# Why do we need to keep lights on?

• GarageDweller
In summary, when a light is turned on, it emits high frequency EM waves, but the light fades when it is turned off because the waves are absorbed by the surrounding. In a perfectly reflecting room, the same would occur as even mirrors are not 100% efficient and eventually the light would be absorbed by the surrounding. In a room with mirrors on the walls, the light would be reduced to 1% after around 20-30 reflections and after 500,000 reflections, it would be considered "gone". Even in a superconducting cavity, where the energy is theoretically capable of being reflected losslessly, there are still challenges due to reflective properties and external factors. Additionally, the conundrum of quantum physics plays a role
GarageDweller
When we turn a light on a wave of high frequency EM waves are emitted, however as soon as we turn the light off the light fades. Why is this?
If we were to do the same thing in a perfectly reflecting room (perhaps a spherical room with mirrors on the walls) will the same occur?

If we were to do the same thing in a perfectly reflecting room (perhaps a spherical room with mirrors on the walls) will the same occur?
I posted a similar question some months ago: Here
Maybe it can offer some help

Look towards the later posts.

My question is why light seems to fade so fast.
Where does it go?edit: After reading the link you provided, I guess the waves are absorbed by the surrounding?

It's absorbed by it's surroundings. Even mirrors aren't 100% efficient. Google says a mirror typically reflects 80-90% of the incident light. The rest is absorbed and turned into heat.

So after around 20-30 reflections only 1% is left. If your mirrored room was 10m in diameter the light would be down to 1% after traveling just 200-300m. Since light travels at 3*10^8m/s it won't take very long for 99% to be absorbed.

According to wikipedia, the best narrow wavelength mirrors reflect ~ 99.999%. Some simple math: F=0.99999
hence F^500 000 ~0.0067...meaning 500,000 reflections is enough to reduce 100% to 0.7%. Light travels at 300 000 000 m/s. Say in a room 3 m across. 3*500,000 ÷ 300 000 000 = 0.005 seconds. In 0.005 seconds the light will be "gone". With more realistic broad wavelength mirrors, reflection is more like 95% and so 100 reflections would get you to 0.6%. Air is composed of electric dipoles we call atoms and molecules. Each of those can interact with light in several ways, look up Rayleigh Scattering. If that weren't true, day and night would be very different. Anyway, even if the mirrors were "perfect", air would absorb the light, eventually. Take out the air, and you'd have more important things to worry about. All matter emits light, the most general is black body radiation. The implication of this is that theoretically, we can see in the dark; all we need is to cool our eyeballs down to absolute zero. As they warm up from zero, they begin to emit light (em radiation) and so begin to compete with the light from our surroundings. So, only bright objects are visible becuz its the difference in light intensity that matters to us.

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One of the Nobel prizes awarded for physics for 2012 included a pair of physicists who had observed the decay of photons being reflected inside of a superconducting cavity that was theoretically capable of reflecting the energy losslessly. Even this idea is not as easy as it sounds, because such a cavity also has reflective properties that may be influenced by things such as external magnetic fields. Also, it is not uncommon for superconducting material domains to be less than optical or electrical perfection, to say the least. As such, the abstract of the experiment does not make for easy reading.

Of course, this question is one of the quantum physics conundrums (like Schroedinger's cat) as well, because in order to determine that the photon(s) are still inside the cavity, you need to observe them in a way that does not absorb its energy prematurely, and this is even more of an experimental challenge.

Danshawen, what does this have to do with the question?

"perfectly reflecting room (perhaps a spherical room with mirrors on the walls"

-- does not necessarily need to be a mirror, does it? Or is there a PF sanction against improving the question?

danshawen said:
One of the Nobel prizes awarded for physics for 2012 included a pair of physicists who had observed the decay of photons being reflected inside of a superconducting cavity that was theoretically capable of reflecting the energy losslessly. Even this idea is not as easy as it sounds, because such a cavity also has reflective properties that may be influenced by things such as external magnetic fields. Also, it is not uncommon for superconducting material domains to be less than optical or electrical perfection, to say the least. As such, the abstract of the experiment does not make for easy reading.

Of course, this question is one of the quantum physics conundrums (like Schroedinger's cat) as well, because in order to determine that the photon(s) are still inside the cavity, you need to observe them in a way that does not absorb its energy prematurely, and this is even more of an experimental challenge.

Thanks for the interesting post.

## 1. Why do we need to keep lights on?

There are several reasons why we need to keep lights on. One of the main reasons is for visibility. Lights allow us to see and navigate our surroundings, especially in the dark. They also provide safety and security by making it easier to spot potential hazards or intruders. Additionally, lights help regulate our sleep patterns and maintain our circadian rhythm.

## 2. How does keeping lights on affect our energy consumption?

Keeping lights on consumes energy, which can have an impact on the environment. The type of light bulbs we use, as well as how long we keep them on, can affect our energy consumption. However, using energy-efficient light bulbs and turning off lights when they are not needed can help reduce our energy consumption and lessen our impact on the environment.

## 3. Are there any health benefits to keeping lights on?

Yes, there are some health benefits to keeping lights on. As mentioned before, lights help regulate our sleep patterns and maintain our circadian rhythm. They can also improve our mood and productivity, as well as reduce eye strain and headaches. However, it is important to find a balance and not keep lights on excessively, as it can also disrupt our sleep and cause other health issues.

## 4. How does keeping lights on impact our daily routines?

Keeping lights on can impact our daily routines in various ways. For example, it allows us to continue activities after the sun sets, such as studying, working, or socializing. It also helps us maintain a sense of normalcy during the winter months when the days are shorter. However, it is important to turn off lights when not needed in order to save energy and reduce costs.

## 5. Can keeping lights on have negative effects on the environment?

Yes, keeping lights on can have negative effects on the environment. As mentioned before, lights consume energy, and the production of this energy can contribute to air and water pollution, as well as greenhouse gas emissions. Overuse of lights can also contribute to light pollution, which can disrupt ecosystems and affect the behavior of animals. Therefore, it is important to use lights responsibly and consider energy-efficient options to reduce our impact on the environment.

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