwatch intervals, all flasks placed together away from draughts

Stand the four identical flasks on a heatproof mat, well spaced and in the middle of the bench, and fit a lid with a snug thermometer hole to each.

Boil water in a kettle and use a measuring cylinder to pour exactly the same volume into every flask.

Place a thermometer in each flask and record the starting temperature, checking that all four are as close as possible to the same value.

Start the stopwatch and read each thermometer at eye level every 30 seconds for about 10 minutes.

Record every reading in a table with one column for each colour.

Repeat the whole investigation at least once and calculate mean temperatures at each time.

1Plot the data

Plot the data Draw temperature on the y-axis against time on the x-axis, with a smooth curve of best fit for each flask on the same axes.

2Compare the gradients

Compare the gradients A steeper curve means the water is losing energy faster.

3Rule out other causes

Rule out other causes Conduction and convection losses are the same for all flasks because the flasks are identical, so colour does not change them.

4Conclude

Conclude Any difference in cooling must be due to infrared radiation: the black flask cools fastest and the shiny silver flask cools slowest.

Reducing errors

Reducing errors Make sure every flask starts at the same temperature,
- because water cools quickly at first and a small difference spoils the comparison.
Keep the thermometer hole small so that thermal energy does not escape through it, and read scales at eye level to avoid parallax error.
A data logger with digital temperature probes gives more accurate and more frequent readings than reading by eye.

Safety

Safety Wear safety goggles when pouring boiling water, do not overfill the kettle and keep water away from electrical sockets.
Keep equipment away from the edge of the bench and work standing up so that you can react quickly to a spill.
Treat any scald by holding the area under cold running water for at least five minutes.

Term	Meaning
Infrared radiation	Electromagnetic waves emitted by all objects because of their temperature; the hotter the object the more it emits.
Emitter	A surface that gives out radiation; dull black surfaces are the best emitters.
Absorber	A surface that takes in radiation; dull black surfaces absorb infrared best.
Control variable	A quantity kept the same in every trial so that only the independent variable affects the result.
Parallax error	A reading error caused by looking at a scale from an angle instead of straight on.
Curve of best fit	A smooth line drawn through plotted points that shows the overall trend.

Saying that black flasks 'attract' heat. Black surfaces emit and absorb infrared better; nothing is being attracted.

Forgetting to control the starting temperature and volume of water, which makes the comparison unfair.

Claiming that radiation is the only way the flasks lose energy. Conduction and convection also occur but are equal for all flasks.

Reading the thermometer from above or below instead of at eye level, which introduces parallax error.

Describing the shiny silver flask as cooling fastest. Shiny surfaces are poor emitters, so that flask cools most slowly.

Last updated: 20 October 2026