Index of Refraction Flashcards
Study for Quiz#2
What are the two basic questions of forensic analyses?
What is this?
- identify class characteristics
Where did this come from?
- Individual characteristics
Why is R.I. important in forensics?
- distinguish glass particles from different sources
- identify minerals and other trace evidence
- understanding the R.I. is key to understanding polarized light microscopy (PLM), phase contrast microscopy and dispersion staining
True or False
The angle of reflection is equal to the angle of incidence.
TRUE
True or False
Light is reflected as it passes from one transparent medium to another.
FALSE
Light is reFRActed as it passes from one transparent medium to another
Snell’s Law
In which direction does the light bends when light passes from low to high density?
- light bends towards the normal
- n1 < n2
- angle1 > angle2
Snell’s Law
In which direction does the light bends when light passes from high to low density?
- light bends away the normal (towards the interface)
- n1 > n2
- angle1 < angle2
What was Alhazen’s contribution to R.I.?
- 1st to propose that refraction was different due to light having different velocities in different materials
- proposed that light traveled slower in denser media
Alhazen’s R.I. equation?
n = (speed of light in a vacuum)/(speed of light in a transparent medium)
n - R.I.
speed of light - approx. 3e8 (m/s)
n should always be greater or equal to 1
Snell’s Law
for large angles
n1sin(theta1) = n2sin(theta2)
n1 = RI of inicial median n2 = RI of 2nd median theta1 = inicial angle theta2 = 2nd angle
Snell’s Law
for small angles
n1(theta1) = n2(theta2)
- variables have same definitions as for the 1st equation
- use radians instead of degrees for angles
Snell’s Law
When n1 = n2, what happens?
special case
- theta1 = theta2
- there is no refraction
- there is no reflection
- 100% transparent
Snell’s Law
What happens to particles that are immersed in a medium having the same index of refraction?
(special case)
- the become invisible
What is the Critical Angle?
- 90 degrees
- the limit to how steep the entry angle (theta1) can be before refraction is no longer possible
What is Brewster’s Angle
tan(theta p) = (n2/n1)
- an angle of incidence at which light with a particular polarization is perfectly transmitted through a transparent dielectric surface with no reflection
Total Internal Reflection
- when a light ray passes from a higher index of refraction to a lower index of refraction (n1 > n2), there is an angle of incidence (critical angle) for which the angle of refraction (theta2) equals 90 degrees
- for theta1 > theta critical, this results
Dispersion
- the variation if index of refraction with wavelength or frequency
What are the two types of Dispersion
- normal
- anomalous
What is Normal Dispersion?
- the R.I. increases with decreasing wavelength
What equations represent Normal Dispersion
- Cauchy equations
- Sellmeier equations
- cauchy equations are only valid in the visible region of the spectrum
- cauchy equations only represent this kind of dispersion
What is Anomalous Dispersion?
- dispersion of some refraction spectra in which the normal order of the separation of components is reversed in the vicinity of certain wavelength
- R.I. rises then falls as the wavelength decreases
- occurs where the material has an absorption band
What is Relief?
- the degree to which a mineral grain or grains appear to stand out from the mounting material
What are the degrees of Relief?
Strong - greater than 0.12 R.I. units
Moderate - between 0.12 & 0.04 RI units
Low - indices differ by or are within 0.04 RI units of each other
What is the Becke Line?
- a bright fringe that appears when
> the indices of refraction of a particle and the surround medium do not match
> the microscope is defocused
True or False
If the microscope is defocused so as to increase the distance between the objective and the specimen, the Becke Line appears to move into the medium having the higher index of refraction.
TRUE
