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.

"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.

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.

"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.

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.

"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.

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.

"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.

"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.

"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.