April 3, 2014
What we call “light” is merely a small fraction of the total
electromagnetic spectrum.
The electromagnetic spectrum
Consists of transverse waves of varying frequencies
Occur due to oscillation of electric and magnetic fields
Do not require a medium
Travel at the speed of light (c = 3.00 X 108 m/s) in a vacuum
Like any wave, an EM
wave carries what?
Energy!
Ancients believe that we could see because of light beams that left
our eyes and hit objects. What day-to-day experiences tell you that
isn’t so?
We see when light when visible light enters our eyes and focuses on
our retinas. The light may come from a luminous source
(something that emits light, such as the sun or a light bulb) or may
bounce (reflect) off an illuminated object.
Light
Luminous
source
Light
Illluminated
object
We have photoreceptor cells in our retinas that send a signal to our
brain whenever light hits them.
• Rods are sensitive to all wavelengths of visible light
 provide black and white vision
• Cones are sensitive to only certain wavelengths
 provide color vision
We have three cones types:
blue, green, red.
Our brain interprets color based on the
combinations of cones that are
triggered by certain light.
… if only the red cone is triggered, we see
red
… if red and green are triggered equally,
we see yellow
Different materials absorb, reflect, and/or transmit different
wavelengths of light.
•
•
•
Absorption – light energy that has the same natural frequency as
the electrons in the object will be absorbed – the energy will be
converted into vibrational motion of the electrons.
Reflection – the light ‘bounces off’ the surface of the object
Transmission – the light goes through the object and out the
opposite side
Which type of light hits our eyes?
Reflected or transmitted light!
Transparent objects transmit most of the light that hits them.
Translucent objects transmit some of the light that hits them.
Opaque objects reflect or absorb most of the light that hits them.
How would the following objects appear?
1. An object that reflects blue and green light; absorbing other
wavelengths. Opaque blue-green
2. An object that reflects all visible light waves.
Opaque white
3. An object that absorbs all visible light waves.
Opaque black
4. An object that reflects pink light but transmits all other
wavelengths.
Translucent pink
5. An object that transmits most of the light hitting it.
•
transparent
Waves of any sort – light, sound, water, etc. –
exhibit different behaviors when they
encounter a barrier.
• Reflection – bouncing back of a wave as it
encounters a new medium
• Refraction – the bending of a wave as it
transmits through a different medium
• Dispersion – The separation of a wave
into its component frequencies
• Diffraction – the spreading of a wave
behind an obstruction
Law of Reflection
Angle of reflection is
equal to angle of
incidence.
i =
r
(the angles are measured to
the normal to the barrier).
All waves, including light, sound, water obey this relationship,
the law of reflection.
When a wave passes from one medium to another, its
velocity changes. The change in speed results in a change
in direction of propagation of the refracted wave.
When a wave passes from one medium to another,
its velocity changes. The change in speed results in a
change in direction of propagation of the refracted wave.
Visualization of refraction
As a toy car rolls from a
hardwood floor onto carpet,
it changes direction because
the wheel that hits the
carpet first is slowed down
first.
Light –
faster in air than water
Sound –
Faster in water than air
v1
v2
f=
=
λ1
λ2
Frequency is determined by the source so it doesn’t
change. Only wavelength changes. Wavelength of the
same wave is smaller in the medium with smaller speed.
We can predict the degree of refraction using Snell’s Law.
Snell’s Law states that the ratio
θ1
θ2
is constant for the given frequency.
The Snell’s law is of course valid
for all types of waves.
Greater speed – greater angle
Slower speed – slower angle
www.le.ac.uk/ua/mjm33/wave2/images/Snell.gif
Turn & Talk
1) Why is there no bending of
light in the first animation?
2) Is the block a slow medium or
a fast medium? Justify your
response.
1) You see a translucent purple ruler. What frequencies of
light are absorbed? What are reflected? What are
transmitted?
2) Draw a reflected wave, labeling the angle of incidence and
angle of reflection. Also compare the size of the two angles.
3) Draw a light wave passing through two different mediums,
first fast then slow. Label the angles of incidence and angles
of refraction.