Transcript METR125: Light and color

METR125: Light, Color, and
Atmospheric Optics
http://apollo.lsc.vsc.edu/classes/m
et130/notes/chapter19/color.html
Why do objects have color?
• objects appear to
have color since they
are able to selectively
absorb and reflect
certain wavelengths
of visible light.....
• The sun emits white
light - the sum of all
wavelengths (colors)
of visible light
together
White Object
• An object will appear
white when it does
not absorb any
wavelength of visible
light.... it is all
scattered
• hence, the object will
appear white
A Red Object
• An object will appear
red when it absorbs
all wavelengths of
visible light except for
red....
• red light is scattered
to our eye, so the
object looks red
A black object
• an object will appear
black when it absorbs
all wavelengths of
visible light.
• therefore, no light is
scattered to our eye.
Processes affecting propagation
of electromagnetic radiation
what can happen to visible light as it passes
through the atmosphere?
Class Discussion
Processes affecting propagation
of electromagnetic radiation
what can happen to visible light as it passes
through the atmosphere?
• Reflection
• Scattering
• Transmission
• Refraction
• Diffraction
Reflection of Light
• light can simply be
reflected
• the incident angle (a)
will equal the
reflected angle (b)
Mie Scattering
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Light can be scattered.....redirected in
many directions
We will consider two types of
scattering that occur in the
atmosphere
Mie Scattering
large particles in the atmosphere are
able to scatter all wavelengths of white
light equally
when all wavelengths of white light are
scattered equally, then Mie scattering
is occurring
this is why clouds appear white....
however, if a cloud is optically thick,
then little light will penetrate through
the cloud....
when little light can penetrate to a
particular location in a cloud, such as
cloud base, how will it look?????
Rayleigh Scattering (Scattering,
continued)
Why is the sky blue?
• It's because of Rayleigh scattering
- the selective scattering of the
shorter wavelengths of visible light
(violet and blue) by atmospheric
gases.
• Note that Rayleigh scattering
involves much smaller scattering
particles than Mie scattering
Class Think;
• if there were no atmosphere, what
color would the sun look like?
Class Participation Activity
Rayleigh and Mie Scattering
what processes explain:
• blue sky
• haze
• white cloud
• dark areas in cloud
Crepuscular Rays
• bright light beams due to Mie scattering by haze
and dust
• seen most often near sunrise and sunset
what causes red sunsets and clouds to
appear orange/red near sunset?
orange/red sunsets in a dirty
atmosphere
• when pollution is present, the
atmosphere contains more
particles such as aerosols
having larger diameters than
the atmospheric gases
• hence, more of the
intermediate wavelengths of
visible light such as yellow and
green are scattered in addition
to the blue light
• what largely remains is red
light...., hence the sun appears
red.
Why do clouds also appear red in
sunset?
Transmission of light
• occurs when light
passes through an
object
Refraction of Light
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is the bending of light as it passes
from one medium to another with
different densities.
amount of refraction depends
upon:
– density of the materials
– angle at which the light enters
the material
– wavelength - causes colors of
white light to separate when
passing through a prism.
light will bend away from the
normal when passing into a less
dense medium
light will bend toward the normal
when passing into a more dense
medium