518. _Why, when we move before a mirror, does the image draw near
to the reflecting surface as we draw near to it, and retire when we
retire?_
Because the lines and angles of _reflection_ are always equal to the
lines and angles of _incidence_.
519. _What is the line of incidence?_
If a person stands in a direct line before a mirror, the line through
which the light travels from him to the mirror is _the line of
incidence_.
_Incidence_--falling on.
[Verse: "Blessed be the Lord, who daily loadeth us with benefits,
even the God of our salvation."--PSALM LXVIII.]
520. _What is the line of reflection?_
The _line of reflection_ is the line in which the rays of light are
returned from the image formed in the glass to the eye of the observer.
_Reflection_--a turning back.
521. _What is the angle of incidence?_
The _angle_ of incidence is the angle which rays of light, falling on a
reflecting surface, make with a line perpendicular to that surface.
[Illustration: Fig. 14.--EXPLAINING THE LINES AND ANGLES OF INCIDENCE
AND OF REFLECTION.]
522. _What is the angle of reflection?_
The _angle_ of reflection is the angle which is formed by the returning
rays of light, and a line perpendicular to the reflecting surface. It
is always _equivalent_ to the angle of incidence.
Take a marble and roll it across the floor, so that it shall
strike the wainscot obliquely. Let A in the diagram represent the
point from which the marble is sent. The marble will not return to
the hand, nor will it travel to the line B, but will bound off, or
be _reflected_, to C. Now B is an imaginary line, _perpendicular
to the reflecting surface_; and it will be found that the path
described by the marble in _rolling to the surface and rebounding
from it_, form, with the line B, two angles that are _equal_.
These represent the angles of _incidence_ and of _reflection_,
and explain why the reflection of a person standing at A before a
mirror, would be seen by another person standing at C. This simple
law in optics explains a great many interesting phenomena, and
therefore it should be clearly impressed upon the memory.
[Verse: "And God made two great lights; the greater light to rule
the day, and the lesser light to rule the night: he made the stars
also."--GEN. I.]
523. _Why do windows reflect the sun in the evening?_
Because the eye of the observer is in the _line of the reflection_.
524. _Why do windows not reflect the sun at noon?_
They do, but our eyes are not then in the _line of the reflection_.
[Illustration: Fig. 15.--SHOWING THE LINES OF INCIDENCE AND REFLECTION
OF THE SUN'S RAYS AT NOON AND AT EVENING.]
It is obvious from the foregoing diagram that the evening rays of
reflection fall upon the eyes of spectators, while the reflections
at noon are so perpendicular that they are lost.
525. _Why do the sun and moon appear smaller when near the meridian,
than when near the horizon?_
Because, when near the horizon, they are brought into _comparison with
the sizes of terrestrial objects_; but when near the meridian they
occupy the centre of a vast field of sky, and as there are no objects
of comparison surrounding them, they _appear smaller_.
[Verse: "There is no darkness nor shadow of death, where the workers
of iniquity may hide themselves."--JOB XXXIV.]
This is one "Reason Why," assigned by some observers. But there is
also another reason to be found in the fact that, when the sun or
moon is near the horizon, we view it through a _greater depth of
atmosphere_ than we do when at the meridian. (_See_ Fig. 13.) A
straight line passed upward through the air, would not be so long
as that which passes to S. Consequently, as the air is generally
impregnated with moisture, at the time when these effects are
observed, the rays of light are caused to diverge more, and the
disc of the sun or moon _appears magnified_. Probably both of
these reasons contribute to the effect. This latter reason also
explains why the disc of the sun or moon may sometimes appear
_oval_ in shape, the lower stratum of air being more loaded with
moisture than that through which we view the upper part of the
disc.
526. _Why do our shadows lengthen as the sun goes down?_
Because light travels only in _straight lines_, and as the sun
descends, the direction of his rays becomes more _oblique_, thereby
causing longer shadows.
527. _What is the cause of the optical illusions frequently observed in
nature?_
There are various kinds of natural optical illusions:--
The _mirage_, in which landscapes are seen reflected in burning sands.
The _fata morgana_, in which two or three reflections of objects occur
at the same time.
The _ærial spectra_, or ærial reflections, &c.
[Illustration: Fig. 16.--ILLUSTRATING THE APPEARANCE OF PHANTOM SHIPS.]
The optical illusions above enumerated owe their origin to various
atmospheric conditions, in which _refractions_ and _reflections_
are _multiplied_ by the different densities of atmospheric layers.
They chiefly occur in hot countries, where, from the varying effects
of heat, the conditions of atmospheric refraction and reflection
frequently prevail in their highest degree.
[Verse: "In the morning ye say, it will be foul weather to-day, for
the sky is red and lowering."--MATT. XVI.]
528. _Why do we have twilight mornings and evenings?_
Because the coming and the departing rays of the sun are _refracted_
and _reflected_ by the upper portions of the atmosphere. (_See_ Fig.
13.)
529. _How long before the sun appears above the horizon does the
reflection of his light reach us?_
The time _varies_ with the refracting and reflecting power of the
atmosphere, from _twenty minutes_ to _sixty minutes_. But the sun's
position is usually _eighteen degrees_ below the horizon when twilight
begins or ends.
530. _Why is the sky blue?_
The white light of the sun falls upon the earth without change; it
is then reflected back by the earth, and as it passes through the
atmosphere portions of it are again returned to us, and this double
reflection produces a _polarised_ condition of light which imparts to
vision the sensation of a _delicate blue_. (_See_ 549.)
531. _Why do the clouds appear white?_
Because they reflect back to us the solar beam _unchanged_.
532. _Why does the sky appear red at sunset?_
Because the light vapours of the air, which are condensed as the sun
sets, refract the rays of light, and produce red rays. The refraction
which produces _red_ requires only a _moderate degree of density_.
533. _Why do the clouds sometimes appear yellow?_
Because there is a larger amount of vapour in the air, which produces a
different degree of refraction, _resulting in yellow_.
534. _Why does a yellow sunset foretell wet weather?_
Because it shows that the air is heavy with vapours. The refraction
that produces _yellow_ requires a greater degree of density.
[Verse: "When it is evening ye say it will be fair weather, for the
sky is red."--MATT. XVI.]
535. _Why does a red sunset foretell fine weather?_
Because the redness shows that the vapours in the air _towards the
West_, or wet quarter, are _light_, as is evidenced by the degree of
refraction of the sun's rays.
536. _Why does a red sunrise foretell wet?_
Because it shows that _towards the East_, or dry quarter, the air is
charged with vapour, and therefore probably at other points the air has
reached _saturation_.
537. _Why does a grey sunrise foretell a dry day?_
Because it shows that the vapours in the air are _not_ very dense.
538. _Why is "a rainbow in the morning the shepherd's warning?"_
Because it shows that _in the West_, or wet quarter, the air is
_saturated_ to the rain point.
539. _Why is "a rainbow at night the shepherd's delight?"_
Because it shows that the _rain is falling in the East_, and as that
is a dry quarter, it will _soon be over_. Rainbows are always seen in
opposition to the sun.