Light Waves
Interacting with Matter
(Chapter 7)
Student Learning Objective
Determine the behavior of electromagnetic
waves as they interact with different types
of matter.
How does “light” interact with matter?
Light interacts differently
with different types of
matter.
1) Surface
2) Material
3) Angle
4) Wavelength
Absorption
Some opaque materials
absorb light.
Practice
1) What is an opaque material?
2) Do all opaque materials absorb light?
3) If a material absorbs radiant energy,
what is the result?
Transmission
Light travels through
transparent materials.
 Absorbed & re-emitted
 Passes through
Air
Light
Ray
Glass
Air
What is the law of reflection?
Reflection
Light may bounce from
a surface back into the
original medium.
Parallel Reflection
Law of reflection
 Incoming light rays and outgoing light rays have the
same angle with respect to the normal line.
I R
Mirror
The law of reflection applies to all reflective surfaces.
Diffuse Reflection
Practice
Does the law of
reflection apply to
diffuse reflection?
Virtual Image
A flat mirror reflects
all light rays in the same
direction.
1)
2)
3)
4)
Upright
Same size
Same distance
Left-right reversed
Curved Mirrors
 Convex mirrors always
produce a smaller image.
 Concave mirrors usually
produce a larger image.
Practice
1) Which mirror has the larger field of view,
concave or convex?
2) What kind of mirror is the side mirror on your
car?
When and how is light refracted?
Refraction
Light will change speed
and may change direction
when it is transmitted
from one material to
another.
• Wavelength
• Angle of incidence
• Material (n = c/v)
• Temperature
Refraction affects where objects appear to be.
Object sizes may be magnified.
Moon Looks large
The Sunset
Practice
1) In which substance, would light travel faster, ethyl
alcohol or air?
2) Calculate the speed of light in ethyl alcohol.
3) As the light goes from air into the ethyl alcohol, will
it bend toward or away from the normal line?
When does total internal reflection occur?
Light approaching a boundary at the critical angle
is reflected rather than transmitted.
Fiber Optics & Diamonds
How is a rainbow formed?
Dispersion
 Visible light is divided
into separate colors.
Each person sees their own set of colors (rainbow)
from a particular set of raindrops.
Sunlight
Practice
1) Which color of light is refracted the most in a
raindrop?
2) How is a secondary rainbow formed?
What do lenses do to light?
Convex Lens
(true image)
Concave Lens
(virtual image)
Question
What type of lens is the human eye?
http://www.mysciencesite.com/optics4.html
What is polarized light?
 Light is many transverse waves vibrating in many directions.
 Polarized light has only one direction of vibration.
Practice
1) How could you determine whether sunglasses
are truly polarized?
2) Do polarized glasses make a difference?
http://www.prescriptionglassesonline.net/cheapglasses/?help122.html
What causes diffraction?
Diffraction
Diffraction occurs when
light waves bend around
corners.
The amount of
diffraction depends on
the how the size of the
wavelength compares to
the size of the barrier.
Shadows
How are colors produced?
Most of the colors you see in the world are due to
selective reflection.
Daytime
Nighttime
Practice
1) What does selective reflection mean?
2) What is selective transmission? What is an
example of this?
3) Why does the sky appear to be blue mid-day?
The sky on Earth appears
blue because blue (and
violet) photons are
scattered as they collide
with air particles.
The sky on Earth appears
red at sunset because the
light must pass through a
lot of atmosphere.
The Sun’s
Rays in Space
Practice
1)Both blue and violet photons are scattered by
our atmosphere. Why does our sky appear to
be mostly blue, and not violet, at mid-day?
2)What color would our sky be if atmospheric
particles were slightly larger?
More Practice
3) Why is the sky black on
the moon?
4) Why do stars twinkle?
Wien’s Law
Some objects have a particular color because of
temperature.
Every object has a specific wavelength (color) at
which it radiates most of its energy.
T(Kelvin) = 3,000,000
lmax (nm)
Practice
1)Our Sun has a surface temperature of 5,800 K.
What is the wavelength of maximum emission
for our Sun? What color is this?
2)What is the surface temperature of a star for
which the wavelength of maximum emission is
425 nm? What color would this star be?
More Practice
3) If the burner on your electric stove is 500 ºF
(260 ºC), what is the wavelength of maximum
emission? What “color” corresponds to this
wavelength?