15.3 Optical Phenomena pp 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 ( θ c ), 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: n i = 1.333, n r = 1.00  sin θ C = 1.00/1.333  θ C = 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

 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

 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)