Download 10.1 - Sources and Nature of Light

SNC 2D
10.1 – Sources and Nature of Light
What is Light?
 A type of radiant energy in the form of a wave that can be detected by the
human eye
 Can be from natural or artificial sources
o Natural: The sun, fireflies, lightning
o Artificial: Light bulbs, fluorescent tubes, candles, lasers, etc.
Sources of Light
Objects that emit (give off) their own light are said to be luminous.
 e.g., the sun, a switched-on flashlight
In luminous objects, some form of energy is changed into light energy. This occurs
when atoms within the material absorb this energy and enter into an excited state by
bumping an electron up one energy level. Then, almost immediately, the excited
atoms release the energy in the form of light when the electron falls back down to its
normal state.
Objects that do not emit light are said to be nonluminous.
 e.g., the moon, a switched-off flashlight, a pencil, a chair, etc.
 These objects can reflect light from a luminous source to allow us to see them
Light from Incandescence:
 Things that are extremely hot become luminous
 The process of emitting light because of a high temperature is called
incandescence
 e.g. light bulb – fine metal wire (usually tungsten) glows white hot
o Very inefficient at producing light – only 5% of electrical energy gets
converted into light, the other 95% is lost as heat
Light from Electric Discharge:
 When electricity passes through a gas, the gas particles can enter
into an excited state and emit light
 This process is called electric discharge
 e.g., lightning, “neon” gas tubes (actually many different gases
used for different colours), street lights
Light from Phosphorescence:
 Certain materials, called phosphors, will give off light for a short time
after you shine light on them
 The process of emitting light for some time after receiving energy from
another source is called phosphorescence
 e.g., glow-in-the-dark paint (like on watch dials)
Light from Fluorescence:
 Fluorescence is the process of emitting light while
receiving energy from another source
 e.g., fluorescent light bulbs, CFLs
o electrical energy hits molecules of mercury
vapour, causing them to emit ultraviolet
(UV) energy
o the UV energy then hits a phosphor coating
on the inside of the tube, causing it to glow
white
o Process is about 20% efficient
Light from Chemiluminescence:
 The process of changing chemical energy into light
energy with little or no change in temperature is
called chemiluminescence
 e.g., glow sticks – two chemicals inside separated
from one another; when mixed, they give off light
until chemicals are used up
Light from Bioluminescence:
 The process of living creatures making themselves
luminous by using a chemical reaction
 Similar to chemiluminescence
 e.g., fireflies, glow worms, some fish
The Nature of Light
Light is known as an electromagnetic wave. Electromagnetic waves are very similar to water waves,
in that both involve the movement of energy from one point to another.
The wavelength of a wave is defined as the distance from one crest (or trough) of a wave to the next
crest (or trough).
Electromagnetic (EM) waves are invisible and can travel through a vacuum; unlike sound waves, for
example, they do not need particles in order to travel from one place to another.
EM waves will travel through a vacuum at the speed of light – 3.00 x 108 m/s
The Electromagnetic Spectrum:
The electromagnetic spectrum is a diagram that illustrates the range, or spectrum, of
electromagnetic waves in order of their wavelength. Notice that the range of visible light is just a tiny
part of the entire EM spectrum – furthermore, the colours of light are just different wavelengths
within the visible range.
The colour red has the longest wavelength of visible light, which is 700 nm (nanometers, or 10 -9
metres). Violet has the shortest wavelength of visible light, at 400 nm.