In our investigations we will be particularly interested in the energy of the light emitted by the gas. Two factors - brightness and color - contribute in very different ways to the energy of a light.
Brightness
When we think about the definition of energy, the brightness makes
sense. A bright light has more energy in it than a dim light.
This conclusion matches the observation from the first tutorial
- as we increased the electrical energy supplied to the lamps,
they became brighter.
Color
The color connection is not quite so obvious. Atoms in small packets
of energy emit light. These packets are called photons. Each individual
photon contains an amount of energy that is related to its color.
So, if we wish to discuss the energy of one of these photons,
we need to know its color.
For light that we can see the energy ranges from red at the
low energy to violet at the high-energy end. Not visible but still
a form of light are infrared photons with an energy lower than
red and ultraviolet photons which have energies higher than violet.
The order of energies for the various colors of photons is shown
below.
Low energy photons: Infrared
Red
Orange
Yellow
Green
Blue
Violet
Higher energy photons: Ultraviolet
Each time an atom produces light, it emits one photon. Each photon of visible light carries a very small amount of energy. This energy ranges from about 2.56 x 10-19 Joules for red light to 4.97 x 10-19 Joules for violet light. In electron volts (eV) 1.6 eV (red) and 3.1 eV (violet).
The brightness of the light is related to the number of photons
emitted. A dim light will emit fewer photons than a bright light.
Thus, we have two measures of energy - brightness and color. Because
color is related to the light from each individual atom, we will
concentrate on it.