CHAPTER XXV.
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.
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_.
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.
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.