Light
27.4-27.6
Review
• Are light waves and sound waves part of the same
scale?
– No. Sound waves are mechanical, need a medium, and
vibrate matter. Light waves are part of the
electromagnetic scale and do not need a medium.
• What is the speed of light?
– 300,000 km/s
• Does light make up a large part of the
electromagnetic spectrum?
– No. Less than 0.1%
Objectives
1. Explain how the f of light affects what
happens when it enters a substance.
2. Interaction of UV rays and glass
3. Opaque vs. transparent materials
4. Solar and lunar eclipses
What is light?
• Light:
– vibrating electric charges
Light interacts with matter
• Just as a sound wave can force a sound receiver into
vibration, a light wave can force charged particles in
materials into vibration
• When incident upon matter --> electrons in matter vibrate
Response of Receiving Material
• Depends on f of light
& natural f of material
• Light acts differently
when it is incident
upon a window
compared to a rock
Transparent Materials
• Transparent:
– Allows light to pass through
• Visualize this…
• Electrons of an atom in glass connected to springs (fig 27.6)
• Light wave hits them they vibrate
Transparent Materials
Can UV light pass through glass?
• E’s in glass have same natural f as UV B light
• What happens when you have two things with identical natural
frequencies?
• Resonance!
• Large amplitude causes atoms to hold on to energy for long time
(100 millionth of a second!)
• Atom make many collisions with other atoms --> gives up energy
in form of heat
• Most UV B light can NOT pass through!
Transparent Materials
Can UV light pass through glass?
1.
2.
3.
4.
5.
UV B & glass --> same natural f
Resonance
Atoms collide
Energy loss --> heat
Most UV B light can’t pass!
Does that mean I can’t get sunburned through glass?
NO!
•You CAN get sunburned through glass
•Let’s learn the basics about UVA and
UVB rays first
UVB rays
•UVB rays:
•Short wavelength (290-320
nanometers)
•Responsible for sunburn
•Main cause of basal cell
and squamous cell skin
cancers
•Most of these rays are
absorbed by glass, but NOT
ALL!
UVA Rays
•UVA rays:
•Long wavelength (320-400
nanometers)
•Penetrates more deeply on
skin
•Responsible for aging,
wrinkling and leathering
(breaks down collagen and
elastin)
•Exacerbates UVB’s effects
•May directly cause
melanoma and other cancers
•Passes through glass!
Bottom Line
• You can get sunburned through glass, just
not as quickly
• Glass is just like sunscreen, it protects you
pretty well, just not perfect
• Most glass blocks out UVB radiation, but not
UVA
• Where sunscreen that protects against both
UVA and UVB radiation!
Transparent Materials
Visible light Has Lower Natural Frequencies than UV light
•
•
•
•
•
Visible light
Smaller amplitudes -->
Atom holds energy for less time
Less collision
Energy reemitted as visible light w/ slight time delay
Transparent Materials (glass)
Light Has Lower Natural Frequencies
•
•
•
•
Visible light -->
Lower f & smaller amplitude -->
Less collisions -->
Reemitted from glass with slight delay
Why the time delay in glass?
Checkpoint
1. What happens when light is incident upon matter?
•
Causes the electrons to vibrate
2. Why are most UVB rays not able to pass through
glass?
•
UV has same natural frequency as glass, atoms resonate
and collide, turned into heat.
3. Why is there a time delay when visible light passes
through glass? And which color, red or violet, takes
longer to get through the glass?
•
there is a delay in between atoms in the glass. Light wave
hits one atoms, it vibrates, then spits out the light to the
next atom . . .etc. Violet takes longer to get through the
glass b/c it’s natural frequency is closer to glass than red
light.
When light is incident upon a
material it can . . .
1. Bounce off (reflect)
2. Be transmitted through
3. Be absorbed by the material
Opaque Materials
• Opaque:
– Absorb light w/o reemission
• What happens if energy is not
reemitted?
• Vibrations given to electrons
by incident light can last for a
long period of time --> many
collisions --> energy
converted to heat
• Material becomes warmer
Atmosphere and Clouds
• Does our atmosphere let
UV light in?
• Some, but most
absorbed by ozone
• Can we get sunburned
on a cloudy day?
• Yes, clouds are
semitransparent to UV
light
Why is the sky blue? Why are
sunsets red?
• Why is the sky blue
and a sunset red?
Water tank/powdered milk
demo
Checkpoint
• Why is a black tar road hotter to the touch than a
pane of window glass?
– Sunlight is absorbed and converted to internal energy
in the road surface, but is transmitted through the
glass to somewhere else
• Why is glass transparent to visible light, but
opaque to most UVB rays?
• Natural frequency of electrons in the glass match that of uv
light --> resonance occurs. This generates heat instead of
wave remission --> opaque to uv light.
• Different frequency for visible light --> transmits, not
absorbed
Shadows
•Ray:
•thin beam of light
•Shadow:
-formed where light
rays can NOT reach
Umbras vs. Penumbras
• Umbra:
– Total shadow
• Penumbra:
– Partial shadow
– Some light blocked,
other light fills in
Solar Eclipse
• Moon between earth and
sun
• Moons shadow barely
reaches earth
• If directly in umbra-->
total darkness
• If in penumbra --> partial
eclipse
• When can I see one?
• Flash animation on cdROM (Ch. 26 Properties
of Light fig. 26-12 and 2613)
Lunar Eclipse
• Earth between sun and moon
• Earth casts shadow to space
• Which are more common?