Transcript Light - Stellenbosch University

Introduction to Light
IN THIS LECTURE
–
–
–
–
–
–
–
–
Light
Electromagnetic Radiation
Wave Nomenclature
Electromagnetic Spectrum
Speed of Light
Wave front and wave normal
Isotropic versus anisotropic minerals
Polarisation of light
Light
•
LIGHT is a form of energy, detectable with the eye, which can be
transmitted from one place to another at finite velocity.
•
Two complimentary theories have been proposed to explain how light
behaves and the form by which it travels.
– Particle theory - release of a small amount of energy as a photon
when an atom is excited.
– Wave theory - radiant energy travels as a wave from one point to
another.
•
Wave theory effectively describes the phenomena of polarization,
reflection, refraction and interference, which form the basis for
optical mineralogy.
Electromagnetic Radiation
Wave Theory tells us that radiant energy like light has both electrical and
magnetic properties and is therefore called Electromagnetic Radiation
Light rays have electric
and magnetic components
that vibrate at right
angles to each other and
to the direction of
propagation.
In optical mineralogy only the electric component, referred to as
the electric vector, is considered and is referred to as the
vibration direction of the light ray
Wave Nomenclature
•
The wave is travelling left to right with velocity (V nm/sec). The
wavelength (l, nm) is the distance between successive wave crests. The
frequency is the number of wave crests that pass some point per
second. Frequency is expressed as cycles per second or Hertz (Hz). The
amplitude (A) is the height of the wave.
•
Velocity, wavelength and frequency are related by the following equation
V
f= /
l
Electromagnetic Spectrum
The electromagnetic Spectrum
classifies different types of
energy in terms of wavelength.
Energy in the form of visible
light, with wavelengths
between about 400nm and
700nm, is a small portion of
this spectrum (1nm = 10-9m).
Note that the energy of light
increases with decreasing
wavelength
Speed of Light
•
The velocity of light depends on the nature of the material through
which it travels and the wavelength of the light
•
In a vacuum the velocity of light is at a maximum: 3 x 1017 nm/sec
In all other material,
the velocity of light
slows down. This is
because the wavelength
of light decreases as
the density of the
material through which
it is travelling increases
but the frequency
remains constant.
Wave front & Wave Normal
A wave front is a surface that connects the same points on adjacent waves
Successive wave fronts are one wavelength apart
A wave normal is right angles to the wave front and represents the direction
the wave is moving in
A light ray is the propagation
direction of the light energy
Isotropic versus Anisotropic
Minerals can be subdivided, based on the interaction of the light ray
travelling through the mineral and the nature of the chemical bonds
holding the mineral together, into two classes:
ISOTROPIC
ANISOTROPIC
•Same velocity of light in all
directions
•Velocity of light is different
in different directions
•Wave Normal and Light Ray
are parallel
•Wave Normal and Light Ray
are NOT parallel
•Examples are volcanic glass
and minerals of the isometric
(cubic) system such as
Fluorite, Garnet and Halite
•Anisotropic minerals belong to
tetragonal, hexagonal,
orthorhombic, monoclinic and
triclinic systems
Isotropic vs Anisotropic
ISOTROPIC
In isotropic minerals the wave
normal and the direction of
propagation of the light rays are
perpendicular to the wave front
ANISOTROPIC
In anisotropic minerals the light
rays are not perpendicular to the
wave front
Polarisation of Light
•
•
Light emanating from some source, sun, or a light bulb, vibrates in all
directions at right angles to the direction of propagation and is
unpolarized.
If the electric vector is constrained to lie within one plane then the
light is plane-polarised and vibrates in only one direction.