42. (Pg. 177) How if divergent?

43. (Pg. 177) How, and why, may concave, become burning mirrors?

44. (Pg. 178) Why may rays of light coming from the sun, be viewed as parallel to each other?

45. (Pg. 178) If a luminous body, as a burning taper, be placed in the focus of a concave mirror, how will the rays from it, be reflected?

(fig. 6.)

46. (Pg. 178) What fact is explained by fig. 7, plate 18?

CONVERSATION XVI.

ON REFRACTION AND COLOURS.

TRANSMISSION OF LIGHT BY TRANSPARENT BODIES. REFRACTION. REFRACTION BY THE ATMOSPHERE. REFRACTION BY A LENS. REFRACTION BY THE PRISM. OF COLOUR FROM THE RAYS OF LIGHT. OF THE COLOURS OF BODIES.

MRS. B.

The refraction of light will furnish the subject of to-day's lesson.

_Caroline._ That is a property of which I have not the faintest idea.

_Mrs. B._ It is the effect which transparent mediums produce on light in its pa.s.sage through them. Opaque bodies, you know, reflect the rays, and transparent bodies transmit them; but it is found, that _if a ray, in pa.s.sing from one medium, into another of different density, fall obliquely, it is turned out of its course. The ray of light is then said to be refracted._

_Caroline._ It must then be acted on by some new power, otherwise it would not deviate from its first direction.

_Mrs. B._ The power which causes the deviation of the ray, appears to be the attraction of the denser medium. Let us suppose the two mediums to be air, and water; if a ray of light pa.s.ses from air, into water, it is more strongly attracted by the latter, on account of its superior density.

_Emily._ In what direction does the water attract the ray?

_Mrs. B._ The ray is attracted perpendicularly towards the water, in the same manner in which bodies are acted upon by gravity.

If then a ray, A B, (fig. 1, plate 19.) fall perpendicularly on water, the attraction of the water acts in the same direction as the course of the ray: it will not, therefore, cause a deviation, and the ray will proceed straight on, to E. But if it fall obliquely, as the ray C B, the water will attract it out of its course. Let us suppose the ray to have approached the surface of a denser medium, and that it there begins to be affected by its attraction; this attraction, if not counteracted by some other power, would draw it perpendicularly to the water, at B; but it is also impelled by its projectile force, which the attraction of the denser medium cannot overcome; the ray, therefore, acted on by both these powers, moves in a direction between them, and instead of pursuing its original course to D, or being implicitly guided by the water to E, proceeds towards F, so that the ray appears bent or broken.

_Caroline._ I understand that very well; and is not this the reason that oars appear bent in the water?

_Mrs. B._ It is owing to the refraction of the rays, reflected by the oar; but this is in pa.s.sing from a dense, to a rare medium, for you know that the rays, by means of which you see the oar, pa.s.s from water into air.

_Emily._ But I do not understand why refraction takes place, when a ray pa.s.ses from a dense into a rare medium; I should suppose that it would be less, attracted by the latter, than by the former.

_Mrs. B._ And it is precisely on that account that the ray is refracted.

Let the upper half of fig. 2, represent gla.s.s, and the lower half water, let C B represent a ray, pa.s.sing obliquely from the gla.s.s, into water: gla.s.s, being the denser medium, the ray will be more strongly attracted by that which it leaves than by that which it enters. The attraction of the gla.s.s acts in the direction A B, while the impulse of projection would carry the ray to F; it moves, therefore, between these directions towards D.

_Emily._ So that a contrary refraction takes place, when a ray pa.s.ses from a dense, into a rare medium.

[Ill.u.s.tration: PLATE XIX.]

_Mrs. B._ The rule upon this subject is this; _when a ray of light pa.s.ses from a rare into a dense medium, it is refracted towards the perpendicular; when from a dense into a rare medium, it is refracted from the perpendicular_. By the perpendicular is meant a line, at right angle with the refracting surface. This may be seen in fig. 1, and fig. 2, where the lines A E, are the perpendiculars.

_Caroline._ But does not the attraction of the denser medium affect the ray before it touches it?

_Mrs. B._ The distance at which the attraction of the denser medium acts upon a ray, is so small, as to be insensible; it appears, therefore, to be refracted only at the point at which it pa.s.ses from one medium into the other.

Now that you understand the principle of refraction, I will show you the real refraction of a ray of light. Do you see the flower painted at the bottom of the inside of this tea-cup? (Fig. 3.)

_Emily._ Yes.--But now you have moved it just out of sight; the rim of the cup hides it.

_Mrs. B._ Do not stir. I will fill the cup with water, and you will see the flower again.

_Emily._ I do, indeed! Let me try to explain this: when you drew the cup from me, so as to conceal the flower, the rays reflected by it, no longer met my eyes, but were directed above them; but now that you have filled the cup with water, they are refracted, and bent downwards when pa.s.sing out of the water, into the air, so as again to enter my eyes.

_Mrs. B._ You have explained it perfectly: fig. 3. will help to imprint it on your memory. You must observe that when the flower becomes visible by the refraction of the ray, you do not see it in the situation which it really occupies, but the image of the flower appears higher in the cup; for as objects always appear to be situated in the direction of the rays which enter the eye, the flower will be seen at B, in the direction of the refracted ray.

_Emily._ Then, when we see the bottom of a clear stream of water, the rays which it reflects, being refracted in their pa.s.sage from the water into the air, will make the bottom appear higher than it really is.

_Mrs. B._ And the water will consequently appear more shallow. Accidents have frequently been occasioned by this circ.u.mstance; and boys, who are in the habit of bathing, should be cautioned not to trust to the apparent shallowness of water, as it will always prove deeper than it appears.

The refraction of light prevents our seeing the heavenly bodies in their real situation: the light they send to us being refracted in pa.s.sing into the atmosphere, we see the sun and stars in the direction of the refracted ray; as described in fig. 4, plate 19., the dotted line represents the extent of the atmosphere, above a portion of the earth, E B E: a ray of light coming from the sun S, falls obliquely on it, at A, and is refracted to B; then, since we see the object in the direction of the refracted ray, a spectator at B, will see an image of the sun at C, instead of its real situation, at S.

_Emily._ But if the sun were immediately over our heads, its rays, falling perpendicularly on the atmosphere, would not be refracted, and we should then see the real sun, in its true situation.

_Mrs. B._ You must recollect that the sun, is vertical only to the inhabitants of the torrid zone; its rays, therefore, are always refracted, in this lat.i.tude. There is also another obstacle to our seeing the heavenly bodies in their real situations: light, though it moves with extreme velocity, is about eight minutes and a quarter, in its pa.s.sage from the sun to the earth; therefore, when the rays reach us, the sun must have quitted the spot he occupied on their departure; yet we see him in the direction of those rays, and consequently in a situation which he had abandoned eight minutes and a quarter, before.

_Emily._ When you speak of the sun's motion, you mean, I suppose, his apparent motion, produced by the diurnal motion of the earth?

_Mrs. B._ Certainly; the effect being the same, whether it is our earth, or the heavenly bodies, which move: it is more easy to represent things as they appear to be, than as they really are.

_Caroline._ During the morning, then, when the sun is rising towards the meridian, we must (from the length of time the light is in reaching us) see an image of the sun below that spot which it really occupies.

_Emily._ But the refraction of the atmosphere, counteracting this effect, we may, perhaps, between the two, see the sun in its real situation.

_Caroline._ And in the afternoon, when the sun is sinking in the west, refraction, and the length of time which the light is in reaching the earth, will conspire to render the image of the sun, higher than it really is.

_Mrs. B._ The refraction of the sun's rays, by the atmosphere, prolongs our days, as it occasions our seeing an image of the sun, both before he rises, and after he sets; when below our horizon, he still shines upon the atmosphere, and his rays are thence refracted to the earth: so likewise we see an image of the sun, previously to his rising, the rays that fall upon the atmosphere being refracted to the earth.

_Caroline._ On the other hand, we must recollect that light is eight minutes and a quarter on its journey; so that, by the time it reaches the earth, the sun may, perhaps, have risen above the horizon.

_Emily._ Pray, do not gla.s.s windows, refract the light?

_Mrs. B._ They do; but this refraction would not be perceptible, were the surfaces of the gla.s.s, perfectly flat and parallel, because, in pa.s.sing through a pane of gla.s.s, the rays suffer two refractions, which, being in contrary directions, produce nearly the same effect as if no refraction had taken place.

_Emily._ I do not understand that.

_Mrs. B:_ Fig. 5, plate 19, will make it clear to you: A A represents a thick pane of gla.s.s, seen edgeways. When the ray B approaches the gla.s.s, at C, it is refracted by it; and instead of continuing its course in the same direction, as the dotted line describes, it pa.s.ses through the pane, to D; at that point returning into the air, it is again refracted by the gla.s.s, but in a contrary direction to the first refraction, and in consequence proceeds to E. Now you must observe that the ray B C and the ray D E being parallel, the light does not appear to have suffered any refraction: the apparent, differing so little from the true place of any object, when seen through gla.s.s of ordinary thickness.