How do I compute a capability index? For example, a homework question is asking about standard deviation and allowable variation and defectiveness.

How do I compute a capability index? For example, a homework question is asking about standard deviation and allowable variation and defectiveness. What does all of this mean? This is the question:

"A process that produces computer chips has a mean of .04 defective chip and a standard deviation of .003 chip. The allowable variation is from .03 to .05 defective. Compute the capability index for the process."