Iceland spar from Iceland: a

key to the nature of light and

its interactions with matter

Leó Kristjánsson,

Emeritus research professor

University of Iceland

A talk given at the annual

conference of the Faculty

of Arts, March 2015

Picture: Iceland spar

decoration in the

ceiling at the entrance

of the main building

of the University

A precise relation between the incident and refracted angles of a light ray

entering a material (Snell’s or Descartes’ Law) was known since ~1630.

R. Bartholin published in 1669 an

essay on the behavior of light

rays in crystals of Iceland spar.

An incident ray was split in two,

one obeying the law and the

other not. The dot A underneath a

spar crystal thus appeared as two

fainter dots.

If the crystal was rotated while

on the table, the dot representing

the “extraordinary” ray rotated

with it.

This behavior was called double

refraction. It seemed to depend

on properties of the material.

Christiaan Huygens of Leiden received some Iceland spar crystals from Bartholin,

and included a chapter about them in his book Traité de la Lumière, 1690.

In the book, Huygens argued that light is a wave

motion in a tenuous substance called aether.

If a particle within a homogeneous material emits

light, it propagated outwards as a spherical wave.

In Iceland spar however, the light forms two wave fronts. One is spherical and the

other is ellipsoidal, with a symmetry axis parallel to a certain direction in the crystal.

Huygens also noted the presence of double refraction in quartz (rock crystal) where

it was much less prominent. The same also applied to most natural minerals.

Iceland spar has therefore always been presented in textbooks and papers as the

type example of double refraction.

Iceland spar is a variant species of the mineral calcite (calc spar, CaCO3), having

exceptional clarity, purity and absence of defects. It occurred in some abundance

only at a single locality in Iceland, where it was quarried at intervals in 1850-1924.

In the book by Isaac Newton, Opticks (1704) he

viewed light as being a stream of particles, emitted

by luminous objects and travelling in straight lines

in homogeneous materials. He proposed a

tentative explanation of double refraction in 1706.

Most scientists accepted Newton’s theory, and not

much happened in optics during the 18th century.

W.H.Wollaston measured in 1802 how the refract-

ion of the extraordinary ray in Iceland spar depends

on its angle of incidence. The results agreed fairly

well with Huygens’ 1690 undulation theory.

Thomas Young published in 1802-07 experimental results where he e.g.

illuminated two narrow closely spaced slits in a screen. Patterns seen on

the other side of the screen indicated that light behaved like waves on

a calm liquid surface, or sound. The colors of thin films supported this.

Young’s papers increased interest in Huygens’ theory, but many remained

sceptical. The French Academy of Science announced a prize competition

on the topic of double refraction.

The year 1809 marks the beginning of a revolution in optics, with Iceland spar

playing a central role. Huygens had recorded but not explained a curious fact:

When a light ray had split in two in an Iceland spar crystal, the emerging rays

were not just half as faint as the original one, but also somehow different.

This was not followed up by others, and it may have been regarded as due to

some peculiarity of the crystals, rather than having a wider significance.

In 1808 E.L. Malus made a most important discovery while working on an

essay to be submitted to the Academy’s prize competition on double refraction.

Biographers record that he happened to look from his rooms at the evening

sunlight reflected from windows of the Luxembourg palace, through an Iceland

spar crystal. He then saw that its intensity varied as he rotated the crystal. He

concluded, following additional experiments, that:

A simple translation:

“Light that is reflected obliquely by a smooth surface of an insulator

or a metal, undergoes by this to some extent the same change as by

passing through a doubly refracting crystal”.

This transformation of the light was called polarization by Malus, who

announced his discovery in 1808-09. He was awarded the Academy

prize for his theoretical and experimental research on Iceland spar.

Both Malus and the famous scientists P.S. de Laplace and J.B. Biot

argued that double refraction could be explained by Newton’s theory.

According to a modern text on the history of science, “Une nouvelle

branche de l’optique était née... Les phénomènes de polarisation

formaient le centre des préoccupations des physiciens” , as Malus’

work had revealed a hitherto unknown fundamental property of light.

New major findings in optics were soon announced, aided in many

significant ways by the Icelandic crystals. See the next slide.

-F. Arago showed in 1811 that thin laminas of

colorless crystals like mica appeared brightly

colored when viewed through Iceland spar.

-Polarized light apparently had some directional

preference. Arago found in 1811 that this direction

changed gradually when a ray of such light passed

through a plate of quartz. Biot discovered in 1815

that this property of “optical activity” also occurred

in various organic liquids and solutions.

-In contrast to light from the Sun or from a lamp, he blue light from a clear sky

showed a degree of polarization properties, depending on the direction of view.

A 6-cm wide colorless

lamina of mica, having

variable thickness. Young explained this color phenomenon in terms

of the wave theory, in 1814. His arguments were

confirmed by A. Fresnel’s experiments in 1821.

A. Fresnel published in 1816-19 papers describing observations on diffraction,

i.e. bending of light around obstacles. The results were much in favor of the

wave theory, but some scientists remained unconvinced.

Fresnel did realize around 1817 that the wave motion in light had to be

transverse to the direction of propagation (rather than longitudinal as in

sound), in order to explain double refraction.

He did not publish this conclusion until 1821, after he and Arago had carried

out in 1819 a crucial experiment with the aid of Iceland spar crystals.

In sunlight and lamplight the aether may be envisaged as oscillating in

irregularly varying directions, whereas the oscillations in the rays emerging

from Iceland spar only take place in two mutually perpendicular planes.

The transverse

motion is very

sensitive to

interactions with

matter.

Fresnel continued proposing new theoretical and experimental approaches to

the transmission of light in ordinary transparent materials like glass or water.

Among other things he derived equations for the proportions of polarized light

reflected and transmitted at a boundary between two such materials. These

equations are still being taught in University courses on optics.

He also considered light waves in crystals, deriving on the basis of certain

assumptions that the propagation of the extraordinary ray agreed with Huygens’

ellipsoid. This was confirmed later by precise measurements on Iceland spar.

It was frequently inconvenient to

have two polarized beams emerging

from a spar crystal, with overlap.

W. Nicol found in 1829 a method

to deflect one beam off to the side.

It is likely that tens of thousands of

such “Nicol prisms” were produced.

In certain classes of crystals, Fresnel’s theory predicted the

occurrence of a peculiar phenomenon known as “conical

refraction”. It was indeed observed by H. Lloyd in 1833.

G.T. Fechner’s book Repertorium der Experimentalphysik II, Leipzig 1832

considers the wave theory indispensable for understanding polarization:

A. Baumgartner states in Die Naturlehre, Supplementband II, Wien 1830 on

the explanations offered for diffraction, interference, and polarization of light:

Understandably, Fresnel’s contributions to wave theory received much support in

France, and they prompted new research into optics and wave motion in general.

In particular, the mathematician A.L. Cauchy published tens of papers on these

subjects in 1830-40. His theoretical approach regarding the transverse motion of

the aether which differed somewhat from that of Fresnel, was also favored by

some others including F.E. Neumann in Germany and J. MacCullagh in Ireland.

In England, the astronomer G.B. Airy deals with the wave theory of light on 162

pages in his Mathematical Tracts, Cambridge 1831. He carried out experiments

with Iceland spar, interpreted them, and invented optical devices. He says:

“The Undulatory Theory of Optics is presented to the

reader as having the same claims to his attention as the

Theory of Gravitation: namely that it is certainly true, and

that by mathematical operations of general elegance

it leads to results of great interest.”

Airy means by this ...”the hypothesis, namely, that light consists of undulations

depending on transversal vibrations”... and we must keep in mind that they

were introduced because longitudinal motions could not account for polarization.

The colored figure seen in quartz plates in polarized light was explained by Airy.

It was considered well into the 19th century that the Sun emitted two types of

radiation in addition to visible light. They were called rayons calorifiques (heat

rays) and rayons chimiques, whose effects included darkening of silver salts.

Some experiments had been made before and around 1830 to test if the above

types had wave properties. The reults do not seem to have been conclusive,

But A.M. Ampère was among the first to suggest in 1832 that both the visual

and the thermal radiations were emitted by vibrating atoms in materials.

M. Melloni investigated the nature of heat radiation in 1831-54 using a sensitive

electrical thermometer. He found that it became polarized by reflection, and in

1836 that the plane of its polarization rotated during passage through quartz.

J.D. Forbes who began a similar series of experiments in 1835 and obtained the

same results as Melloni regarding the polarization of heat, said already in 1836:

The pioneer Melloni who had originally intended to discover a difference in

character between light and heat rays, finally agreed with Forbes in 1842:

Melloni adds here the chemical rays to the two other types. J. Sutherland had

observed polarization of these with the aid of Iceland spar in 1841, concluding:

By the mid-1840s, Nicol prisms were a common tool in optical research. In

addition to producing polarized light, they also could be used in analyzing

precisely the state of polarization in a light beam. This applied both to the

direction of vibration of the light, its intensity, and delays (phase changes).

One of the most important discov-

eries in optics during the 1840s

was made by M. Faraday in 1845.

A polarized ray of light from the

mirror 2 passed through a block

8 of heavy glass in the strong

magnetic field created by the

electro-magnet F. With the Nicol

prism 5, Faraday found that the

direction of polarization in the

light had rotated by a small angle.

This was the first-ever indication that electric or magnetic fields had anything

to do with light. It generated much interest and new research activity in optics.

H. Fizeau and L. Foucault demonstrated the wave-nature of thermal radiation

in 1847, using Iceland spar in their experiments. H. Knoblauch confirmed its

double refraction in Iceland spar in 1848. The figure shows the 1849 setup of

F. de la Provostaye and P. Desains who found the Faraday effect in heat rays.

These scientists and others studied further properties of heat radiation around 1850

with the aid of Iceland spar, including the rotation of its plane of polarization in organic

liquids. This established its identity with visible light, apart from a greater wavelength.

C and C are

Nicol prisms,

D is a glass bar,

E is a sensitive

thermal detector

Iceland spar continued serving in important research into the nature of light.

One question was: for how long does a monochromatic ray of light stay “in

step” with itself before any breaks in it (at stars in the diagram) occur?

* * * *

Three pieces of such a ray are shown, with n = about 11, 5, and 8 whole periods.

H. Fizeau and L. Foucault showed in 1850 that n was at least 7000 on average in

two narrow wavelength intervals of sunlight. J. Stefan doubled this number for a

third wavelength interval in 1864. E. Mascart found in 1872 that n in the yellow

light from a sodium flame reached at least 105000. All of these used Iceland spar,

Mascart even stating that it was by far the best material to use in his experiment.

Different methods were applied in later estimates of these lengths, which remained

of considerable interest when quantum theories were applied to light after 1905.

Iceland spar was involved in L. Pasteur’s

1848 revelations regarding optical activity.

He found that those compounds which in the

liquid state rotate the plane of polarization of

light, form crystals in mirror-image versions.

This discovery was a great step forward

in the understanding of interactions of

light and matter. It led directly to ideas

in 1874 about mirror-image versions of

active molecules, due to the tetrahedral

arrangement of carbon’s chemical bonds.

The figure shows the amino acid alanine.

This brings us to J.C. Maxwell’s theory from 1865, on

light as a wave of transverse electric and magnetic fields.

Maxwell was quite familiar with polarized light, having ex-

perimented with it since a schoolboy. He had been given

a couple of Nicol prisms by W. Nicol himself, and he had

published papers on the electric and magnetic properties

of Iceland spar and other crystals.

The Faraday effect which Maxwell had written about in

1862, was among the chief pieces of evidence supporting

his theory. He associated it with molecular current loops.

Thus, J. Kerr found in 1875 that liquids became doubly refracting in electric fields:

Many contemporary colleagues ignored the

electromagnetic theory, as it did not explain

various phenomena any better than the old

view of an elastic aether. However, some

new clues supporting it appeared gradually.

Around 1870, Nicol prisms contributed to fundamental discoveries on the so-

called scattering of light by small particles. J. Tyndall sent unpolarized white-light

beams through glass vessels containing smoke, dust, and liquids with colloids.

He saw that the light scattered sideways was polarized, and that it also became

more and more bluish in hue as the particles were smaller.

This therefore provided

a joint explanation of

the color and the pol-

arization of skylight.

Before long, J.W. Strutt

(Lord Rayleigh) derived

theoretical equations

describing scattering,

including his famous law

that its relative intensity

is proportional to the

inverse fourth power of

the wavelength of light.

H. Hertz carried out from 1888 experiments on “Strahlen elektrischer Kraft”,

i.e. electromagnetic waves of wavelengths around 1 m. Hertz proved that

they had many characteristics of the transverse visual and thermal radiation.

Above, Hertz describes how he found the direction of oscillation

In his waves, using a metal-ring detector. F. Trouton polarized

such waves in 1889, by reflecting them from a large piece of wax.

The Iceland spar rhomb K played a role in O. Wiener’s experiment

in 1890. A polarized light ray was reflected from a polished metal

plate R which had been coated with a layer of light-sensitive gel.

The light caused the formation of dark striations in the gel. Their

distances from the metal surface indicated that the electric field

of Maxwell’s waves had a much greater effect than the magnetic

field, and pointed in the direction that Fresnel had assumed to be

the vibration direction of the aether. P. Drude and W. Nernst found

equivalent results from a different setup in 1891.

A.Righi found in 1894 that Hertz’ waves underwent double refraction in wood

(which has different properties in different directions, like crystals).

The above and various other pieces of evidence increased the credibility of the

the electromagnetic theory, although some scientists still ignored it in the 1890s.

The theory did not agree with Newton’s mechanics, for moving observers. So....

The disagreement led to an increased efforts to measure the speed of the Earth

with respect to the aether which was supposed to be stationary in the Universe.

Of the many attempts carried out (with rather negative results), at least seven in the

period 1860-1905 and three in 1906-28 made some use of Iceland spar prisms.

All these experiments were much discussed at the time, and they were known to

A.Einstein. However, all are forgotten now except the one by A.A. Michelson and

E.W. Morley described in 1887. It did not use polarized light.

D.B. Brace tried in 1904 to detect double refraction in water in a 4-m long trough;

such an effect might in some interpretations be caused by the Earth’s movement

through the aether. Items 4 and 11 are Nicol prisms.

Iceland spar prisms were involved in the application of Einstein’s quantum

theory of 1905 to the structure of atoms and their interactions with light.

One example concerned wavelengths in the line-spectrum of light emitted by

atomic hydrogen. Only four of these were known in the visible range by 1885,

and technical problems prevented observations in the ultra-violet.

W. Huggins used a triangular spar

prism (e) to analyse stellar spectra,

as it was better suited than glass

or quartz. His photographic plates

(f) from bright whitish stars included

(from 1877) several ultraviolet lines.

Their wavelengths appeared to

follow a regular progression.

J.J. Balmer found in 1885 that the formula B.n2/(n2-4) with n = 3, 4, 5,...agreed

very well with all of the visual and ultraviolet lines. Wavelengths with higher n’s

fitting the formula were soon found in stellar, solar and laboratory spectra.

The above formula

and others for spectral

series of hydrogen

were basic to N. Bohr’s

1913 quantum model

of the permitted orbits

for an electron around

the hydrogen nucleus.

The atom emits light

when the electron

jumps from an outer

orbit to an inner one.

Hydrogen atom

Further: atoms of the elements emit line spectra reaching from the visual range to

the ultraviolet, infrared and beyond. In all except hydrogen the wavelengths of the

lines are distributed very irregularly, as shown below for helium.

P. Zeeman discovered in 1896 that when atoms emitted light while

in a strong magnetic field, each of their spectral lines split into 3 to

9 fainter lines. A full explanation took over 30 years to find.

Research on the Zeeman effect greatly accelerated progress in the understanding of

line spectra. This research included observations on the polarization of the emitted

light and on the absorption of polarized light in metal vapors with Nicol prisms (N).

E is a magnet

with a hole

The laws of the quantum theory restrict the energies and angular momenta of

electrons in atoms to certain discrete values (as shown here for sodium).

The wavelength of a light quantum emitted when an electron moves to a lower

energy level, depends on the difference in energy between these levels. The

polarization of this light quantum depends on changes in its angular momentum.

Iceland spar also was

essential in research

related to the Zeeman

effect, for instance on

so-called resonance

radiation which led to

the development of

atomic clocks.

The last topic to be treated here concerns the spectral analysis of X-rays. They

are emitted when energetic electrons impact on atoms of the heavier elements.

This analysis required (from 1915) the use of very perfect crystals, acting like a

prism to disperse the rays into different directions according to their wavelengths.

It resulted in precise tables of all the highly complex energy levels of the atoms.

K is the analysing crystal in the

X-ray spectrometer shown. For

this role, Iceland spar (from

Iceland or from elsewhere) was

quite commonly chosen until

1950 at least, along with quartz,

common salt and gypsum.

Results also included confirma-

tion of Einstein’s predictions

regarding the energy of light

particles, and their momentum

(A.H. Compton,1923).

M. Siegbahn’s spectrometer, 1920s

Several other examples of the use of Iceland spar crystals fundamental research

on the nature of light and light-matter interactions might be mentioned, such as:

-Various aspects of solar and stellar radiation

-Direction-dependent properties (anisotropy) as the cause of double refraction

-Selective absorption of polarized light in some solids (dichroism) and liquids

-The effect of pressure on light refraction in solids (photoelasticity)

-Optical activity in ferromagnetic materials (Kerr magneto-optic effect)

-Absorption of some parts of the spectrum in solids and liquids (Beer’s law)

-Electro-optic phenomena in crystals (Pockels effect, modulation of light)

-Emission of electrons from metals when exposed to light (photoelectric effect)

-The Stark effect, fluorescence, black-body radiation, photochemistry, vision,...

Further compilations on Iceland spar in science are accessible at www.raunvis.hi.is/~leo