Index of Refraction

Question 1

What are the two basic questions of forensic analyses?

Answer 1

What is this?
- identify class characteristics
Where did this come from?
- Individual characteristics

Question 2

Why is R.I. important in forensics?

Answer 2

• 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

Question 3

True or False

The angle of reflection is equal to the angle of incidence.

Answer 3

TRUE

Question 4

True or False

Light is reflected as it passes from one transparent medium to another.

Answer 4

FALSE

Light is reFRActed as it passes from one transparent medium to another

Question 5

Snell’s Law

In which direction does the light bends when light passes from low to high density?

Answer 5

• light bends towards the normal
• n1 < n2
• angle1 > angle2

Question 6

Snell’s Law

In which direction does the light bends when light passes from high to low density?

Answer 6

• light bends away the normal (towards the interface)
• n1 > n2
• angle1 < angle2

Question 7

What was Alhazen’s contribution to R.I.?

Answer 7

• 1st to propose that refraction was different due to light having different velocities in different materials
• proposed that light traveled slower in denser media

Question 8

Alhazen’s R.I. equation?

Answer 8

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

Question 9

Snell’s Law

for large angles

Answer 9

n1sin(theta1) = n2sin(theta2)

n1 = RI of inicial median
n2 = RI of 2nd median
theta1 = inicial angle
theta2 = 2nd angle

Question 10

Snell’s Law

for small angles

Answer 10

n1(theta1) = n2(theta2)

• variables have same definitions as for the 1st equation
• use radians instead of degrees for angles

Question 11

Snell’s Law

When n1 = n2, what happens?
special case

Answer 11

• theta1 = theta2
• there is no refraction
• there is no reflection
• 100% transparent

Question 12

Snell’s Law

What happens to particles that are immersed in a medium having the same index of refraction?

(special case)

Answer 12

• the become invisible

Question 13

What is the Critical Angle?

Answer 13

• 90 degrees

- the limit to how steep the entry angle (theta1) can be before refraction is no longer possible

Question 14

What is Brewster’s Angle

Answer 14

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

Question 15

Total Internal Reflection

Answer 15

• 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

Question 16

Dispersion

Answer 16

• the variation if index of refraction with wavelength or frequency

Question 17

What are the two types of Dispersion

Answer 17

• normal

- anomalous

Question 18

What is Normal Dispersion?

Answer 18

• the R.I. increases with decreasing wavelength

Question 19

What equations represent Normal Dispersion

Answer 19

• Cauchy equations
• Sellmeier equations
• cauchy equations are only valid in the visible region of the spectrum
• cauchy equations only represent this kind of dispersion

Question 20

What is Anomalous Dispersion?

Answer 20

• 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

Question 21

What is Relief?

Answer 21

• the degree to which a mineral grain or grains appear to stand out from the mounting material

Question 22

What are the degrees of Relief?

Answer 22

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

Question 23

What is the Becke Line?

Answer 23

• a bright fringe that appears when
  > the indices of refraction of a particle and the surround medium do not match
  > the microscope is defocused

Question 24

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.

Answer 24

TRUE