Learning Aims:
  • Triggering the students’ interest in light
  • Differentiating between objects that are sources of light and those that are not
  • Understanding that sources of light have different properties
  • Understanding that light may not necessarily be visible to the human eye
Candle, Torch, Infrared TV remote control, Overhead projector/acetate or Whiteboard/marker, Mobile phones (with cameras)
Suggestions for use:

Hand out Worksheet 1.1. The students should be asked to consider what objects they can see in the classroom, and a (brief!) list made of their choices. The discussion should then turn to whether these objects are ‘sources of light’.

Next, the students should subsequently attempt to describe the differences between the candle and the torch on the basis of physical characteristics (i.e. is the intensity of light constant, what colour does the source produce, is the source hot, does the source require a battery, etc). Having developed a list of criteria and expanded this to a number of different light sources, the students should then discuss whether objects that are sources of light have similar properties to those that are not.

This is undoubtedly a difficult exercise and intentionally so! The problem students will face is that other than the obvious ‘light sources emit light’ it is difficult to find a unifying principle that distinguishes light sources from other objects. This is a relatively robust way of challenging any preconceptions students may have: for example, that light sources need to be electrical in nature or that all objects are sources of visible light because we can see them.

Finally, the IR remote control should be introduced. The students can examine whether this is a source of light using mobile phone cameras and imaging the remote control LED while the teacher presses a button. Although invisible to the naked eye, the sensors used in mobile phones are typically sensitive to the IR light produced.

Possible questions:
  • Which of the light sources are also hot? Are all light sources hot?
  • Which of the light sources are solids, liquids, or gases?
  • Which of the light sources involve chemical reactions?
  • If we can see walls, tables, and chairs, are they also sources of light? If not, why can we see them?
  • Is there a single physical characteristic that explains why some objects are sources of light and some are not? Does energy play a role in some way?