Conversations on Natural Philosophy, in which the Elements of that Science are Familiarly Explained Part 44

_Mrs. B._ The ray which falls in the direction of the axis of the mirror, is reflected back in the same line; but let us draw two other rays from the focus, falling on the mirror at B and F; the dotted lines are perpendicular to those points, and the two rays will, therefore, be reflected to A and E.

_Caroline._ Oh, now I understand it clearly. The rays which proceed from a light placed in the focus of a concave mirror fall divergent upon it, and are reflected, parallel. It is exactly the reverse of the former experiment, in which the sun's rays fell parallel on the mirror, and were reflected to a focus.

_Mrs. B._ Yes: when the incident rays are parallel, the reflected rays converge to a focus; when, on the contrary, the incident rays proceed from the focus, they are reflected parallel. This is an important law of optics, and since you are now acquainted with the principles on which it is founded, I hope that you will not forget it.

_Caroline._ I am sure that we shall not. But, Mrs. B., you said that the image was formed in the focus of a concave mirror; yet I have frequently seen gla.s.s concave mirrors, where the object has been represented within the mirror, in the same manner as in a convex mirror.

_Mrs. B._ That is the case only, when the object is placed between the mirror and its focus; the image then appears magnified behind the mirror, or, as you would say, within it.

_Caroline._ I do not understand why the image should be larger than the object.

_Mrs. B._ This results from the convergent property of the concave mirror. If an object, A B, (fig. 7.) be placed between the mirror and its focus, the rays from its extremities fall divergent on the mirror, and on being reflected, become less divergent, as if they proceeded from C: to an eye placed in that situation, the image will appear magnified behind the mirror at _a b_, since it is seen under a larger angle than the object.

You now, I hope, understand the reflection of light by opaque bodies. At our next meeting, we shall enter upon another property of light, no less interesting, and which is called _refraction_.

Questions

1. (Pg. 168) What is meant by the angle of vision, or the visual angle?

2. (Pg. 169) Why do objects of the same size appear smaller when distant, than when near?

3. (Pg. 169) Why do not two objects, known to be equal in size, appear to differ, when at different distances from the eye?

4. (Pg. 169) How is this exemplified, by a house seen through a window?

5. (Pg. 170) Why do rows of trees, forming an avenue, appear to approach as they recede from the eye, until they eventually seem to meet?

6. (Pg. 170) In drawing a view from nature, what do we copy?

7. (Pg. 170) What is the difference in sculpture, in this respect?

8. (Pg. 170) Excepting the rays from an object enter the eye, under a certain angle, they cannot be seen; what must this angle exceed?

9. (Pg. 170) What two circ.u.mstances may cause the angle to be so small, as not to produce vision?

10. (Pg. 170) Motion may be so slow as to become imperceptible, what is said on this point?

11. (Pg. 170) Under what circ.u.mstances may a body, moving with great rapidity, appear to be at rest?

12. (Pg. 170) Upon what does the real velocity of a body, depend?

13. (Pg. 171) What must be known, to enable us to ascertain the real s.p.a.ce contained in a degree?

14. (Pg. 171) What is explained by fig. 2, plate 17?

15. (Pg. 171) What is said respecting the evidence afforded by our senses, and how do we correct the errors into which they would lead us?

16. (Pg. 171) An image of a visible object is formed upon the retina of each eye, why, therefore, are not objects seen double?

17. (Pg. 172) By what experiment can you prove that a separate image of an object is formed in each eye?

18. (Pg. 172) Under what circ.u.mstances are objects seen double?

19. (Pg. 172) Why is not the image of an object inverted in the common mirror?

20. (Pg. 172) Your whole figure may be seen in a looking-gla.s.s, which is not more than half your height; how is this shown in fig. 3. plate 17?

21. (Pg. 173) Why is the image invisible to the person, when not standing directly before the gla.s.s?

22. (Pg. 173) In what situation may a second person see the image reflected?

23. (Pg. 173) In what direction will an object always appear to the eye?

24. (Pg. 173) How is this explained by fig. 4, plate 17?

25. (Pg. 173) What is it that reflects the rays in a looking-gla.s.s?

26. (Pg. 174) All opaque bodies reflect some light, why do they not all act as mirrors?

27. (Pg. 174) What substances form the most perfect mirrors, and for what reason?

28. (Pg. 174) What are the three kinds of mirrors usually employed for optical purposes?

29. (Pg. 174) How are the rays of light affected by them?

30. (Pg. 175) What is the form of a convex mirror, and how do parallel rays fall upon it, as represented in fig. 1, plate 18?

31. (Pg. 175) What is represented by the dotted line in the same figure?

32. (Pg. 175) Explain by the figure, how the parallel rays will be reflected.

33. (Pg. 175) At what distance behind such a mirror, would an image, produced by parallel rays, be formed?

34. (Pg. 175) What is that point denominated?

35. (Pg. 176) What is meant by a focus?

36. (Pg. 176) Why is the point behind the mirror, called the _imaginary focus_?

37. (Pg. 176) Why does an object appear to be lessened by a convex mirror, (fig. 2.)?

38. (Pg. 176) What is a concave mirror, and what its peculiar property?

39. (Pg. 176) How are parallel rays reflected by a concave mirror, as explained by fig. 3, plate 18?

40. (Pg. 177) Where is the focus of parallel rays, in a concave mirror?

41. (Pg. 177) If rays fall on it convergent, how are they reflected?