METR125: Light, Color, and Atmospheric Optics

http://apollo.lsc.vsc.edu/classes/met130/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 • 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?

Rayleigh and Mie Scattering

what processes explain:• blue sky • haze • white cloud • dark areas in cloud

Class Participation Activity

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 • 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