Question

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency \( 'A' \times 10^{12} \, \text{hertz} \) and that has a radiant intensity in that direction of \( \frac{1}{'B'} \, \text{watt per steradian} \). 'A' and 'B' are respectively:

• A. 540 and \( \frac{1}{683} \)
• B. 540 and 683
• C. 450 and \( \frac{1}{683} \)
• D. 450 and 683

Answer 1

The Correct Option is B

The question asks about the definition of the candela, which is one of the seven base units in the International System of Units (SI). A candela is defined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation.

Let’s break down the given information:

• The frequency of the monochromatic radiation is given as \( A \times 10^{12} \, \text{hertz} \).
• The radiant intensity in that direction is \( \frac{1}{B} \, \text{watt per steradian} \).

The problem provides options for values of A and B. To find the correct answer, we need to recall the standard definition:

The candela is defined by the frequency of radiation being 540 × 10¹² hertz and has a radiant intensity of 1/683 watt per steradian.

Thus, the correct values for A and B from the options given are:

• Frequency: 540 (in terms of 540 × 10¹² hertz)
• Radiant Intensity: 683 (where the radiant intensity is defined as \( \frac{1}{683} \, \text{watt per steradian} \))

Therefore, the correct option is 540 and 683.

The incorrect options can be discounted because:

• Options with 450 do not match the standard frequency for defining a candela.
• The option with \( \frac{1}{683} \) as B does not match the radiant intensity needed for a standard candela.

Thus, the correct answer is 540 and 683 according to the SI unit definition of candela.

Answer 2

The definition of candela specifies:
- $A = 540 \times 10^{12}$ hertz is the frequency of the monochromatic light.
- $B = 683$ is the luminous efficacy in terms of watts per steradian.
These values are based on the standard SI definition of candela.