502. _What is the difference between the refraction and the reflection
of light?_
_Refraction_ is the deviation of rays of light from their course
through the interference of a _different_ medium; _reflection_ is
the return of rays of light which, having fallen upon a surface, are
repelled by it.
503. _What is the radiation of light?_
The _radiation_ of light is its _emission in rays_ from the surface of
a _luminous body_.
504. _Do all bodies radiate light?_
All bodies radiate light; but those that are not in themselves primary
sources of light, are said to _reflect it_.
505. _Do black bodies reflect any light?_
Black bodies _absorb_ the light that falls upon them. But they reflect
a _very small_ degree of light.
506. _Why is glass transparent?_
Because its atoms are so arranged that they allow the vibrations of
light to continue through their substance.
[Verse: "As in water face answereth to face, so the heart of man to
man."--Proverbs xxvii.]
507. _Does glass obstruct the passage of any portion of light?_
Glass _reflects_ (sends back) a very small portion of light. This may
be observed by holding a piece of paper, or a hand, a few inches from
a window, when a faint reflection of it will be visible. Probably the
small amount of light _reflected by transparent glass, which gives a
passage to the greater part of the rays_, may serve to illustrate the
small amount of light reflected from _black surfaces_, which _absorbs
the greater portion of light_.
Instead of a piece of white paper, hold a piece of _black cloth_
two or three inches from the window-pane, and you will have two
reflections so weak that the image of the cloth will be almost
lost. The first reflection is that of the very small amount of
light from the black surface on to the glass, and the second
reflection is that of the inconceivably small amount returned by
the glass, and by which the faint image of the black cloth is
produced. But put the black cloth outside of the window-pane, and
then hold an object before them, and you will find that the _two
weak reflectors, acting together_, produce an improved image, or
reflection.
508. _Why, if a book is held between a candle-light and the wall, does
a shadow fall upon the wall?_
Because the rays of light are _intercepted_ by the book.
509. _Why do the rays pass over the edges of the book in a direct line
with the flame of the candle?_
Because light always travels in _straight lines_.
510. _Why is there some amount of light even where shadows fall?_
Because, _as all objects reflect light_, some of them throw their light
into the field of the shadow.
511. _Why are some substances opaque to light?_
Because the arrangement of their particles will not admit of the
_vibrations of the luminous ether_ passing through them.
Opaque--impervious to rays of light.
512. _Why do we see our faces reflected in mirrors?_
Because the rays of light from our faces are _reflected_ by the surface
of the _quicksilver_ at the back of the glass.
[Verse: "The day is thine, the night also is thine: thou hast
prepared the light and the sun."--PSALM LXXIV.]
513. _Why does the quicksilver reflect the rays of light?_
Because, being _densely opaque to light_, and presenting also a bright
surface, it is a good reflector, and it _throws back the whole of the
rays_.
514. _What has the glass to do with the reflection?_
The glass has _nothing to do with the reflection_, except that it
affords a field upon which the reflecting surface of the quicksilver is
spread; and it keeps the air and dirt from _dulling the quicksilver_.
The parts of a mirror from which the quicksilver is rubbed away
give no reflection that could assist the reflecting power of the
quicksilver. That the surface of the glass does not reflect the
image, is shown by the fact, that if you put the point of any
object against the glass, the thickness between the point and the
place where the reflection of it begins, will _show the exact
thickness of the glass_.
515. _Why does a compound mirror (a multiplying mirror) exhibit a large
number of images of one object._
Because all objects reflect rays of light in _every direction_, and
therefore the different mirrors, being at _various angles_, receive
_each a reflection_ of the same object.
516. _Why does a window-pane appear to be a better reflector by
candle-light than by day-light?_
The reflecting power of glass is precisely the same by night as by
day, and is always very feeble. But it appears to be greater by night,
_because the surrounding darkness increases the apparent strength of
the reflection_.
517. _How do we know that objects reflect light in every direction?_
Because if we _prick a hole in a card with a pin_, and then look
through that small hole upon a _landscape_, we can see some miles of
country, and some thousands of objects; every part of every object
throughout the whole scene, must have sent rays of light the small hole
pricked in the card.
[Verse: "Such knowledge is too wonderful for me; it is high, I
cannot attain unto it."--PSALM CXXXIX.]
At one extremity of the landscape, viewed through the hole in the
card, there may be a forest of trees; in the distance there may be
hills bathed in golden light, and overhung with glittering clouds;
in the mid-distance there may be a river winding its course along,
as though it loved the earth through which it ran, and wished,
by wandering to and fro, to refresh the thirsty soil; in the
foreground may be a church, covered by a million ivy leaves; and
grouping towards the sacred edifice may be hundreds of intending
worshippers, old and young, rich and poor; flowers may adorn the
path-ways, and butterflies spangle the air with their beauties;
yet every one of those objects--the forest, the hills, the clouds,
the river, the church, the ivy, the people, the flowers, the
butterflies--must have sent rays of light, which found their way
through the little hole in the card, and entered to paint the
picture upon the curtain of the eye.
This is one of the most striking instances that can be afforded
of the wonderful properties of light, and of the infinitude of
those luminous rays that attend the majestic rising of the sun.
Not only does light fly from the grand "ruler of the day" with
a velocity which is a million and a half times greater than
the speed of a cannon-ball, but it darts from every reflecting
surface with a like velocity, and reaches the tender structure of
the eye so gently that, as it falls upon the little curtain of
nerves which is there spread to receive it, it imparts the most
pleasing sensations, and tells its story of the outer world with a
minuteness of detail, and a holiness of truth. Philosophers once
sought to _weigh_ the _sunbeam_; they constructed a most delicate
balance, and suddenly let in upon it a beam of light; the lever
of the balance was so delicately hung that the fluttering of a
fly would have disturbed it. Everything prepared, the grave men
took their places, and with keen eyes watched the result. The
sunbeam that was to decide the experiment had left the sun eight
minutes prior to pass the ordeal. It had flown through ninety-five
millions of miles of space in that short measure of time, and it
shot upon the balance with unabated velocity: but the lever moved
not, and the philosophers were mute.