1 Light Bulb and 3 Switches Puzzle

The one light bulb and three switches puzzle has been asked several times in interviews to evaluate candidate’s analytical skills.

Puzzle

There is a light bulb 💡 inside a closed room (no windows) with one door. You cannot see the light bulb is on or off as the door is closed. However, you can assume that the light is off initially. There are three light switches outside the room. One of the switch belongs to the light bulb inside the room. You can turn on or off the switches however you want, but you are allowed to open the door only once. You are not allowed to touch the switches once you open the door.

How do you figure out which switch belongs to the light?

Think for a while before looking at the solution.

Solution

We are assuming that all the switches are off initially. Turn on switch number 1 and wait for two minutes. After two minutes, turn off the switch number 1 and turn on the switch number 2 and open the door to see light bulb

• If the light bulb is on, then switch number 2 belongs to the bulb.
• If the light bulb is off, then touch the bulb with your hand. If the bulb is hot, then switch number 1 belongs to the bulb otherwise switch number 3 belongs to the bulb.