CHAPTER 16. OPTICAL PHENOMENA; PROPERTIES OF MATTER 16.2
For the electron to receive energy, it absorbs a photon and gets its energy. When an
electron loses energy todrop to a lower level, it emits (gives off) a photon with that
energy.
Interaction of light withmetals ESCHD
When light encountersor passes through a material, the photons of the light interact
with the atoms or molecules of the material. Depending on the strengthof the interac-
tions and how often theyhappen, the light will pass through the material or be scattered
in some other direction.
Each wavelength of light relates to a particular energy, and the closer that energy is to
the energy difference between two of the levelsof the atom, the likelierthe photon is
to interact with the atom.
When visible or ultraviolet (UV) radiation shineson a metal, the photonsare absorbed
by the electrons in the metal. The electrons are then excited up to a higherenergy level.
When an electron returns to a lower energy level, another photon is emitted. This is
how light appears to be reflected off a metal surface.
In previous chapters, you have studied geometrical optics, which tells uswhat happens
to rays of light when they are reflected off a surface or refracted through a lens. That
tells us what happens to light rays, made up of many photons, on a large scale. If
you look at a smaller level, i.e. on a microscopic scale, then reflectionand refraction
happen by all the photons interacting with the atoms of the lens or mirror. The photons
get absorbed and re-emitted many times beforeemerging as the finals rays of light that
we see.
Scattering of light is responsible for many effectsin everyday life. We seethat certain
materials are red or blue, for example, since they contain materials that have energy
level differences that correspond to the energies of the photons that make up red or blue
light. These materials then reflect the red or bluelight and absorb the other wavelengths
in the visible spectrum.White objects reflect photons of all wavelengthsin the visual
spectrum, while black objects absorb these photons.
FACT
Because a truly black ob-
ject absorbs all the visual
wavelengths of light, and
does not re-emit photons
at visual wavelengths,
we can say that ’black’
is not a colour itself, but
rather a lack of colour!
Also, since black ob-
jects absorb visual light,
they heat up more than
objects of other colours
which reflect light at cer-
tain wavelengths.Activity: Reflection and absorption
Aim:
Investigate the interaction of light with differently coloured metal objects
Apparatus:
Find some differently coloured metal objects (atleast 5) which will not be dam-
aged if left in the sun for 15 minutes. Make sure to include at least onewhite
item and one black item.
Method:
At the start of your lesson set out your objectsin direct sunlight. Leave them
there for around 15 minutes.