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Why Do Light Bulbs Glow When Switched Off?

Here’s a portion of interesting food for thought.

The magic of olden times that fascinated audiences were actually just hidden physics and chemistry at play.

And even today, there might be unexplained phenomena we still get scared about.

But those events basically have science at their root cause.

An example is mysterious glowing light bulbs that continue to shine even after they have been switched off!

Why do they glow?

Light bulbs only shine faintly and briefly when off, due to the wiring or the internal components. In some cases, bulbs continue to shine due to a high temperature. Other times, it can be residual electricity or vaporizing, depending on the type of bulb.

Let’s take a closer look at these phenomena.

What Types Of Bulb Can Glow After Switched Off?

Sometimes electrical equipment, including bulbs, can act in unexpected ways.

One of the things that happen is that light bulbs continue to shine faintly and briefly even after they have been turned off.

This can be pretty annoying to deal with. For example, even a faint glow in a bedroom bulb will bother you all night.

The bulbs may flicker, pulse, or glow steadily. There might even be a buzzing or humming noise.

Remember, the switch is off!

You would be surprised to know that almost all light bulbs have an afterglow even after you turn them off.

These strange occurrences are mainly due to the various components inside the bulbs.

LEDs, CFL (compact fluorescent lamps), and incandescent bulbs have been observed to emit a faint glow, to varying degrees.

The glow can last from a few seconds to several minutes.

However, all the different bulbs have different reasons for the continued glowing. You can rest assured, there is most likely nothing dangerously wrong with your bulbs.

And there are probably no ghosts at play!

Below I am going to explore the science behind each of the causes of the glow.

The Main Causes Of Glowing

power consumption

A lot goes into correct wiring and quality components of electrical equipment. While there can be many different reasons for the glowing, I will look at the most common causes.

Residual Electricity

When a low current level is still present within your LEDs diode, the bulb will pick it up and convert it into lighting.

This phenomenon is called residual electricity. LEDs are most likely to pick up residual electricity due to their high efficiency.


Sometimes the wiring is at fault. If the neutral wire is not earthed or is at very high resistance, it can leak a small current to the bulb. This faintly powers the bulbs continuously.

Fancy Switches

Some switches have features like an indicator light, motion detection, timer, etc. These need some power to be on stand-by.

Due to the wiring, LEDs can draw on this small power and still stay on faintly even when you’ve flipped the switch off.

Operating Temperature

The filament of an incandescent bulb gets extremely hot to give off white-hot light.

When you turn off the power to the bulb, the filament takes a few seconds to start cooling down. So it continues to glow slightly as the filament is still quite hot for a few seconds.

Vaporizing Inside Of The Bulb

The primary culprit behind this kind of a residual glow is fluorescent bulbs.

There is some persistence in the interaction of mercury and phosphorus that are inside the fluorescent tubes a while after the light has been switched off.

Why Do CFL Bulbs Glow When Turned Off?

CFL bulbs work due to the interaction of mercury vapor and the phosphorus coating inside the tube. This is called the vaporizing effect.

What happens is that the mercury vapor gets ionized when the electric current starts flowing through the tube after you turn on the switch.

As the mercury tries to ground and return to an un-ionized state, its electrons emit energy in the form of UV radiation.

Following that, this UV radiation excites the phosphorus coating. This interaction then emits photons of visible light.

When the switch is turned off, the current stops instantly, but the gas does not immediately return to its ground state. It takes a few seconds for the electrons to stop releasing energy.

Within these seconds, the ionized mercury’s ultraviolet radiation is still faintly reacting with the phosphorus, emitting residual photons of visible white light.

This causes a lingering glow.

However, the vaporizing effect quickly dissipates, and the gas cools down. Therefore the tube no longer glows.

Glow-in-the-dark objects also use this principle of slow release of energy to excite phosphorus.

Why Do LED Lights Keep Glowing After Switching Off?

LEDs also continue to faintly glow at times after being turned off. However, they don’t have any chemical processes going inside as fluorescents do.

Sometimes the electrical socket connected to the lamp or fixture of LED light does not turn all the way off, instantly.

There will still be some residual electricity. It can last from a few seconds to a couple of minutes.

The minuscule current can be coming from the built-in capacitor that stores electrical charge in the bulb. When you switch off the power, the capacitor emits the residual current from it.

In both these instances, there is a very low level of current emitted.

But LEDs are highly efficient.

Although minuscule, the efficiency of LED light converts even minimal energy into visible light. This energy makes the bulb appear to glow.

You will notice that when you remove the LED from the socket and put it back, it will now stop glowing as the power was completely cut off.

You can also swap out the fancy switches for regular switches to stop the trickle flow of current, or amend the wiring to have both hot and neutral wiring within the switch.

Final Words

Out of all the lighting technologies on the market, LEDs have the highest savings while being the safest.

Still, it is essential to keep yourself informed of specific issues that can arise with LEDs that result in an afterglow. As you saw, most of these can be easily fixed.

Do you have 2-way or timer switches?

Have you experienced any bulb glowing faintly after turning it off?

Now that you know why they might be glowing share your thoughts in the comments below.

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