Light Flashcards
Nature of light, EMR, Phase and Interference
What is the range of wavelengths in the visible spectrum
400-700 nm
Components of an electromagnetic wave
- Electrical*** - up and down directions. (Interacts with shells of e-)
- Magnetic - side to side direction.
What is the relationship between the frequency and velocity of light?
Frequency and velocity are directly proportional : ƒ≈ V
What is the relationship between frequency and wavelength of light?
Frequency and wavelength are inversely proportional : ƒ ≈ 1/lambda
What remains constant when light travels through materials and what changes?
Energy and frequency remain constant ; Wavelength and velocity change.
What is a wave front?
It is a plane which connects equivalent points in adjacent waves.
What is the wave normal?
A line perpendicular to the wave front
What is the light ray?
The direction of propagation of light energy.
What is an isotropic medium?
In an isotropic medium, the wave normal and light ray are parallel to one another. (Isometric minerals, glass, gases)
What is an anisotropic medium?
In an anisotropic medium, the wave normal is not parallel to the light ray. (Anisotropic minerals)
What does retardation refer to in phase?
The distance one wave lags behind another. (∆)
What kind of interference between 2 waves (A + B), results in a higher amplitude R wave?
Constructive interference. (∆ = I*lambda)
Describe destructive interference.
Interference between 2 waves (A + B) which are exactly out of phase. The waves cancel one another out. ∆ = (i+1/2)lambda
How does wavelength affect the perception of colour?
One wavelength - Monochromatic
Multiple wavelengths - polychromatic
Entire spectrum - white light
colours - selective absorption
Opaque - complete absorption of visible spectrum
What does a blue filter do?
Cancels out certain wavelengths of light so that light appears pure/more white.
How does the number of electrons in a mineral effect its interaction with light?
More e- = slower velocities.
1. Polymorphs: Same composition, different densities.
- The more dense the structure (more e- per unit distance), the slower light travels.
2. Solid Solution: e.g. Olivine end members. Fayalite and Forsterite.
- The more Fe (more e- per unit distance) the lower V is, slower the light travels
What is the critical angle?
It is the angle of incidence where refraction is 90 degrees.
Why does refraction occur?
Refraction occurs because of the change in velocity and wavelength as light enters a new medium (no change in energy, no change in frequency)
Snell’s Law
The sin of the angle of incidence/sin of the angle of refraction = refractive index 2/refractive index 1
When does total internal reflection occur?
When the angle of incidence is larger than the critical angle, light cannot be refracted into the material. Light bounces off.
What is the difference between isotropic and anisotropic media?
Isotropic: isometric minerals and materials with random distribution (glass and gases)
Anisotropic: most minerals. velocity of light varies with crystallographic direction and n varies with wave direction.
What do the Fraunhofer lines tell us?
nF (486nm)
nD (587nm)
nC (656nm)
nF-nC is the coefficient of dispersion.
High value = strong dispersion
negative value = abnormal dispersion
What is a main difference between polarized and unpolarized light?
Unpolarized light vibrates in all directions, and polarized light vibrates in a single plane, circular shape or elliptical shape.
Which type of polarization is related to the Brewsters angle?
Reflection
Does polarization by reflection occur parallel or perpendicular to the reflecting surface?
Parallel.
What is pleochroism?
When one direction of light is more strongly absorbed than another, resulting in different colours at different angles.
Which method of polarization is associated with splitting of light into two rays with perpendicular polarization?
Selective absorption.
How does double refraction work?
Two polarizers at 90º lead to a split in a light ray, with resulting rays travelling in different directions.
