15.3 Optical Phenomena

1. 15.3 Optical Phenomena pp. 580 - 585 Mr. Richter

2. Agenda • Warm Up • Discuss Lab • Notes: • Total Internal Reflection • Atmospheric Refraction and Mirages • Dispersion and Rainbows

3. 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.

4. Warm-Up: • Which material has the higher index of refraction?

5. Total Internal Refraction

6. 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.

7. 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.

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

9. 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°

10. 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.

11. Atmospheric Refraction

12. 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.

13. 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.

14. Dispersion

15. 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.

16. Rainbows • Dispersion is most easily observed in nature as rainbows. • Water droplets act as prisms and disperse components white light at different angles.

17. 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.

18. Homework • p 585 #1-4 • Review Lens Slides (15.2)