III. A further application of localized white-light fringes of superposition

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III. A further application of localized white-light fringes

of superposition

S. Tolansky

To cite this version:

III. A FURTHER APPLICATION OF LOCALIZED WHITE-LIGHT FRINGES OF SUPERPOSITION

By

S. TOLANSKY.

Sommaire. 2014 Les

franges de superposition localisées qui apparaissent en lumière blanche dans un dispositif à franges de Fizeau sont utilisées pour mettre en évidence de faibles inclusions et changements

d’indices dans le mica. On coupe en deux une feuille de mica argentée sur les deux faces et l’on utilise

les deux morceaux _{pour produire} un système de franges. Une différence de marche de 2,5 A seulement

pent être détectée à l’0153il. Cette technique se recommande par sa _simplicité.

LE JOURNAL DE PHYSIQUE ET LE RADIUM. I TOME

It,

JUILLET

4930,

PAGE 4:1().

Introduction. - The

principle

of

_{superposition}

of

_fringes

has been

long

in use, since in fact their

discovery

early

last

_century

_by

Brewster. Two

types

can be

used,

either

_fringes

of

_equal

incli-nation or

_fringes

of

_equal

thickness,

and

_they

can be used either with two beams or with

_multiple’

beams. The Brewster and Jamin

_fringes

are

_typical

two beam

_arrangements

and the _classical

super-position

system

of

_equal

inclination

_fringes

used

by

Fabry,

Perot and Buisson in the metre

deter-mination was the first use of

_{multiple-beams}

for such

_fringes.

More

_recently

Sears and Barrell determined the refractive index of air with a

_closely

similar

_optical

_arrangement.

By

far the

_larger

number of studies

_using

multiple-beam

_fringes

of

_{superposition}

have been made with

the

_fringes

of

_equal

_inclination,

and

_apart

from

very brief mention

by

Fabry

in his book " Les

appli-cations des

_{interfirences}

lumineuses "

very little use of the

_system

_{using fringes}

of

_equal

thickness appears to have been made.

It is the _{purpose of this} Note to describe such an

application, highly specific

it is

true,

yet

none the

less of much interest because of its

_great

_power

and

_simplicity.

The method is

_applied

to

_studying

"

invisible "

inclusions in mica sheets. The illus-trations used were taken for me

_by

_my

_assistant,

Miss Austin.

Experimental. -

The

_arrangements

_{are ~of}

the

simplest

possible.

The mica is cleaved and silvered

on both sides

_{(reflectivity}

go -

_g5

_per

cent).

The

piece

of mica is cut in

two,

and the

_pieces

held

close

_together,

i. e.

effectively parallel.

This is

illuminated with a

_parallel

beam of white

_light

from an arc or

_pointolite

and

following

the mica

is a lens of focal

length

25 -~ 5o cm. On

_placing

the eye at the

principal

focus localised white

_light

fringes

of

_{superposition}

are seen.

The results are

_striking.

If the mica had consisted

of a

_{truly parallel-sided}

_sheet,

then one would

_only

have seen a

_uniformity

of illumination. But in

general,

mica sheets have minute

_steps

on

_them,

cleavage

steps,

and it has

_already

been established that these are

_integral

_multiples

of . the

_crystal

lattice

_spacing

20

Á,

often

only

a

_single

lattice

spacing (see

M. B.

_{I.) yet}

_possibly

_extending

over

quite large

areas. Thus in the field of view one

sees

_beautifully

tinted areas,

separated

by

cleavage

lines. Plate 5 is a

_photograph

of such a

_region

and

_{gives only}

a _{very poor}

_impression

indeed of

what the eye

actually

sees, for the different

regions

have

_great

_purity

of colour and are also _very

brightly differently

coloured,

e. g. green,

red,

blue,

etc.

~

PL 5.

It is

_immediately

obvious from the

_{superposition}

principle

that that in effect one has

_white-light

multiple-beam

interference over the

_path

_lengths

which are the minute

_differences

in thickness between the two mica

_sheets,

i.e. over the

_cleavage

areas. It has been shown elsewhere

_by

the author

(see

M. B.

_I.)

that when

_using

_single

_{parallel-sided}

stepped

films

_(in

the 11

crossed Fizeau

_fringe

tech-nique

")

a

_change

of

_height

of but one

_crystal

lattice

_spacing

in mica can make a _{5 o per cent}

change

in the

_intensity

of transmission.

Indeed,

it

_requires

_only

the

_remarkably

small alteration

of

3.5 A

in

_{optical path}

difference to

_produce

a

detectable 10 _{per cent}

_change

in

_intensity

of illu-mination. Thus even with a

_single

double-silvered

film,

and monochromatic

_light,

a

_single

lattice

_change

is

_easily

detected

_providing

one

_adapts

the

_angle

of

incidence to

_give

correct

transmission,

as has been

already fully

discussed.

Now it is well know from the

_theory

of the

437

compound Fabry-Perot

interferometer

_{(see Tolansky,}

High

Resolution

_{Spectroscopy,}

Methuen that

the

_fringe

width of a double

interferometer,

both

identical,

is

/

times that of either alone. Hence

V2

the

_sensitivity

of the

_fringes

of

_{superposition}

to small

_changes

in

_path

is

_{V 2}

times as

_great

as in the

case of a

_single

mica film.

Furthermore,

one

_always

finds a suitable colour

_available,

and in

_addition,

the eye is

certainly

far more sensitive to a

_slight

colour-lint alteration than to a

slight

_intensity

alteration.

Pl. 6.

One

_recognises

therefore that the

_sensitivity

of the method is such that one should be able to detect

_optical

_path

differences of

_only

2.5

A,

i.e.

_{1 /2}

ooo th of a _green

_light

wave. This

_being

so, even _very

_slight

variations in _chemical compo-sition of mica should _{show up.}

_Any

11

invisible" inclusions should be revealed with _very

_great

contrast if

_they

have a refractive index

_differing

even

_slightly

from that of mica. In

_{plate 5}

a mass of

_unsuspected

_secondary

inclusions _{appear. These} cannot be seen

normally.

Plate 6 shows

_typical

11

invisible "

inclusions. Two features _may be remarked _upon,

_{namely, (a)}

the

_fringe

_{sharpness, (b)}

the

_reproduction

is

_only

in monotone and

_gives

a _very

_inadequate

_picture

of what is

_actually

seen

_by

_{the eye.}

Plate 7

is another characteristic

_example.

Here

one sees that the inclusions have often a small

crystalline

nucleus,

and one

notices,

too,

the

_revealing

secondary

structure

_surrounding

the nucleus. On other

_plates

_{continuous variations appear in}

_patchy

areas,

indicating

secondary changes

in chemical

composition.

Pl. 7.

Conclusion. -- The method has

many obvious

applications

to the

_study

of thick and thin films.

For

_example,

if films are

_put

down in a selected

piece

of mica and then

_silvered,

a means for

stu-dying

film

_uniformity

becomes available. This

should find

_application

in

_evaporated

films,

grown

films,

even

_{crystalline deposits}

and

_{probably organic}

monolayers.

It also becomes a means of

_matching

thick

_optical

specimens

for exact

_{equality (or equating}

two

_equal

Fabry-Perot interferometers),

for it is the differential

effect which leads to the

_{interference,}

but with thick

_specimens

collimation will

_be.

critical. The fact that a

_special

monochromatic source ’is not

needed is a considerable attraction.

The

_rapidity

and

_simplicity

of the

_technique

has

much to commend it. White

_light

sources

_(arcs)

are so

_bright

that one can tolerate

_quite

_heavy

silverings

and thus obtain maximum

_sensitivity.

With thin

_specimens

_very crude collimation can be used with little loss in

_sensitivity.

It is

_{re-emphasised}

that the monotone

repro-ductions

_given

here fail

_entirely

to

_give

an idea of the real

_sensitivity

a colour

_reproduction

reveals.

Intervention de M. 0. S. Heavens. The limit to the

_sharpness

of the

fringes

of

_equal

chromatic order would appear to be set

by

the statistical fluctuations in thickness of the

_deposited

silver film.

This will in turn set a limit to the

magnification

which may be used between the interferometer

and the

_dispersing

_system.

Intervention de M. Kuhn. This beautiful method of

_detecting

local

inclu-sons and variations in refractive index

is,

of course,

closely

related to the interference

_microscope.

These

_fringes

which are of the Brewster

_type

are,

however,

entirely

different from those used in the double interferometer. In the latter

instrument,

no interference takes

_place

_{of any two rays which}