NCERT Solutions Curiosity Chapter 3 Activity 3.4: Let us observe — Electric lamp
Book page 26 Updated on2026-09-05
Q1.
Take the torch and examine its lamp. What do you see? Do you notice a thin wire fixed in the middle of the glass bulb?
Answer
Yes. Inside the small glass bulb there is a very thin, coiled wire held in the middle, supported at its two ends by two much thicker wires.
What you see, from outside inwards:
A clear glass bulb.
Two thick wires rising from the base — they only hold and feed the thin wire.
The thin wire between them, usually wound like a tiny spring.
A ring of insulator at the base that keeps the two terminals from touching.
Q2.
Now, switch on the torch. Which part of the lamp glows?
Answer
Only the thin wire inside the glass bulb glows. This glowing thin wire is called the filament of the lamp.
The thick supporting wires, the glass bulb and the metal base stay dark and comparatively cool.
Why it happens: The same current passes through the thick wires and the thin filament, but the thin filament resists the current far more. That extra opposition makes the filament heat up until it is white-hot and gives out light. A lamp that works this way is called an incandescent lamp — 'incandescent' means 'glowing because it is hot'.
Q3.
Take out the lamp with the help of your teacher and inspect it from all sides. How is the filament fixed?
Answer
The filament is fixed between two thicker wires that both support it and carry current to it (Fig. 3.4a). Those two thick wires lead down to the two terminals of the lamp:
Thick wire
Where it ends
Name of that terminal
One
The metal case at the base of the lamp
One terminal
The other
The metal tip at the centre of the base
The other terminal
A ring of insulating material separates the metal tip from the metal case, so the two terminals never touch each other (Fig. 3.4b).
Why it happens: If the tip and the case touched, the current would take that easy short path and skip the filament altogether — the lamp would not glow. The insulator forces the current to go up one thick wire, through the filament, and down the other.