In Physics, there are exactly seven fundamental base quantities in the International System of Units (SI). While length (meter), mass (kilogram), and time (second) are very famous, the base quantity related to light is often asked in exams because it is less common.
A standard modern lighthouse beam has a luminous intensity of millions of candelas, which is why ships can see it through thick fog from miles away at sea.
The SI base unit of Luminous Intensity is the Candela.
In simple terms, Luminous Intensity is the measure of the visible brightness of a light source in a specific, given direction. It tells us how intense the light beam is when it travels straight into the human eye.
Why 'Candela'? The word comes from the Latin word for 'candle'. Historically, 1 Candela was defined as the exact amount of light emitted by one standard wax candle. So, a 100-candela lightbulb shines about 100 times brighter than a single candle in a specific direction.
Students often confuse these two light measurements:
The 7 base units are: Meter (Length), Kilogram (Mass), Second (Time), Ampere (Electric Current), Kelvin (Temperature), Mole (Amount of substance), and **Candela (Luminous Intensity)**.
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