472. _What is the refraction of light?_
When rays of light fall _obliquely_ upon the surface of any
_transparent medium_, they are slightly diverted from their course.
This alteration of the course of the rays is called _refraction_, and
the degree of refraction is influenced by the difference between the
_densities_ of the mediums _through which light is transmitted_.
[Verse: "Let your light so shine before men, that they may see your
good works, and glorify your father which is in heaven."--MATT. V.]
473. _If a ray of light falls in a straight line upon a transparent
surface, is it then refracted?_
In that case the ray pursues its course--_there is no refraction_.
474. _Is the direction in which the rays are bent, or refracted,
influenced by the relative densities of the media?_
A ray of light falling slantingly upon a _window_, in passing through
it is slightly brought to the _perpendicular_; and if it then falls
upon the surface of water, it is still further brought to the
perpendicular in _passing through the water_.
475. _Is light refracted in passing from a dense medium to a thinner
one?_
It is; but the _direction of the refraction_ is just the opposite to
the instance just given; a ray of light passing through _water_ into
_air_, does not take a more _perpendicular course_, but becomes more
_oblique_.
[Illustration: Fig. 11.]
476. _Why, if a rod or a spoon be set in an empty basin, will it appear
straight, or of its usual shape?_
Because the rays of light that are reflected from it all pass through
the same medium, the _air_.
477. _Why if water be poured into the basin will the rod or spoon
appear bent?_
Because the rays of light that pass through the _water_ are _reflected
in a different degree_ to those that pass through the air.
[Verse: "Evening, and morning, and at noon, will I pray, and cry
aloud; and he shall hear my voice."--PSALM LV.]
Place in the bottom of an empty basin (Fig. 11.) a shilling;
then stand in such a position at the point B that the line of
sight, over the edge of the basin, just excludes the shilling
from view. Then request some one to pour water into the basin,
until it is filled to C (Fig. 12.), keeping your eye fixed upon
the spot. The shilling will gradually appear, and will soon come
entirely in view. Not only will the shilling be brought in view,
but also portions of the basin before concealed. This is owing to
the rays of light passing from the bottom through the water in a
direction _more perpendicular_ than they would have done through
the air; but on leaving the water they become more _oblique_, and
hence they convey the image of the shilling _over the edge of the
basin_, which otherwise would have obstructed the view.
[Illustration: Fig. 12.]
478. _Why is it that in cloudy and showery days we see the sun's rays
bursting through the clouds in different directions?_
Because, in passing through clouds of _different densities_ the rays
are _bent out of their course_.
479. _Why is the apparent depth of water always deceptive?_
Because the light reflected from the objects at the bottom is
_refracted_ as it leaves the water.
480. _How much deeper is water than it appears to be?_
About _one-third_. A person bathing, and being unable to swim, should
calculate before jumping into the water, that if it _looks two feet
deep_, it is quite _three feet_.
481. _Why can we seldom at the first attempt touch anything lying at
the bottom of the water with a stick?_
Because we do not allow for the _different refractive powers_ of water
and of air.
[Verse: "I do set my bow in the cloud, and it shall be for a token
of a covenant between me and the earth."]
482. _Why do we see the sun before sunrise, and after sunset?_
Because of the refractive effects of the atmosphere. Rays of light,
passing obliquely from the sun through the air to the earth, are
refracted three or four times by the varying density of the medium.
Each refraction bends the rays towards the _perpendicular_; and hence
we see the sun _before it rises_ and _after it sets_.
[Illustration: Fig. 13.--DIAGRAM EXHIBITING THE REFRACTION OF THE SUN'S
RAYS IN PASSING THROUGH THE ATMOSPHERE.]
483. _Why do figures, viewed through the hot air proceeding from
furnaces, and from lime-kilns, appear distorted and tremulous?_
Because the ever varying density of the air which is flying away in hot
currents, and succeeded by cold, _constantly changes the refractive
power_ of the medium through which the figures are viewed.
484. _Why do the stars twinkle?_
Because their light reaches us through _variously heated and moving
currents of air_. In this case the earth is the _kiln_, and the _stars_
the _object_ that is _viewed through the refractive medium_.
485. _Why does much twinkling of the stars foretell bad weather?_
Because it denotes that there are _various ærial currents_ of different
temperatures and densities, producing _atmospheric disturbance_.
[Verse: "And it shall come to pass, when I bring a cloud over the
earth, that the bow shall be seen in the cloud."--GENESIS IX.]
486. _What causes the rainbow?_
The _refraction_ of the sun's rays by the _falling rain_.
487. _Why does the rainbow exhibit various colours?_
The colours belong to the _elementary rays of light_; and these rays
having _different degrees of refrangibility_, some of them are bent
more than others; they are therefore separated into _distinct rays of
different colours_.
488. _Why are there sometimes two rainbows?_
Because the rays of _refracted_ light, reflected upon other drops of
rain, are _again_ refracted, and then _reflected again_, forming a
secondary bow.
489. _Why are the colours of the secondary bow arrayed in the reverse
order of the primary bow?_
Because the secondary bow is _a reflection_ of the primary bow, and,
like all reflections, is reversed.
490. _Why are reflections reversed?_
Because those rays which _first reach_ the reflecting surface are the
_first returned_. If you hold your open hand towards the looking-glass,
the light passing from the point of your finger will reach the
reflector and be returned before the rays that pass from the back parts
of the hand. Hence the image of the hand will present the reflection of
the finger point towards the point of the finger.
491. _Why are the colours of the secondary rainbow fainter than those
of the primary?_
Because they are derived from the _refraction and reflection_ of rays
which have _already_ been refracted and reflected, and thereby _their
intensity has been diminished_.
492. _What is a lunar rainbow?_
A _lunar rainbow_ is caused by the light of the _moon_, in the same
manner as the _solar rainbow_ is caused by the light of the _sun_.
[Verse: "I am come a light into the world, that whosoever believeth
in me should not abide in darkness."--JOHN XIII.]
493. _Why is the lunar rainbow fainter than a solar rainbow?_
Because the _light of the moon_ is the _reflected light of the sun_,
and is therefore _less intense_.
494. _What is a halo?_
A halo is a _luminous ring_, which forms between the eye of the
observer and a luminous body.
Haloes may appear around the disc of the sun, moon, or stars. But in
this country the _lunar_ haloes are the most remarkable and frequent.
495. _What is the cause of the luminous ring?_
The _refraction of light_ as it passes through an intervening _cloud_,
or a stratum of _moist_ and _cold air_.
496. _Why are haloes sometimes large and at other times small?_
Because they are sometimes formed _very high_ in the atmosphere, at
other times _very low_. Being high, and farther removed from the
spectator, and nearer the source of light, they appear _smaller_; while
the nearer they are, the _larger they appear_.
497. _Why do haloes foretell wet weather?_
Because they show that there is a great amount of atmospheric moisture,
which will probably form _rain_.
498. _Why do glass lustres and chandeliers exhibit "rainbow colours"?_
Because they _refract the rays of light_ in the same manner as the rain
drops.
499. _Why does a soap bubble show the prismatic colours?_
Because, like a large rain drop, it _refracts the rays of light_, and
shows the elementary rays.
500. _What causes the rich tints displayed by "mother-of-pearl?"_
The _refraction of the light_ that falls upon the surface of the pearl.
[Verse: "Light is sown for the righteous, and gladness for the
upright in heart."--PSALM XCVII.]
501. _What causes the brilliant colours of the diamond?_
The _refraction_ of the rays of light by the various _facets_ of the
diamond.
The refraction of light, and the production of prismatic colours,
surrounds us with most interesting phenomena. The laundress,
whose active labours raise over the wash-tub a soapy froth,
performs inadvertently one of the most delicate operations of
chemistry--the chemistry of the imponderable agents--and the
result of her manipulations manifests itself in the delicate
colours that dance like a fairy light over the glassy films that
follow the motion of her arms. The laughing child, throwing a
bubble from the bowl of a tobacco pipe into the air, performs
the same experiment, and produces a result such as that which
filled the philosophic Newton with unbounded joy. The foam of the
seashore, the plumage of birds, the various films that float upon
the surface of waters, the delicate tints of flowers, and the rich
hues of luscious fruits, all combine to remind us, that every ray
of light comes like an angelic artist sent from heaven, bearing
upon his palette the most celestial tints, with which to beautify
the earth, and show the illimitable glory of God.