15.3 Optical Phenomenapp. 580 - 585

Mr. Richter

Agenda

Warm Up Discuss Lab Notes:

Total Internal Reflection Atmospheric Refraction and Mirages Dispersion and Rainbows

Objectives: We Will Be Able To… Describe the conditions in which light undergoes total

internal reflection. Recognize the atmospheric conditions that cause

refraction. Explain dispersion and phenomena such as rainbows in

terms of the index of refraction and wavelength.

Warm-Up:

Which material has the higher index of refraction?

Total Internal Refraction

Total Internal Reflection Total internal reflection:

the ray of light within one material is reflected at a boundary as if it were a perfect reflector.

This only occurs when light travels from a medium with a higher index of refraction to a lower index of refraction. Water to air, for example.

Total Internal Reflection

At the critical angle (θc), the light is refracted parallel to the boundary.

If the angle of incidence is greater than the critical angle, the light is internally reflected.

Calculating the Critical Angle

An adaptation of Snell’s law can be used to calculate the critical angle. Use Table 15.1 p564

Practice Problem

Find the critical angle for a water-air boundary.

Given: ni = 1.333, nr = 1.00

sin θC = 1.00/1.333

θC = sin-1 (1.00/1.333) = 48.6°

Fiber Optics

Fiber optic cables are thin, bendable pieces of glass wound together.

The glass allows total internal reflection of light over long distances, with very little loss of light.

Allows a very quick transfer of information with very little loss of clarity.

Atmospheric Refraction

Atmospheric Refraction

When we can’t actually see the sun, but we can still see light.

The atmosphere has a different index of refraction than the vacuum of space.

Light from the sun refracts through the atmosphere to our eyes.

Mirages When air in one place is a

significantly hotter than the air above it (like a desert) it is as if there are two different materials. Boundary!

The light will refract through the hotter air. Our brains assume this inverted

image is a reflection off of water, which it isn’t.

Dispersion

Dispersion

The index of refraction depends on the wavelength of light.

Or in other words: Light bends differently depending on its wavelength, and by extension, its color.

This is how prisms separate light. Blue light has a shorter

wavelength than red, so it bends at a greater angle of refraction.

Rainbows

Dispersion is most easily observed in nature as rainbows.

Water droplets act as prisms and disperse components white light at different angles.

Wrap-Up: Did we meet our objectives?

Describe the conditions in which light undergoes total internal reflection.

Recognize the atmospheric conditions that cause refraction.

Explain dispersion and phenomena such as rainbows in terms of the index of refraction and wavelength.

Homework

p 585 #1-4 Review Lens Slides (15.2)