Turn an incandescent lamp on and off quickly while you are standing near it. You feel it's heat, but you find when you touch the bulb that it is not hot. Explain why you felt the heat from the lamp?
The infrared heat and light radiation from the bulb goes instantly to your skin, but it takes time for the bulb to warm up because of its heat capacity.
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When an incandescent lamp is turned on, it emits light by converting electrical energy into both visible light and heat. The bulb filament inside the lamp is made of a metal (typically tungsten), which heats up rapidly when an electric current passes through it. The filament reaches extremely high temperatures, sometimes exceeding 2500 degrees Celsius.
Now, let's understand why you felt the heat from the lamp even though the bulb itself didn't seem hot to touch. When the lamp is on, the emitted light energy primarily consists of visible light and infrared radiation. Infrared radiation corresponds to electromagnetic waves with longer wavelengths than visible light, and it carries heat energy.
As you stood near the lamp, your body absorbed the emitted infrared radiation. Infrared radiation can transfer heat energy directly to your body, even if the surrounding air temperature remains unchanged. That's why you felt the sensation of heat. So, the heat you experienced was not due to the bulb itself being hot, but rather from the infrared radiation being absorbed by your body.
When you touched the bulb, you likely felt that it was not hot because most incandescent lamps have a protective glass or plastic cover surrounding the filament. This cover acts as a barrier, preventing direct contact with the hot filament. It reduces the risk of burns and electrical shock while still allowing the emitted visible light and infrared radiation to pass through. Therefore, even though the filament is extremely hot, the outer surface of the bulb remains relatively cool to touch.