This document discusses the nature and propagation of light. It covers topics such as: - Light behaves both as a particle (photon) and wave (electromagnetic wave) - The speed of light in a vacuum is approximately 300,000 km/s - Sources of light include incandescent sources like the sun and luminescent sources like fluorescent lights - Light propagates via reflection, when it changes direction at interfaces between mediums, and refraction, when it passes from one medium to another at an interface.

1. Sonny P. De Leon
Physics 2 Teacher
The Nature and
Propagation of
Light

2.  Optics- branch of physics which deals with the
behavior of light and other electromagnetic waves.

3. Light, form of energy visible to the human
eye that is radiated by moving charged

4. What is the nature of light?
 Scientists have learned through experimentation
that light behaves like a particle at times and like
a wave at other times.
 The particle-like features are called photon.
 Photons are different from particles of matter in
that they have no mass and always move at the
constant speed of about 300,000 km/sec
(186,000 mi/sec) when they are in a vacuum.
 When light diffracts, or bends slightly as it passes
around a corner, it shows wavelike behavior.
 The waves associated with light are called
electromagnetic waves because they consist of
changing electric and magnetic fields.

5. The Real Nature of Light
 The propagation of light is best described by a
wave model, but understanding emission and
absorption requires a particle approach.

6. The Nature of Light
Wave Model of Light Particle Model of Light

7. Absorption and Emission of
Light
When a photon, or packet of
light energy, is absorbed by
an atom, the atom gains the
energy of the photon, and
one of the atom’s electrons
may jump to a higher energy
level. The atom is then said
to be excited. When an
electron of an excited atom
falls to a lower energy level,
the atom may emit the
electron’s excess energy in
the form of a photon. The
energy levels, or orbitals, of
the atoms shown here have
been greatly simplified to
illustrate these absorption
and emission processes.

8. Sources of Light
Sources of light differ in how they provide
energy to the charged particles, such as
electrons, whose motion creates the light.
If the energy comes from heat, then the
source is called incandescent. If the
energy comes from another source, such
as chemical or electric energy, the source
is called luminescent.

9. Sources of Light
 Incandescent
source of light
--- Sun and all
other stars,
candle light,
incadescent bulb,
black body and all
other thermal
sources.
 Luminescent source of
light
--- Flourescent light that
make use of chemical
compounds called
phosphors, light in T.V
set, aurora borealis and
aurora australis
• Chemiluminescence occur
s when a chemical
reaction produces
molecules with electrons
in excited energy levels
that can then radiate light.
• When chemiluminescence

10. Electrons in the solar wind that
sweeps out from the Sun
become deflected in Earth’s
magnetic field and dip into the
upper atmosphere near the
north and south magnetic poles.
The electrons then collide with
atmospheric molecules, exciting
the molecules’ electrons and
making them emit light in the

11. Other sources of Light
 In a synchrotron light source, electrons are
accelerated by microwaves and kept in a circular
orbit by large magnets. The whole machine,
called a synchrotron, resembles a large artificial
atom. The circulating electrons can be made to
radiate very monochromatic light at a wide range
of frequencies.
 A laser is a special kind of light source that
produces very regular waves that permit the light
to be very tightly focused. Laser is actually an
acronym for Light Amplification by Stimulated
Emission of Radiation.

12. No matter what is source,
electromagnetic radiation travels in a
vacuum at the speed of light.

13.  The first demonstration that the speed of light
is not infinite was made in 1876 by the Danish
Astronomer Ole Romer, from the observation
of one Jupiter’s satellite
 The first successful terrestrial measurement of
the speed of light was made by French
scientist Armand Fizeau in 1849 using a
reflected light beam by a notched rotating
disk.
 More refined versions of this experiment were
carried out later in the 19th century by Jean
Foucault in France and Albert A. Michelson in
the United States.

14.  From analysis of all measurements up to
1831, the most probable value for the
speed of light at that time was
c = 2.99792458 x 108 m/s.

15. Reflection and Refraction
 When a light wave strikes a smooth interface
separating two transparent materials ( such as air
and glass or water and glass), the wave in
general partly reflected or refracted ( transmitted)
into second material.
 Interface – is the line or plane between two
different material mediums.

16. Example: When you look into a restaurant
window from the street, you see a reflection of
the street scene, but a person inside the
restaurant can look out through the window at
the same scene as light reaches him by
refraction.

17. Optics- branch of physics which deals with the
behavior of light and other electromagnetic
waves.
Geometric Optics – branch of optics
representing waves in terms of rays.
Physical optics- branch of optics dealing
specifically with wave behavior.

18.  If the interface is rough, both the transmitted and
reflected light are scattered in various directions,
and there is no single angle of transmission or
reflection. Scattered reflection from a rough
surface is called diffuse reflection.

19.  Reflection at a definite
angle from a very
smooth surface is called
specular reflection.

20. Geometric optics
Light from the
source is
called
incident ray,
the ray that is
reflected is
the reflected
ray. The
reference line
used for
measuring
angles ia a
line that is
normal or
perpendicular
to the
surface. The
line is normal
line.