If a magnet is held near the screen of a CRT (see Sec. 16.5), the picture is distorted. [Don't try this—see part (b).] 

(a) Why is the picture distorted? 

(b) With a color CRT, the distortion remains even after the magnet is removed. Explain. (A color CRT has a metal mask just behind the screen with holes to line up the electrons from different guns with the red, green, and blue phosphors. Of what kind of metal is the mask made?)