Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Absorbing radiant energy with different surfaces

Class practical

Using the hand as a detector to compare the absorption of radiation.

Apparatus and materials

Radiant heater

Insulating sheet with hole

Retort stand, boss, and clamp

Crystallizing dishAluminium leaf (not kitchen foil)

Vegetable black

Paint brush

Methylated spirit

Health & Safety and Technical notes


Only aluminium leaf should be used. Do not use the thicker aluminium kitchen foil, which has a higher thermal capacity.
 
Some pupils may burn their hands without realizing the danger, unless an appropriate warning is given. Appropriate words are given in the "Procedure" below.

Procedure


Radiant heater and insulating sheet with hole

a Set up the heating element with the insulating sheet in front of it. Ask a student to place the back of one hand near the hole for about 5 seconds. You might say:

Hold your hand close to the hole in the screen, with the back of your hand towards the red-hot heater. Notice what you feel. Only do this for a short time to decide what you feel - one . . . two . . . three . . . four . . . five ... - about that long. Do not hold your hand longer than this in case you burn it.
 
b Cover the back of the student's hand with aluminium leaf and ask them to repeat this for a further 5 seconds. Now coat the aluminium leaf with a vegetable black paint. When the paint is dry, ask the student to place the back of the hand again near the hole for about 5 seconds.

 

Teaching notes


Student's hand with aluminium leaf

1 To put the leaf on the student's hand, get them to clench their fist and moisten the back of their hand with clean water until wet all over. Lay a sheet of aluminium leaf gently on the wet skin, blowing on it to push it on to the skin. Ask the student to unclench their hand a little to stop the leaf from cracking. 

Blacken leaf

2 To blacken the aluminium leaf, mix some vegetable black with methylated spirit to the consistency of thick soup. Apply with a soft paint brush on top of the leaf.

running tape to get rid of paint and leaf
3 To get rid of the paint and leaf, students should hold their hand under a running tap. Students should not try to rub off the paint or leaf. This will rub soot into the hands which will be difficult to remove.
 
4 When radiation is absorbed or stopped by any surface, the surface is warmed up.
 
The hand absorbs most energy when it is painted black, a medium amount when it is not coated with anything, and the least when coated with a shiny surface. In the last case the energy is being reflected away.
 
5 White paper rather than aluminium foil stuck on the back of the hand and brought up to the heater may give a surprising result. The white paper and blackened foil absorb nearly the same amount of energy, even though the white paper reflects most of the energy. What is it about the respective properties of paper and aluminium foil?
 
6 How Science Works extension: No measurements are taken in this experiment, but it is still possible to draw a valid conclusion about the nature of a surface and the absorption of radiant energy. You could:
 

  • Use the experiment to illustrate the concept of valid evidence.
  • Point out that, in a comparative analysis, numerical data is not always necessary.
  • Ask students whether this experiment would still provide valid evidence if three or more colours were investigated. If they say it would not, ask them how the experiment could be adapted so that it did.
  • Ask students to illustrate the findings of this data-free experiment in an appropriate way.


This experiment was safety-checked in January 2007