A Guide To The Scientific Knowledge Of Things Familiar Part 83

A. The line of the _angle of incidence_.

Q. _When a marble REBOUNDS back again, what is the path it THEN describes called?_

A. The line of the _angle of reflection_.

Q. _When the light of our face goes TO the GLa.s.s, what is the path through which it goes CALLED?_

A. The line of the _angle of incidence_.

Q. _When the light of our face is reflected BACK again from the mirror, what is this RETURNING path called?_

A. The line of the _angle of reflection_.

Q. _Why does our reflection in a mirror seem to APPROACH us as we walk TOWARDS it, and to RETIRE FROM us as WE retire?_

A. Because the line _of the angle of incidence_ is always _equal_ to the _line and angle of reflection_.

[Ill.u.s.tration: Here CA, EA and DB, FB are the lines of the angle of incidence; and GA, KA and HB, LB are the lines of the angle of reflection. When the arrow is at CD, its shadow will appear at GH, because the line CA=GA and the angle CAB=angle GAB, &c.; and the same may be said about the point D.]

Q. _Why can a man see his WHOLE PERSON reflected in a LITTLE MIRROR not 6 inches in length?_

A. Because the _line of the angle of incidence_ is always equal to the _line and angle of reflection_.

Take the last figure--CD is much larger than the mirror AB; but the head of the arrow C is reflected obliquely behind the mirror to G; and the barb D appears at H.--Why? Because the line CA=AG and the angle CAB=angle GAB, &c. The same may be said of the point D.

Q. _Why does a SHADOW in WATER always appear TOPSY-TURVY?_

A. Because the _line of the angle of incidence_ is always equal to the _line and angle of reflection_.

[Ill.u.s.tration: Here the arrow-head A strikes the water at F, and is reflected to D; and the barb B strikes the water at E, and is reflected to C.

If a spectator stands at G, he will see the reflected lines CE and DF, produced as far as G.

It is very plain that the more elevated object A will strike the water, and be projected from it more perpendicularly than the point B, and therefore the shadow will seem inverted.]

Q. _When we see our SHADOW in WATER, why do we seem to STAND on our HEAD?_

A. Because the _line of the angle of incidence_ is always equal to the _line and angle of reflection_.

Suppose our head to be at A, and our feet at B; then the shadow of our head will be seen at D, and the shadow of our feet at C. (_See last figure._)

Q. _Why do WINDOWS seem to BLAZE at SUN-RISE and SUN-SET?_

A. Because gla.s.s is a good _reflector of light_; and the rays of the sun (striking against the window gla.s.s) _are reflected_, or thrown back.

Q. _Why do NOT windows reflect the NOON-DAY rays also?_

A. They do, but the _reflection is not seen_.

Q. _Why is the reflection of the RISING and SETTING sun seen in the window, and NOT that of the NOON-DAY sun?_

A. As the angle of _incidence_ always equals the _angle of reflection_, therefore the rays of the noon-day sun enter the gla.s.s _too perpendicularly_ for their reflection to be seen.

[Ill.u.s.tration: Here AB represents a ray of the noon-day sun striking the window at B; its reflection will be at C:

But DB (a ray of the rising or setting sun) will be reflected to E (the eye of the spectator).]

Q. _Why can we not see the REFLECTION of the SUN in a WELL, during the day-time?_

A. Because the rays of the SUN _fall so obliquely_, that they _never reach the surface of the water_ at all, but strike against the brick sides.

[Ill.u.s.tration: Let BDEC be the well, and DE the water.

The ray AB strikes against the brick-work _inside_ the well; and

The ray AC strikes against the brick-work _outside_ the well.

None will ever touch the water DE.]

Q. _Why do we see the MOON reflected in a WELL very OFTEN?_

A. As the rays of the MOON are not so _oblique_ as those of the sun, they will often reach the water. _(See next figure.)_

Q. _Why are the STARS REFLECTED in a WELL, although the SUN is NOT?_

A. As the rays of the STARS are not so _oblique_ as those of the sun, they will often reach the water.

[Ill.u.s.tration: Here the moon's rays AB, AC, both strike the water DE, and are reflected by it.]

Q. _In a sheet of water at noon, the sun appears to s.h.i.+ne upon only ONE spot, and all the REST of the water seems DARK,--WHY is this?_

A. Because the rays (which fall at various degrees of obliquity on the water) are _reflected at similar angles_; but as only those which _meet the eye of the spectator_ are visible, all the sea will appear dark but _that one spot_.