Summary of Part_3 (English)

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3-8: Summary of Part 3

Let us summarize

what we have

learned in Part 3.

(1)

It was clarified

that the law of equipartition of energy

that is inevitably

derived from

the classical theory

(Newtonian mechanics

and Maxwellian electromagnetism)

is not satisfied

for the heat capacities

of solids

at low temperature

and for the cavity radiation.

This was a deadlock

which the classical theory

came to.

(2)

It was made clear

that the deadlock

would be overcome

by introducing Planck's

hypothesis of energy quanta,

i.e.,

the energy is

not a continuous quantity

but exists in a form

of "grains"

(energy quanta).

(3)

Extending Planck's hypothesis,

Einstein proposed

the hypothesis

of light quanta,

i.e., light exists

in a form of "particles"

and it is

absorbed into atoms

as a whole unit instantaneously.

This particle nature of light was confirmed

by experiment.

The particle of light

was called photon,

and has been admitted

into the brotherhood

of "particles" like

electrons or protons.

The particle nature

of light can never

be explained with

the classical theory.

It is an amazing property

from the point of view

of the classical theory.

[The Particle Nature and the Wave Nature of Light]

Although the particle nature

of light was made clear,

yet its wave nature

has not been denied.

The fact that

light causes

the diffraction

and the interference

cannot be understood

on the basis

of the particle nature.

Let us consider

Young's experiment

with a double-slit

by which

T. Young

(UK, 1773 - 1829)

confirmed the wave nature

of light

for the first time.

A schematic drawing

of Young's experiment

is given in the following

figure.

Young's double-slit experiment

A monochromatic light

from a point source passes

through the two thin slits,

S₁

and

S₂,

and makes an interference

fringes

(striped pattern)

on the screen.

An example of

the interference fringes

is shown in the following

picture.

The above picture (A)

represents the picture

of the pattern

on the screen

when only one of

the slits is open,

and

(B)

shows the picture

of the striped pattern

on the screen

when both the slits are open.

The reason why such

a striped pattern appears

on the screen

in the double-slit experiment

is as follows:

Path difference in Young's experiment

As seen in the above figure,

the incident monochromatic light

(electromagnetic wave)

passes through both

the slits separately.

The two groups (beams)

of light,

one passing through

the slit S₁

and the other the slit

S₂,

interfere

with each other

after having passed

through each slit.

Let us assume

that the distance

between the two slits is d.

Here we think

of the path of beams

from the slit

S₁

(or S₂)

to a point A on the screen.

If the path difference

is equal to

an integral multiple

of the wavelength,

then the two beams

reinforce each other

at the point A

and constructive

interference occurs.

If the path difference

is equal to

where n

is an odd integer,

destructive interference occurs

and a dark fringe results.

Thus there appears

an striped interference pattern

on the screen.

If we assume

that light is "particles",

one particle

cannot pass

through two slits

simultaneously

so that such a interference

pattern as in

the above picture (B)

never appears.

The experimental fact that

there actually appear

the interference fringes

on the screen

implies that

light must be waves

and a wave passes

through the double slits

partially and simultaneously

to make an interference pattern.

Looking at these facts,

we have to consider that

light must be

particles

at one moment

and waves

at another moment.

Then, when is it particles?

And

when is it waves?

This is a quite

difficult question.

What is the true

feature of light?

The answer to this question

has first been given

by the construction

of Quantum Mechanics.

Until then,

physicists could not help

changing their attitudes

depending on

what kind of problem

they met with.

They adopt the particle

theory on one occasion

and the wave theory

on the other.

Someone told a joke

that they obey

the particle theory

on Monday, Wednesday and Friday

in a week,

and the wave theory

on Tuesday, Thursday

and Saturday.

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