Chapter 7

Optical Microscopy

I.                   Nature of light

a.     Wave theory

                                                             i.      Visible light, as part of the electromagnetic spectrum travels in a straight line with a transverse wave motion. 

                                                          ii.      Vibrates at right angles to the direction of propagation like a drop in water

                                                       iii.      Wavelength is the distance from crest to crest

                                                        iv.      Amplitude is the height of the wave

                                                           v.      Frequency is the # of waves per second

                                                        vi.      Velocity is the frequency X wavelength.  

b.     Corpuscular theory – light is made of particles called photons

c.     Reflection and Refraction

                                                             i.      Reflection in the bouncing of light off of the surface of a different medium and Refraction is the bending of light through a medium

                                                          ii.      Laws of reflection

1.    The angle of incidence (i) equals the angel of reflection (r0, when both angles are measured from the surface normal

2.    The incident and reflected rays lie in the same plane

d.     Refractive index and Snell’s Law

                                                             i.      When light passes from one material into another it is either speeded up of slowed downs as a function of the differing atomic structures of the two materials

                                                          ii.      The refractive index (n or RI) of a substance is defined in terms of wave velocity:

1.    n = Wave velocity in a vacuum/ wave velocity in the material = c(constant) / v

2.    Air is similar to a vacuum  and n in a vacuum is 1 so n=1/v

3.    Snell’s law  sin I / sin r = n2/n1 = v1/v2

4.    Remember, v = frequency X wavelength, therefore, the longer the wavelength the greater the velocity. 

a.     Red light has a higher velocity than violet light

b.     N for red is less than n for violet

e.     Total Reflection and the Critical Angle

                                                             i.      When going from a more dense material to air, reflection is more common

                                                          ii.      The angle of incidence at which the rays graze the surface is called the critical angle

f.      Isotropic and Anisotropic Crystals

                                                             i.      Isotropic group includes isometric minerals, where light moves in all directions with equal velocity and only 1 index of refraction

                                                          ii.      Anisotropic minerals have more than one index depending on the orientation of the mineral

g.     Polarized light

                                                             i.      Remember, light travels in all directions.  Polarizing light restricts it so a single plane

                                                          ii.      Polarize light by absorption

                                                       iii.      Polarized light by reflection

II.                Microscope

a.     Immersion method

b.     Becke lines