The document summarizes key concepts in optics and optical properties of materials. It discusses topics like electromagnetic radiation spectrum, optical classifications of materials as transparent, translucent or opaque. It also covers concepts like reflection, refraction, absorption, transmission and how they relate to the band structure and band gaps of materials. Specific phenomena like fluorescence, phosphorescence, photoelasticity and their working principles are defined. Applications of optics like lasers, optical data storage are also briefly mentioned.

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.