Nature and Behaviour of Light

Polarized and Non polarized light

• Vibrations take place at right angles to the direction in which light is traveling.

• Light can be polarized (made to vibrate in one plane) by selective absorption (OR) or by reflectance (OL)

Reflection and Refraction of Light

• When light passes from a low density medium (e.g. air) into a higher density non-opaque medium (e.g. a mineral), part will be reflected and part will be pass through, but be bent and slowed – refracted.

• Angle of reflection (r’) equals the incident angle (i)

• Angle of refraction (r) will differ from the incident angle depending

on the change in velocity between the two substances

Mineral properties: relief

• Relief is a measure of the relative difference in n between a mineral grain and its surroundings • Relief is determined visually, in PPL• Relief is used to estimate n

garnet: n = 1.72-1.89quartz: n = 1.54-1.55epoxy: n = 1.54

Garnet has high relief

Quartz has low relief

Mineral properties: Index of refraction (R.I. or n)

Light is refracted when it passes from one substance to another; refraction is accompanied by a

change in velocity

n1

n1n2

n2

n2>n1 n2<n1

n =velocity in air

velocity in mineral

• n is a function of crystallographic orientation in anisotropic minerals

isotropic minerals: characterized by one RI uniaxial minerals: characterized by two RI biaxial minerals: characterized by three RI

• n gives rise to 2 easily measured parameters: relief & birefringence

O E Double imagesDouble images

Ray Ray 2 rays with 2 rays with different propagation different propagation and vibration directionsand vibration directions

Each is polarized ( Each is polarized ( each other)each other)

Fig 6-7 Bloss, Optical Crystallography, MSA

Anisotropic crystalsCalcite experiment and double refraction

Double Refraction

Mineral’s power of splitting a ray of entering light into two.

Nicol is made of a clear variety of calcite known as Iceland Spar.

Light entering perpendicular to one of the cleavage planes is split in to two rays, and, in consequence, any object viewed along such a perpendicular is doubled; two images reach the eye.

• Ordinary ray and Extraordinary ray.