Light

Question 1

What is the range of wavelengths in the visible spectrum

Answer 1

400-700 nm

Question 2

Components of an electromagnetic wave

Answer 2

1. Electrical*** - up and down directions. (Interacts with shells of e-)
2. Magnetic - side to side direction.

Question 3

What is the relationship between the frequency and velocity of light?

Answer 3

Frequency and velocity are directly proportional : ƒ≈ V

Question 4

What is the relationship between frequency and wavelength of light?

Answer 4

Frequency and wavelength are inversely proportional : ƒ ≈ 1/lambda

Question 5

What remains constant when light travels through materials and what changes?

Answer 5

Energy and frequency remain constant ; Wavelength and velocity change.

Question 6

What is a wave front?

Answer 6

It is a plane which connects equivalent points in adjacent waves.

Question 7

What is the wave normal?

Answer 7

A line perpendicular to the wave front

Question 8

What is the light ray?

Answer 8

The direction of propagation of light energy.

Question 9

What is an isotropic medium?

Answer 9

In an isotropic medium, the wave normal and light ray are parallel to one another. (Isometric minerals, glass, gases)

Question 10

What is an anisotropic medium?

Answer 10

In an anisotropic medium, the wave normal is not parallel to the light ray. (Anisotropic minerals)

Question 11

What does retardation refer to in phase?

Answer 11

The distance one wave lags behind another. (∆)

Question 12

What kind of interference between 2 waves (A + B), results in a higher amplitude R wave?

Answer 12

Constructive interference. (∆ = I*lambda)

Question 13

Describe destructive interference.

Answer 13

Interference between 2 waves (A + B) which are exactly out of phase. The waves cancel one another out. ∆ = (i+1/2)lambda

Question 14

How does wavelength affect the perception of colour?

Answer 14

One wavelength - Monochromatic
Multiple wavelengths - polychromatic
Entire spectrum - white light
colours - selective absorption
Opaque - complete absorption of visible spectrum

Question 15

What does a blue filter do?

Answer 15

Cancels out certain wavelengths of light so that light appears pure/more white.

Question 16

How does the number of electrons in a mineral effect its interaction with light?

Answer 16

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

Question 17

What is the critical angle?

Answer 17

It is the angle of incidence where refraction is 90 degrees.

Question 18

Why does refraction occur?

Answer 18

Refraction occurs because of the change in velocity and wavelength as light enters a new medium (no change in energy, no change in frequency)

Question 19

Snell’s Law

Answer 19

The sin of the angle of incidence/sin of the angle of refraction = refractive index 2/refractive index 1

Question 20

When does total internal reflection occur?

Answer 20

When the angle of incidence is larger than the critical angle, light cannot be refracted into the material. Light bounces off.

Question 21

What is the difference between isotropic and anisotropic media?

Answer 21

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.

Question 22

What do the Fraunhofer lines tell us?

Answer 22

nF (486nm)
nD (587nm)
nC (656nm)
nF-nC is the coefficient of dispersion.
High value = strong dispersion
negative value = abnormal dispersion

Question 23

What is a main difference between polarized and unpolarized light?

Answer 23

Unpolarized light vibrates in all directions, and polarized light vibrates in a single plane, circular shape or elliptical shape.

Question 24

Which type of polarization is related to the Brewsters angle?

Answer 24

Reflection

Question 25

Does polarization by reflection occur parallel or perpendicular to the reflecting surface?

Answer 25

Parallel.

Question 26

What is pleochroism?

Answer 26

When one direction of light is more strongly absorbed than another, resulting in different colours at different angles.

Question 27

Which method of polarization is associated with splitting of light into two rays with perpendicular polarization?

Answer 27

Selective absorption.

Question 28

How does double refraction work?

Answer 28

Two polarizers at 90º lead to a split in a light ray, with resulting rays travelling in different directions.