Photoelectric Effect

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 .
• The radiant intensity in that direction is .

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 )

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 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.

In the photoelectric effect, electrons are ejected from a material when light of a frequency greater than the threshold frequency strikes it.
Option A: When yellow light (which has a frequency greater than the threshold frequency) is focused on caesium, electrons are ejected, and current flows. Therefore, this statement is correct.
Option B: Dimming the brightness of the yellow light reduces the number of photons, which in turn decreases the number of electrons ejected. Hence, the current in the ammeter is reduced. This statement is correct.
Option C: Red light has a frequency lower than the threshold frequency of caesium, so it does not have enough energy to eject electrons. Hence, no current will be produced. This statement is incorrect.
Option D: Blue light has a frequency greater than the threshold frequency, so it will eject electrons and form current in the ammeter. This statement is correct.
Thus, the correct answer is B, C, and D Only.