Optics

Val Bennington

October 2008

Path of Radiation (Light)

Can be modified:

• Reflected

• Scattered

• Absorbed

Or path may remain the same:

• Transmitted

Reflection

• If reflected, all wavelengths hitting an object at one angle will be reflected along the same path

• Angle of incidence = Angle of reflection• Does not spread the colors of visible light!

Refraction - How Light is Scattered

• Separates different colors!!!• Path through object DEPENDS ON

WAVELENGTH !!!

Review on Visible Light

• Violets and blues are higher energy• Red has longest wavelength• Blue / violet has shortest wavelength

Snell’s Law

• Light slows when entering medium of higher refractive index

• This causes the path of the light ray to bend toward the normal

n1sin1 = n2sin2

Snell’s Law Continued…

n1sin1 = n2sin2

n depends on wavelength !

When exiting a medium, light bends AWAY from normal (reverse the ray directions)

Why is the sky Blue?

• The atmosphere scatters visible light, so we receive direct light (from the direction of the sun) and diffuse light (from all directions).

• The atmosphere more effectively scatters shorter wavelengths (blue) than longer wavelengths (red).

• Thus when the sun is high in the sky, away from the sun the sky appears blue.

• At sunset, sunlight must pass through a larger amount of atmosphere, enough so that red light is also scattered resulting in a red sky/clouds near the horizon.

Rainbows

Rainbows

• Notice that violet and blue light are most bent (scattered) in the raindrop

• Blue light and shorter wavelengths are MOST SCATTERED by raindrops and other aerosols in the atmosphere

• Red light is least bent or scattered

Rainbows cont.

Dispersion

• Light is dispersed when traveling through the raindrop

• In other words, colors are separated

• The path of each wavelength is changed, spreading the colors

Halos

• Light travels through ICE CRYSTALS when a halo is formed

• Ice crystals randomly oriented (not all facing one direction)

• Sun’s rays bent 22˚

• Sun’s rays appear TO YOU to be coming from around the sun

Sundogs (mock suns)

• If ice crystals mostly or all oriented with flat faces horizontal, sundogs appear

Hand covers actual sun

Other Phenomena

• Mirages (due to temperature gradient)• Green Flash• Sun Pillar• Coronas• Glories• Broken Bow

Extra Credit• Up to five points to write a paragraph about one of: Sun Pillar, Coronas, Glories, Broken Bow, or Green Flash• Draw a diagram of the observer, the light source, and how the

light is bent, reflected, or scattered• Expect more than the couple sentences in the book, although

feel free to use the textbook as a start• Make sure to explain how and why the light is bent and why

light appears to be coming from a certain direction to the observer

• List references on your extra credit sheet (make sure if it is from the internet, that it is from a university or research website)

• Hand in your Extra Credit at start of class next week