Question

a) A 15-W compact fluorescent light (CFL) is on 5 hours per day. The cost of electricity in your area is 14 cents per kWh. What will it cost per month to operate this light? b) Now that you have the light on, you are having trouble reading up close. Words are blurry if they are closer than 0.5 m in front of your eyes. Is this myopia or hyperopia? c) What power of eyeglasses are needed to correct his vision, so that he can clearly see objects 25.0 cm away? The eyeglasses rest 1.50 cm away from his eyes.

Answer 1

a)

energy dissipated E = P*t

number of days operated = 30 days

t = time for which bulb is operated = 30*5 hours = 150 hours = 150*60*60 s

energy dissipated E = 15*150*60*60 = 8100 kWh

cost per month = 8100*14 = 113400 cents = 113.4 dollars

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(b)

hyperopia

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c)

object distacne s = 25-1.5 = 23.5 cm

image distance s' = -(50 - 1.5) = -48.5 cm

from lens equation

1/s + 1/s' = 1/f

1/23.5 - 1/48.5 = 1/f

f = 45.59 cm = 0.4559 m

power P = 1/f ( in meters)

power P = 1/0.4559 = +2.19 D