In the circuit diagram, the battery, diodes \( D_1 \) and \( D_2 \), bulbs \( B_1 \) and \( B_2 \) are connected as shown. Which of the following statements is true regarding bulbs \( B_1 \) and \( B_2 \) ?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Only \( B_2 \) will glow
A p-n junction diode conducts only when it is forward biased, that is, when current enters at the triangle side (anode) and leaves at the bar side (cathode). In the reverse direction it blocks the current.
In the cell symbol the longer plate is the positive terminal, so the top of the cell is positive and both diodes get current from that top junction.
Option (1) Only \( B_1 \) will glow — Incorrect. The bar of \( D_1 \) faces the cell, so the current from the positive terminal meets its cathode end. \( D_1 \) is reverse biased and blocks, so \( B_1 \) stays off.
Option (2) \( B_1 \) and \( B_2 \) both will glow — Incorrect. Only one of the two diodes is forward biased.
Option (3) Only \( B_2 \) will glow — Correct. The current from the positive terminal enters the triangle side of \( D_2 \), passes through it and \( B_2 \), and returns to the negative terminal. \( D_2 \) is forward biased, so \( B_2 \) glows.
Option (4) Both \( B_1 \) and \( B_2 \) will not glow — Incorrect. \( B_2 \) gets a forward current and glows.