Can you use a 60 watt bulb in a 100 watt?

Light output isn't measured in Watts. A 100 Watt incandescent light bulb doesn't emit "100 Watts of light," because there's no such thing. Instead, it draws 100 Watts of power and delivers a fairly consistent amount of light.

Because the relationship is at least somewhat linear, though, people think of the number of Watts as representative of the amount of light produced by a conventional bulb. CFL (Compact Fluorescent Lamps) complicated matters quite a bit, because a CFL that draws relatively little power can produce as much light as an incandescent that draws much more. This is, in fact, the primary reason for using them: they save power.

Sockets are rated for a particular wattage for two reasons. First, the contacts, switch, cord, and other parts are generally capable of handling a specific amount of current and no more, and this limits the wattage of the bulb that can be used. Second, the socket is generally built to handle a specific amount of heat, and higher-wattage bulbs produce more heat as well as more light.

Now for the good news. A CFL with a "light output" (arbitrary marketing term) of "100 Watts" in actuality draws less than 30 Watts, and produces very little heat. You can, in fact, use a "100 Watt" CFL in a socket rated for a 60 Watt or even 40 Watt bulb, so long as it will physically fit.