Everything Science Grade 12

16.2 CHAPTER 16. OPTICAL PHENOMENA; PROPERTIES OF MATTER

Alternatively, if it is a sunny day, you can useyour teachers’ cars for this

experiment - as long asthere are some cars of different colours and theyhave

been standing in directsunlight for the same length of time!

After 15 minutes is up,touch each of the items/cars (be careful not to burn

yourself!) and comparetheir temperatures (ratethem from 1 to 5 with 1being

cold and 5 being very hot) in a table such as theexample table below:

Object Colour Temperature rating

e.g. car 1 e.g. red e.g. 3

Questions:

1. Which object was the hottest and what was it made of?


2. Which object was the coolest and what wasit made of?


3. How did the temperatures of the black and white objects compare toeach
   other? (which one was hotter and which was cooler?)


4. Try to explain the reasons for the different temperatures of the objects with
   respect to their colours and the materials of which they are made.

Metals generally reflectmost wavelengths of visible light, but they willreflect the light
in a certain direction, given by the laws of reflection in geometrical optics. This is
different to most materials, like wood or fabric, which reflect light inall directions.
Metals have this property since they have electrons that are not boundto atoms and
can move freely throughthe metal. This is unlikemost other materials that have their
electrons bound closelyto the atoms. These freeelectrons in metals canthen absorb
and reflect photons of awide range of energies.

Ultraviolet light (which has shorter wavelengths than visible light) will pass through
some substances, suchas many plastics, because they do not have theright energy
levels to absorb it and re-emit it. X-rays (also short wavelengths) will also pass through
most materials, since the energies of X-rays correspond to the energy levels of atomic
nuclei. Such nuclei aremuch smaller than atoms, so it is much less likely for an X-ray
to hit a nucleus insteadof the whole atom.

Most materials will absorb infrared radiation (longer wavelengths than visible light),
since the energies of that radiation often correspond to rotational or vibrational energy
levels of molecules.

Photon Transmission Photon Scattering

photon

incoming outgoing

photon photon

incoming outgoing

photon

Figure 16.1: Diagrams of photon transmission (left) and scattering (right).