3. Define refraction. Mention two illustrations of this action that you have observed out of school. 4. Why does the moon look larger near the horizon? 5. Is your reflection seen in a pool of water upside down? Why? 6. Why does it whiten molasses candy to pull it? 7. When looking at a building through the ordinary glass of a window why do straight lines of the building appear to be so distorted? What makes them appear to move as you move your head slightly? 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_. [Illustration: 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. [Illustration: FIG. 377.--Forms of Lenses. 1. double convex; 2. plano convex; 3. concavo convex; 4. double concave; 5. plano concave; 6. convexo concave.] [Illustration: FIG. 378.--The action of a burning glass.]