Optical Properties of Materials

ELECTROMAGNETIC WAVE

A transverse wave of mutually perpendicular, time-

varying electric and magnetic fields that propagate at

constant speed, c, in vacuum

electromagnetic radiation can have both wave-like and

particle-like properties

Travels as a wave

Reflection, refraction, diffraction, interference

Interacts with matter like a particle, photon

Photoelectric effect

LIGHT

Light Interactions with Solids

So materials are broadly classified astransparent: relatively little absorption and reflectiontranslucent: light scattered within the materialopaque: relatively little transmission

Optical Properties of METALS• almost any frequency of light can be

absorbed.

• practically all the light is absorbed within about 0.1μm of the surface.

Optical Properties of METALS• So what happens to the excited atoms in the surface layers

of metal atoms?– they relax again, ………… – a photon (as REFLECTED LIGHT)

• The energy lost by the descending electron is the same as the one originally incident

• So the metal reflects the light very well – metals are both opaque and reflective– the remaining energy is usually lost as heat

Optical Properties of Non-METALS

Non-metals can be opaque or transparent to visible light‒ Reflection and Absorption‒ Refraction and Transmission

Optical Properties of Non-METALSSemiconductors and insulators behave essentially the same

way, the only difference being in the size of the ………...

‒ If Egap < 1.8 eV

‒ full absorption; color is

black (Si, GaAs)

‒ If Egap > 3.1eV

‒ Transmission ;

transparent (diamond)

‒ If 1.8 eV < Egap < 3.1eV

‒ partial absorption;

material has a color.

Optical Properties of Non-METALSTRANSMISSION

refers to the passage of light through a medium• For an incident beam I0 that impinges on the front surface of a

specimen with thickness l and absorption coefficient β the transmitted intensity IT is

• Transmitted light depends on losses incurred by absorption and reflection

• Intensity of transmitted light decreases with distance travelled (thick pieces less transparent!)

l

T eRII 22

0 1

Optical Properties of Non-METALS

• Transmitted light distorts electron clouds.

Light is slower in a material medium than in vacuum.

AirNew Medium

Speed = C

Speed = V

REFRACTION

• Bending of light due to a change in velocity

-Adding large, (Lead) heavy ions can decrease the speed of light.

vmaterial ain light of speed

vacuumain light of speed (n) refraction ofIndex

c

Optical Properties of Non-METALS

Optical Properties of Non-METALS

INDEX OF REFRACTION

cc

vn

where λ is the wavelength and ν is the frequency

When light is refracted

• its speed and wavelength λ are changed

• the frequency ν does NOT change

Optical Properties of Non-METALS

REFRACTION

c

v

cn

RED LIGHT – longest λ, smallest n, least refracted

VIOLET LIGHT - smallest λ, greatest n, most refracted

n depends on the crystal structure of the material

Optical Properties of Non-METALS

DISPERSION

medium 1medium 2

n1

n2

1q

angle ofincidence

2q

angle ofrefraction

Snell’s Law: n1sinq1 n2sinq2

Optical Properties of Non-METALS

n1, smaller n2, larger

Ray bends toward normal

Ray bends away

from normal

n1, larger n2, smaller

Optical Properties of Non-METALS

REFLECTION

2

12

12tyReflectivi

nn

nn

Optical Properties of Non-METALS

ABSORPTION

Mechanisms:

1. Electron polarization

2. Valence band-conduction band transition

Optical Properties of Non-METALS

ABSORPTION

Optical Properties of Non-METALS

• Color determined by sum of frequencies of--transmitted light,--re-emitted light from electron transitions.

Ex: Ruby = Sapphire (Al2O3) + (0.5 to 2) at% Cr2O3

-- Pure sapphire is colorless(i.e., Egap > 3.1eV)

-- adding Cr2O3 :• alters the band gap• Result: Ruby is deep

red in color.

Optical Properties of Non-METALS

OPACITY AND TRANSLUCENCY IN INSULATORS

• Even after the light has entered the material, it might yet be reflected again due to scattering inside the material• so a beam of light will spread out or an image will become blurred•In extreme cases, the material could become opaque due to excessive internal scattering

Scattering can come from obvious causes:• in poly-crystalline materials•fine pores in ceramics•different phases of materials

• http://www.columbia.edu/itc/chemistry/chem-c1403/lectures/_C1403_Lecture7_100404.ppt

• http://en.wikipedia.org/wiki/Ultraviolet_catastrophe

• http://nbsp.sonoma.edu/resources/teachers_materials/physical_01/light/light.ppt

• How Things Work by Louis Bloomfield

• http://www.molphys.leidenuniv.nl/monos/smo/basics/images/wave_anim.gif

• users.encs.concordia.ca/~mmedraj/mech221/lecture%2024.pdf

• http://www.kumc.edu/ophthalmology/timberlake/lectures/1-Light%20&%20Refraction.ppt

• www.kyc.edu.hk/studteach/teacher/hlt/userfiles/11-4_refraction_of_light.ppt

• Materials Science and Engineering: An Introduction, 6th Edition by Callister