8. Explain the phenomenon which one observes when looking at an object through the air arising from a hot stove or radiator.
9. Frequently the horizontal diameter of the setting sun appears to be greater than the vertical. Explain.
10. Explain why one observes several images of a luminous body like a lighted candle when the reflected light from a thick glass mirror enters the eye, the angle of reflection being large.
(5) The Formation of Images by Lenses
382. Uses of Lenses in Optical Instruments.—The use of instruments that employ lenses in their operation, such as spectacles, reading and opera glasses, and the camera, microscope, and telescope, is familiar to most students of physics. The part played by the lenses, however, is not generally understood. Consequently the study of the formation of images by lenses is of general interest and importance.
383. Forms of Lenses.—While a lens may be formed from any transparent solid it is commonly made of glass. It may have two curved surfaces or one curved and one plane surface. Most lenses are spherical lenses, since their curved surfaces form a part of the surface of a sphere. Fig. 376 represents a spherical lens with a curved surface coinciding with that of a sphere whose center is at C. This center is called the center of curvature, while the radius of the sphere R, is the radius of curvature.
Fig. 376.—Formation of a spherical lens.
There are two classes of lenses: those thick in the middle are called convex, while those thick at the edges are concave. The mode of constructing the six forms of spherical lenses is shown in Fig. 377. These are named as follows: (1) double convex, (2) plano convex, (3) concavo-convex, (4) double concave, (5) plano concave, (6) convexo-concave.