L4 - Optics

Optics in Petrographic Microscopy

Most minerals are small, so we use petrographic microscopes to study thin sections of rocks (30 µm thick).

Light as an Electromagnetic Wave

Light is an electromagnetic wave with electric (E) and magnetic (B) components vibrating perpendicular to each other and to the direction of travel. Normal light is unpolarized, with random orientations of the vibration plane. Polarized light has all photons vibrating in the same plane, achieved using a polariser with parallel slits.

Petrographic Microscope Components

The petrographic microscope has two polarisers:

  • Polariser: At the base, between the light source and thin section.

  • Analyser: At the top, between the thin section and the eye, oriented 90° to the polariser.

Crystals interact with polarised light, providing information on composition, structure, and origin.

Observations with the Polariser Only

With only the polariser, we can observe:

  • Habit: Crystal's external shape (e.g., euhedral, subhedral, anhedral).

  • Shape: Crystal shape (e.g., tabular, fibrous, acicular).

  • Microstructure: Features within the crystal (e.g., cracks, cleavage, twins, zoning).

  • Colour: Selective wavelength absorption, orientation-dependent due to anisotropy. Opaque minerals can’t be seen.

  • Relief: How strongly a mineral stands out, controlled by the refractive index and orientation.

By knowing the characteristic properties of minerals, we can identify the rock's composition using a microscope.

Birefringence with the Analyser

Adding the analyser introduces birefringence. Light splits into two permitted vibration directions within the crystal. In anisotropic crystals, these waves move at different speeds. In isotropic media, such as liquids, glass, or cubic crystals, light moves at the same speed in all directions, so there is no rotation.

Wave Behavior

In an isotropic medium, the peaks and troughs of the waves remain in sync, and the resultant vector moves back and forth diagonally, remaining constant. If the waves move at different speeds, the peak of one wave aligns with the trough of the other, potentially allowing the resultant wave to pass through the analyser.

Light Rotation

Light can rotate in an anisotropic medium and be observed. Birefringence = Δn{\Delta}n (difference in refractive indices) multiplied by thickness (30 µm), Δ=t⋅(n1−n2)\Delta = t \cdot (n_1-n_2) , describes this behavior. If the birefringence equals an integer number of wavelengths, the waves recombine, and light is blocked by the analyser. If it equals a half-integer number, light rotates 90° and passes through perfectly. If it is between these values, a component of the wave passes through the analyser

Birefringent Colour

Different wavelengths pass through the analyser, resulting in colors characteristic of each mineral. Birefringence is orientation-dependent.

The order of birefringence is found using a Michel-Levy Chart.

Extinction Position

When a crystal is oriented with a vibration direction parallel to the polarization plane, no splitting occurs; thus, no rotation, and light is blocked. This is the extinction position, which can be straight, symmetric, inclined, or unclear, and is characteristic of each mineral.

Optical Indicatrix

The optical indicatrix, an ellipsoid, visualizes refractive index variations. For example, in a tetragonal crystal, properties perpendicular to the c-axis are identical due to symmetry, creating a circular slice through the indicatrix. A slice containing the c-axis is an ellipse, reflecting different optical properties due to varying bond orientations along the c-axis.