A 40-watt light bulb is 4 times more luminous than a 10-watt light bulb. Both bulbs will appear equally bright

Question

if...
A) the 10-watt bulb is 2 times further away than the 40-watt bulb
B) the 10-watt bulb is 4 times further away than the 40-watt bulb
C)the 40-watt bulb is 2 times further away than the 10-watt bulb
D) the 40-watt is 4 times further away than the 10-watt bulb
E) the 40-watt bulb is 8 times further away than the 10-watt bulb

Answer 1

If you double the distance then intensity is 2 squared less (=4 times less -inverse square law) so the answer is c

Answer 2

This is one of those "it depends" questions. One could argue that they would appear equally "bright" at the same distance if quite close to the observer. If the bulbs had long filaments in a clear bulb and were made so that the 40W filament was just 4 times the length of the 10W filament, then the two filaments would be emitting the same amount of light per unit length and would appear equally "bright".

However, if each bulb was then surrounded by a diffusing translucent bulb of the same size, then the light output would be added up over the filament and then distributed over the surface of the bulb. The "brightness" of the 40W builb would then be 4 times that of the 10W bulb. The apparent luminosity to an observer then follows the inverse square law so to appear to have the same luminosity, the 40W bulb would have be twice as far away as the 10W bulb (answer C).

Answer 3

C. The light intensity is inversely proportional to the square of the distance.

Answer 4

the wattage does not determine the brightness. wattage is power consumption, an inefficient bulb of the same wattage will give out less light.

Answer 5

c