2. Optical Properties

3. Electromagnetic Wave Propagation

6. Applications of various waves/photons

8. Optical Classification Intensity of the incident beam =Sum of the intensities of the transmitted , absorbed , and reflected beams. Materials with little absorption and reflection are transparent . You can see through them. Materials in which light is transmitted diffusely are translucent . Objects are not clearly distinguishable. Materials where light is absorbed and reflected are opaque .

12. Absorption in Metals • Absorption of photons by electron transition: • Absorption is usually very small (less than 5%) • Metals have a fine succession of energy states. • Near-surface electrons absorb visible light. Adapted from Fig. 21.4(a), Callister 6e .

13. Absorption in Metals Most of the absorbed radiation is re-emitted from the surface, less than 0.1 micron. Only very thin films of metals are transparent to visible light. Metals are only “transparent” to high frequency radiation ( x - and gamma -rays). A bright silvery color when exposed to light indicates that the metal is highly reflective: number & frequency of incoming photons is ~ equal in the incident and reflected beam (Al, Fe, Ti, Ag) . In some metals, short wavelength radiation ( green , blue , violet ) is not re-emitted. They appear red-orange or yellow (Cu, Au).

15. More semiconductors/insulators • Absorption by electron transition occurs if h  > E gap • If E gap < 1.7eV, full visible absorption, black or metallic • If E gap > 3.1eV, no visible absorption, transparent • If E gap in between, partial visible absorption, colors incident photon energy h  3.1 eV 1.7 eV

19. Methods of Photon Absorbtion

21. LIGHT INTERACTION WITH SOLIDS • Incident light is either reflected, absorbed, or transmitted: If photons of a certain color are absorbed, they obviously aren’t being transmitted (or reflected) to your eyes. This will affect the material color.

25. Light reflected and absorbed going through a material. “non-reflected beam:” “non-absorbed beam:” non-reflected beam 2:

26. COLOR OF NONMETALS in transmission • Color determined by sum of frequencies of --transmitted light, --re-emitted light from electron transitions. • Ex: Cadmium Sulfide (CdS) -- E gap = 2.4eV, -- absorbs higher energy visible light (blue, violet), -- Red/yellow/orange is transmitted and gives it color. • Ex: Ruby = Sapphire (Al 2 O 3 ) + (0.5 to 2) at% Cr 2 O 3 -- Pure sapphire is colorless (i.e., E gap > 3.1eV) -- adding Cr 2 O 3 : • alters the band gap • blue light is absorbed • yellow/green is absorbed • red is transmitted • Result: Ruby is deep red in color.