Epilogue: The End (English)

Microscopic World -1- (Mysteries of Atomic World)

Epilogue : Opening to Quantum Mechanics

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Throughout all the pages of

the present Seminar,

Microscopic World -1-,

we have learned that

matter and

light in the

microscopic world

possess both the

particle nature and the

wave nature.

In addition,

we could understand

that the Bohr model of

atomic structure

is constructed

by considering

both the natures.

By using this model,

it has become possible

to explain the stability

of atoms and

the atomic spectra

which was impossible

to be explained with the

classical theory

(Newtonian mechanics

and Maxwellian electromagnetism).

It seems that

the double nature

(the duality)

of matter and light,

i.e. the wave

and particle natures,

is a very fundamental

property of "things"

or "beings"

existing in our world.

In other word,

the duality seems

to be a fundamental philosophy

for the "existence"

of things.

This looks quite

strange and mysterious

for us,

because we are very much

used to the thinking way

in the classical theory.

[Young's Experiment]

Let us look back

at Young's experiment

by which the wave nature

of light was confirmed

for the first time.

For the sake

of convenience,

the contents of

Young's experiment

presented in the page

3-8: Summary of Part 3

are shown here again.

Schematic drawing of Young's experiment

The monochromatic light

from the source passes

through the two slits,

S₁

and

S₂,

and produce an interference

pattern (fringes)

on the screen.

An example of

the picture of the pattern

is represented in

the following picture (A)

and (B).

In these pictures,

(A) shows the result when

one of the slits is closed,

and (B) the result when

both of the slits are opened.

We will discuss

on some details

of these pictures below.

Before that,

let us study

a little about

the relation between the basic

mechanism of

photography

and the particle nature

of light.

[The Mechanism of Photography and the Light Particle]

In Young's experiment,

the reason that

the photographic plate

(or film)

can response

to light is due to

the particle nature of light.

Let us explain this below.

Photography is based

on the chemical reactions

brought about

by the action of light

(photochemical reactions).

Namely, the molecules

of halogenated silver

are dissociated

by the illumination

of light

and silver atoms

are educed.

For example,

silver bromide

(AgBr) that is

one of the halogenated silvers

is dissociated by light as

AgBr

-->

Ag + Br

to be silver

and bromine atoms.

After being treated

with special chemicals,

only the silver atoms

remain to be black coloured

and to make

a photographic negative.

To dissociate a molecule

of halogenated silver,

roughly speaking,

more energy than

about 2 eV

is necessary.

Namely, one molecule

of silver bromide

(AgBr)

or silver iodide

(AgI)

can be dissociated

only by the accumulation

of more energy than

about 2 eV

This amount of energy

is almost the same as

the value of the

work function

which is the

necessary minimum energy

to bring about

the photoelectric effect.

(Refer to the page

3-6: The Hypothesis of Light Quanta and the Photoelectric Effect.)

Hence, the discussion

on that page

about the time

to make the photoelectric effect

take place

is valid for the present

photochemical reaction

as well;

i.e., if we consider

the time necessary

for the dissociation

of the halogenated silver

within the classical theory,

it should take

a time longer

than the shutter speed

of an ordinary camera

that is usually shorter

than 1/100 s or 1/1000 s.

Consequently,

we cannot help

considering that

light can dissociate

the halogenated

silver molecules

being absorbed instantly

as a particle of light

(photon).

[The Mysterious Particle-Wave Duality]

The image of

the interference fringes

recorded on the

photographic plate

(film)

is consists of

huge number of silver atoms

educed by the photochemical

dissociation of

halogenated silver molecules.

These silver atoms collect

together to make the

interference pattern

on the film.

We recognize this collection

of the silver atoms

as a picture of the

interference pattern.

Namely, we are looking at

the collection of

marks or

traces of photons.

Each mark denotes

a point where

a particle of light

(photon) has struck.

The collection

of these huge number

of marks

constitutes a interference

fringes which characterize

the wave nature of light.

How can we understand

this mechanism?

If light were

simply particles,

one light particle

could not pass through

two slits

simultaneously,

so that it could never

produce interference fringes.

If light were simple

particles,

then the picture

on the screen

should be what

is obtained by superposing

such a picture

in the case of

a single slit opened

as shown in

the above picture (A)

with another picture

obtained by displacing it

by the interslit

distance d.

However, we have

an interference pattern

as shown in

the picture (B)

in practice.

Hence, we must consider

that light is not

a simple particle,

but possesses some kind

of wave nature as well.

If light were

pure waves,

we could not have

a picture of

the interference fringes.

Because light has

the duality

of wave nature

and particle nature

simultaneously,

we can have the picture

of the interference pattern.

We therefore cannot

deny the dual

nature of light.

Then, what part of light

is a wave?

And what part

of light is a particle?

You might suspect that

two different photons

interfere with each other

after they pass through

two slits separately

and consequently

they produce the

interference pattern

on the screen.

However this idea

is not valid

by the following reason:

If the intensity

of the incident light

to the double-slit

experiment is so weak

that only one photon

runs at every

moment of time,

two different photons

are impossible

to pass through two slits

at one time

and

no interference occurs

among different photons.

In spite of such extreme

weakness of the intensity

of the incident light,

the same interference pattern

is still obtained

on the photographic plate

by exposing it for

an extremely long time.

Such an experiment of

three-month exposure time

was carried out

in 1909 by a British student,

and he got a clear

interference pattern

that is exactly the same as

those in usual

Young's experiment.

This tells us that

the interference pattern in Young's experiment is not caused by the interference between different photons.

Then, why does the

interference pattern

come into existence?

[Opening to Quantum Mechanics]

As mentioned above,

we have encountered

a serious discrepancy

between

the particle nature

and the wave nature

of light.

This discrepancy

is not only

for light but also

for electrons.

In order to perfectly

overcome this discrepancy,

people have had

to construct a completely

new theory

getting over the

difficulties in the

classical theory.

This is nothing else

than Quantum Mechanics

proposed by

W. K. Heisenberg

(Germany, 1901 - 76)

and

E. Schroedinger

(Austria, 1887 - 1961).

By this quantum mechanics,

the mystery of the particle

and wave natures

of light and electrons

were solved and

the atomic structure

was clarified.

You are strongly expected

to challenge to study

Quantum Mechanics

proceeding to

the Second Part of

the present seminar,

Microscopic World -2- (Introduction to Quantum Mechanics).

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