Colour, Light, Optics Flashcards
Light
Electromagnetic radiation
- behaves both like waves and particles (photons)
- travels at a constant speed (speed of light - c)
Speed of light
c
from Latin celeritas, speed
3.0 x 10^8 m/s (in a vacuum)
How does light behave like waves?
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 (
How does light behave like a particle?
Travelling as photon particles
- more intense light is composed of a greater number of photons with a higher frequency of incidence
Colour
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
Electromagnetic spectrum
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)
What determines our interpretation of colour?
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
Subtractive colour theory
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
Why do we see magenta if it doesn’t correspond to a single wavelength in the electromagnetic spectrum?
Overlapping combinations of wavelengths
Subtract yellow and cyan; result is predominance of blue and red (magenta!)
Illumination
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
Reflection
When light passing through one medium strikes another medium, part is reflected (like a mirror)
angle of incidence = angle of reflection
Refraction
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
Refractive index (n)
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
Total internal reflection
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)
Isotropic minerals
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)
Anisotropic minerals
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
Birefringence
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
Dispersion
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
Idiochromatic
gemstones are “self-coloured” from an essential elemental constituent
eg., peridot, turquoise, lazurite
Allochromatic
aka other-coloured
gemstone colour comes from an impurity
eg., emerald
Pseudochromatic
aka false-coloured
gemstone colour produced by physical optics
effects include asterism, chatoyancy, iridescence, opalescence, and labradorescence
Chromophore
an element responsible for colouration of a mineral
typically one of the transition elements (eg., Fe, Ti, Cu, Co, Mn…)
Asterism
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
Cabochon
a shaped and polished gem, as opposed to faceted
Chatoyancy
occurs when many fine fibre inclusions are oriented in a parallel manner –> cat eye effect
- cut as cabochons
Iridescence / opalescence
Internal scattering of light off of fine particles causes a play of colour
- common in sunstone and opal
Labradorescence
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)
Pleochroism
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
Transparency
describes how light transmits through a medium
- transparent
- semi-transparent
- translucent
- semi-translucent
- opaque
Transparent
objects can be viewed through the medium
- glass, diamond, beryl
Translucent
objects cannot be viewed through the medium, but light will pass through with lesser intensity
- jade, opal, agate
Semi-translucent
objects cannot be viewed through the medium, and light will only pass through the medium if it is thin
*confusing term, not necessary
Opaque
objects cannot be viewed through the medium, and no light will pass through
- pyrite, malachite, galena
