Why is the fluorescent screen bulged in a CRT?

Answer 1

A CRT functions by generating electron beams in the tube's neck and firing them at the tube's face, which has a phosphor coating on its inner side. The beam or beams are electronically swept back and forth across the tube's face causing a tiny area of the coating to light up. The beams are swept extremely rapidly, relying on our eyes' phenomenon of persistence of vision to "paint" an image.

If the tube's face were flat it would be much more difficult to form a consistent image. First, the beam would have to travel longer distances when it swept the tube's edges and corners versus when it swept the center. Second, as it reached the edges and corners it would hit the screen at an angle which would cause a non-circular area of the phosphor to light up. It would be much more difficult if not impossible to control the beam in a way that it would produce a uniform, smoothly-defined image. Making the tube face slightly spherical both limits the change in distance between the tube's neck and the phosphor surface and ensures that the beam will fall at or near a 90-degree angle.

To make an analogy, think of putting a paint brush in your hand and trying to paint a canvas by only swinging your arm from the shoulder. If the canvas were flat, you could paint parts of it only by contracting your arm or getting a different-size brush; in addition, it would be much harder to do a clean job painting parts of the image at the canvas's edge and corners. However if you were inside a large hemisphere you would have no difficulty reaching any part of it.