LIGHT
CHAPTER 12
SECTION 2
WHAT IS LIGHT?
 All
frequencies or wavelengths of
electromagnetic radiation
 Seven bands of electromagnetic waves,
based upon differences in frequency and
wavelength.
 http://www.colorado.edu/physics/2000/wav
es_particles/
 http://imagers.gsfc.nasa.gov/ems/waves3.h
tml
 http://imagine.gsfc.nasa.gov/docs/science/k
now_l1/emspectrum.html
WHAT CAUSES ELECTROMAGNETIC
WAVES (OR LIGHT)?
Vibrating electrically charged particles
 A changing electric field sets up a changing
magnetic field, which sets up a changing electric
field, and so on.

Electromagnetic wave (a
combination of electric
and magnetic fields
ELECTROMAGNETIC WAVE
http://www.nrc-cnrc.gc.ca/obj/inmsienm/images/research_images/optical_comb/COMBFIG1.gif
27.3 Electromagnetic Waves
Light is a portion of the family of electromagnetic waves that
includes radio waves, microwaves, and X-rays.
The range of electromagnetic waves is called the electromagnetic
spectrum.
HOW DO WE MODEL LIGHT?
TWO


MODELS
Wave Model
Particle Model
 Physicists
recognize the dual nature of
light (light travels like a wave, but hits
like a particle).
 Photon: a massless particle of light
(carries energy—greater the frequency,
the greater the photon energy)
27.4 Light and Transparent Materials
A light wave incident upon a pane of glass sets up vibrations in
the atoms. Because of the time delay between absorptions and
reemissions, the average speed of light in glass is less than c.
INTENSITY OF LIGHT
An increase in the number of waves or photons
(or the amount of light)
 Brightness of light means intensity.
 Not related to frequency or the energy per
photon!

Same frequencies
of light, just
different
intensities!
100 Watts
40 Watts
SPEED OF LIGHT
Nothing in the Universe travels faster than the
speed of light.
 Speed

3 x 108 m/s
 300,000 km/s
 186,000 mi/s

WHAT AFFECTS THE SPEED OF
LIGHT?
Speed of light is affected by the type of medium it
travels through.
 In general, light travels fastest through a
vacuum, then gases, liquids and solids.
 This is opposite of mechanical waves. Sound
waves travel fastest through solids, not gases.

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PHOTOELECTRIC EFFECT: THE EJECTION OF
ELECTRONS FROM CERTAIN METALS WHEN EXPOSED
TO CERTAIN FREQUENCIES OF LIGHT.
Electromagnetic radiation
(visible light range)
Electrons emitted
Photovolatic cell
(solar cell)
Produces electron flow (electricity)
ENERGY TRANSFORMATIONS OBSERVED WITH
PHOTOVOLTAIC CELL, MOTOR AND PROPELLER
Visible light absorbed by photvoltaic cell (light
energy to electrical energy)
 Electric motor causes propeller to spin (electrical
energy to mechanical energy)

LIGHT AS A WAVE

Light interacts with light.


Interference
Light interacts with matter.
Reflection
 Refraction
 Diffraction

LAW OF REFLECTION
Angle of incidence = angle of reflection
29.2 The Law of Reflection
Incident rays and reflected rays make equal angles with a line
perpendicular to the surface, called the normal.
• The angle between the incident ray and the normal is the
angle of incidence.
• The angle between the reflected ray and the normal is the
angle of reflection.
• Angle of incidence = Angle of reflection
29.4 Diffuse Reflection
Diffuse reflection allows us to see most things around us.
a. Light is diffusely reflected from paper in many directions.
b. Light incident on a smooth mirror is only reflected in one
direction.
Ordinary paper has a rough surface
when viewed with a microscope.
CONCAVE AND CONVEX MIRRORS
Concave mirror causes
light rays to diverge.
Convex mirror
causes light rays to
converge.
29.8 Refraction of Light
As a light wave passes from air
into water, its speed decreases.
29.12 Total Internal Reflection
Optical Fibers
Optical fibers, sometimes called light pipes, are transparent fibers that pipe light
from one place to another.
They do this by a series of total internal reflections.
Optical fibers are useful for getting light to inaccessible places.
Mechanics and machinists use them to look at the interiors of engines, and
physicians use them to look inside a patient’s body.
31.1 Diffraction and Huygens’ Principle
The extent to which the water waves bend depends on the size of the
opening.
LIGHT ACTS AS A PARTICLE
Light contains energy, and the energy is located
in a particular place and can be thought of as
particles.
 These discrete, packets of light energy are called
photons.
 Photons have no mass.
 As the frequency increases, so does the energy
per photon.
