Chapter 16, Problem 77A

To determine

To explain:The details of the standard used to set the value of $1\text{ cd}$ .

Explanation of Solution

Introduction:

Consider a lamp is used to illuminate a room, the luminous flux of the lamp is the measure of contribution of the lamp in illuminating the room. It is the total amount of the visible light emitted by the lamp. The SI unit of luminous intensity of the lamp (light source) is lumen (lm). The luminous intensity of the lamp (light source) is the ratio of the luminous flux emitted by the lamp per unit solid angle as shown below:

The SI unit of luminous intensity of the light source is known as Candela (cd).

Light is a form of energy and it can travel through the vacuum. It does not need a proper channel or medium for propagation.Light can travel through the vacuum at a speed of $3\times {10}^8\text{m}/\text{s}$ .The SI unit candela is the unit of luminous intensity. $1\text{ cd}$ is the amount of energy from the light emitted by thorium from an opaque material. The opaque material has an opening of one-sixteenth of a square centimeter in size.