Delta cephei is one of the most visible stars in the night sky. its brightness has periods of 5.4 days, the average brightness is 4.0 and its brightness varies by ±0.35. find a formula that models the brightness, b, of delta cephei as a function of time, t, with t=0 at peak brightness.

The focal length of a relaxed human eye is approximately 1.7 cm. when we focus our eyes on a close up object, we can change the refractive power of the eye by about 16 diopters. (a) does the refractive power of our eyes increase or decrease by 16 diopters when we focus closely? explain. (b) calculate the focal length of the eye when we focus closely.