10.1 Sources and Nature of Light
Light Emissions
• For all light sources,
atoms absorb some
form of energy.
• Excited atoms release
energy, often in the
form of light.
Light from the Sun
• H atoms fuse together
to form He (fusion
reactions).
• Energy released from
fusion excites gas atoms
on outer layers.
• Excited atoms emit
light.
Light from Incandescence
• Incandescence = light
emitted from material
because of high
temperature
• Incandescent light bulbs
have tungsten (W)
wires.
• 95% of electrical energy
released as heat.
Light from Electric Discharge
• Sodium vapour bulbs
(som streetlights) have
Na + a small amount of
Hg vapour inside.
• Electric current moves
between electrodes at
each end , exciting gas
atoms.
Fluorescence
• Fluorescent bulbs are
coated with phosphor &
have Hg vapour inside.
• Electric current exites
Hg atoms, which emit
UV light (not visible).
• Phosphor absorbes UV
& emits visible light
(phosphorescence).
Efficiency of Fluorescent Lights
Uses of Fluorescence
Luminescence
• Luminescence is light
generated without
heating an object, e.g.,
fluorescence.
• Phosphorescence is
light emitted due to
exposure to UV light,
that continues to be
emitted.
Chemiluminescence
• Light generated by
energy released in a
chemical reaction.
Bioluminescence
• Light produced by living
organisms due to
chemical reactions in
the living cells.
The Nature of Light
• Light energy travels in
electromagnetic waves
(involve electric &
magnetic fields).
• Wavelength = distance
from one crest of a
wave to the next
Electromagnetic Spectrum