An individual white led (light-emitting diode) has anefficiency of 20% and uses 1.0 w of electric power.
a) how many leds must be combined into one light sourceto give a total of 1.6 w of visible-light output (comparableto the light output of a 40 w incandescent bulb)?
b) what total power is necessary to run this led lightsource?

a) There must be 8 LED's combined into one ligth source to give a total of 1.6W of visible-light output.

b) The total power necessary to run this LED lightsource is 8.0W

Explanation:

a) According to this problem, each white LED will have an efficiency of 20%. This means that each LED will use only 20% of all the inputed power and turn it into visible light. The rest of the power will turn into heat loss.

So taking this into account, let's take the total amount of power each LED will consume and let's multiply it by it's efficiency.

Where N is the efficiency, so we get:

So from this equation we know now that each LED will turn 0.20W of the energy consumed into light.

Once we have this data we can use conversions to find the number of LED's needed to give a total of 1.6W of visible-light output. So we get:

So the we need 8 LED's to get a total of 1.6W of visible-light output.

b) Since we need 8 LED'S on our lightsource and we know that each LED will consume a total of 1.0W, then all we need to do is multiply the number of LED's by the number of Watts consumed to get the total of Watts consumed by the LED's.

So the total power necessary to run this LED lightsource is 8.0W