Colour, Light, Optics

Question 1

Light

Answer 1

Electromagnetic radiation
- behaves both like waves and particles (photons)
- travels at a constant speed (speed of light - c)

Question 2

Speed of light

Answer 2

c
from Latin celeritas, speed
3.0 x 10^8 m/s (in a vacuum)

Question 3

How does light behave like waves?

Answer 3

Inverse relationship between wavelength (distance from peak to peak) and frequency (wave crests that pass through a point per second)
ie., when frequency of light is decreased, wavelength must increase
–> energy transmitted by light increases with increasing frequency or decreasing wavelength

Important wave equations:
λ = c/f (velocity/frequency)
E = hf (

Question 4

How does light behave like a particle?

Answer 4

Travelling as photon particles
- more intense light is composed of a greater number of photons with a higher frequency of incidence

Question 5

Colour

Answer 5

Brain’s interpretation of light interacting with the object we are looking at

Light in the visible spectrum is composed of wavelengths (colours)
- white is an even mixture of light of wavelengths across the visible range

Question 6

Electromagnetic spectrum

Answer 6

The range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies

• high intensity gamma rays (short wavelength, high frequency)
• mid range (white light) –> visible region (350-750 nanometers)
  - short end - violet (~400 nm)
  - long end - red (~700 nm)
• low intensity radio waves (long wavelength, low frequency)

Question 7

What determines our interpretation of colour?

Answer 7

A material’s absorption of light

Eg., white light shined on a surface that appears red
–> electromagnetic radiation in the ‘red region’ is most effectively reflected

Question 8

Subtractive colour theory

Answer 8

Colour is the result of absorption and transmission of certain wavelengths of light
- colours (wavelengths) that we see are complementary to the colours that are absorbed in the object

Eg.,
- if blue-violet light is absorbed, resulting wavelengths will make the object appear yellow
- if all wavelengths other than blue and red are absorbed, the object would appear purple/magenta
- if all colours are all absorbed, we see black

Question 9

Why do we see magenta if it doesn’t correspond to a single wavelength in the electromagnetic spectrum?

Answer 9

Overlapping combinations of wavelengths

Subtract yellow and cyan; result is predominance of blue and red (magenta!)

Question 10

Illumination

Answer 10

Different sources of energy will emit electromagnetic radiation at different intensities across the spectrum
- LED (light emitting diode) - only emit one wavelength (colour)
- polychromatic light sources can have widely different spectral emittance curves depending on the composition of the source of energy

Question 11

Reflection

Answer 11

When light passing through one medium strikes another medium, part is reflected (like a mirror)

angle of incidence = angle of reflection

Question 12

Refraction

Answer 12

When light passing through one medium strikes another medium, part is refracted (like what you see through a fish tank)
- the speed of the light changes, causing the light to bend/change direction
- the degree to which the light is slowed and bent relates to the difference in the refractive indices between the two media and the angle at which the light path meets the medium

Question 13

Refractive index (n)

Answer 13

A measure of how much it will refract light of a specific wavelength (incident light); ie, how much the light will slow when it enters the new medium compared to when it travels in a vacuum

• refractive index of a medium is also dependent on wavelength
• orientation of multiple refractive indices (anisotropic minerals) is related to the unique crystal structure of each mineral

Question 14

Total internal reflection

Answer 14

All light is reflected back without being transmitted through the new medium
- can occur when light travels from a medium with a high refractive index (gemstone) to one with a low refractive index (air), if the angle of incidence is greater than a critical angle (dependent on the r.i. of the gemstone and surrounding material)

Question 15

Isotropic minerals

Answer 15

Minerals that belong to the cubic system
- one refractice index that is applicable in all 3-D orientations
- all three axes are equal in length, and are all perpendicular to one another
eg., diamond (n=2.419), spinel (n=1.725)

Question 16

Anisotropic minerals

Answer 16

Minerals from crystal systems other than cubic
- more than one refractive index
(tetragonal and hexagonal –> 2;
monoclinic, triclinic, orthorhombic –> 3)

• light that enters is split in two distinct light rays

Question 17

Birefringence

Answer 17

the absolute difference between refractive indices

birefringence = refractive index 1 - refractive index 2
Δn = n1 - n2

high birefringence: difference is large, difference in light’s path is significant –> light transmitted through appears doubled

low birefringence: difference is small –> light transmitted through appears blurry

Question 18

Dispersion

Answer 18

when white light enters or leaves a material at angles other than 90 degrees, individual spectral wavelengths (colours) will be refracted by different amounts

• longer wavelengths (red) are refracted the least
• shorter wavelengths (violet) are refracted the most
• this phenomenon gives gemstones their fire

Question 19

Idiochromatic

Answer 19

gemstones are “self-coloured” from an essential elemental constituent

eg., peridot, turquoise, lazurite

Question 20

Allochromatic

Answer 20

aka other-coloured
gemstone colour comes from an impurity

eg., emerald

Question 21

Pseudochromatic

Answer 21

aka false-coloured
gemstone colour produced by physical optics

effects include asterism, chatoyancy, iridescence, opalescence, and labradorescence

Question 22

Chromophore

Answer 22

an element responsible for colouration of a mineral

typically one of the transition elements (eg., Fe, Ti, Cu, Co, Mn…)

Question 23

Asterism

Answer 23

a prominent star shape (usually 6 sided) appears when crystallographically oriented mineral inclusions exist in the host mineral

• cut as cabochons
• most famous are in sapphire and ruby

Question 24

Cabochon

Answer 24

a shaped and polished gem, as opposed to faceted

Question 25

Chatoyancy

Answer 25

occurs when many fine fibre inclusions are oriented in a parallel manner –> cat eye effect

• cut as cabochons

Question 26

Iridescence / opalescence

Answer 26

Internal scattering of light off of fine particles causes a play of colour

• common in sunstone and opal

Question 27

Labradorescence

Answer 27

Diffraction of light interacting with very thin intergrown layers of feldspars generates a range of colours; width of the layers defines the colour generated during diffraction

• most common in labradorite (a species of feldspar)

Question 28

Pleochroism

Answer 28

different colours (or saturation of colours) is displayed depending on the crystallographic direction of the stone being viewed (how the crystal’s axes are lined up with the direction it is viewed from)

caused by differential absorption of light according to the orientation of the crystal

• best viewed using dichroscope
• Tanzanite, which displays three colours that align with the three different crystal axes (brown, purple, blue)
• Iolite - violet-blue and colourless

Question 29

Transparency

Answer 29

describes how light transmits through a medium
- transparent
- semi-transparent
- translucent
- semi-translucent
- opaque

Question 30

Transparent

Answer 30

objects can be viewed through the medium

• glass, diamond, beryl

Question 31

Translucent

Answer 31

objects cannot be viewed through the medium, but light will pass through with lesser intensity

• jade, opal, agate

Question 32

Semi-translucent

Answer 32

objects cannot be viewed through the medium, and light will only pass through the medium if it is thin

*confusing term, not necessary

Question 33

Opaque

Answer 33

objects cannot be viewed through the medium, and no light will pass through

• pyrite, malachite, galena