Final answer:
A bulb marked 100 watts indicates its power consumption rate, using 100 joules of energy per second when on. While wattage once related to brightness, lumens are a better measure of how bright a bulb will appear.
Step-by-step explanation:
When a bulb is marked 100 watts, it denotes the power consumption of the bulb when it is switched on. This measurement indicates the rate at which the light bulb uses energy. A 100-watt bulb uses 100 joules of energy per second. This is an important aspect to understand when considering the electricity usage and energy efficiency of lighting in your home.
It is also worth noting that wattage used to be a common way to gauge the brightness of a bulb, although it's not the most accurate measure since it does not directly indicate how much light a bulb emits. Instead, lumens are a better measure of brightness as it reflects the luminous flux, or the perceived brightness to the human eye. Thus, a light bulb with a higher lumen output will appear brighter than one with a lower lumen output, regardless of the wattage. Meanwhile, bulbs with the same wattage may actually have different brightness levels if they have different luminous efficacy.