When You Switch Off the Light, Where Does the Light Go?
― It Didn't Vanish. The Walls Soaked It Up and Turned It into Heat
Flip the switch and the room goes dark in an instant. But where did all that light go, the light that filled the room a moment ago? The truth is that it didn't "disappear". Each time it hits a wall or floor, a little more of it changes into heat, and it is all used up in about one hundred-millionth of a second.
It's night, and you turn off the bedroom light before going to sleep. Click, and the bright room goes pitch black.
Has a child ever asked you something that made you stop and think? "Where did the light go?"
Water runs away. Sound fades to nothing. So where did the light flow to? Did it slip out through the window?
Two facts make the answer clear
Light can't stay put. It is always racing along at about 300,000 kilometres per second. It crosses a room from one side to the other in less than one hundred-millionth of a second.
When light hits a wall or floor, some bounces back and the rest is absorbed. The absorbed part doesn't just vanish. It becomes heat that warms the wall by a tiny amount.
So in a room after the switch is off, the leftover light keeps bouncing from wall to wall at tremendous speed, and with every bounce more of it is turned into heat. Let's go through it step by step.
Light bounces around the room
Not all the light from a lamp travels straight to your eyes. Most of it hits the walls, ceiling, floor and furniture, bounces off and scatters all over the room. The room feels bright because you are seeing this bounced light.
These bounces repeat far faster than you might imagine. One trip across the room takes about one hundred-millionth of a second. By the time you notice that it has gone dark, the light has already crossed the room dozens of times. See Figure 1.
With every bounce, light turns into heat
Whitish wallpaper is said to bounce back about half the light that hits it and absorb the other half. Absorbed light becomes heat: a slight jiggling of the molecules in the wall.
Halving each time means that after 10 bounces only about one thousandth of the original is left. After 20 bounces, one millionth. And since one bounce takes about one hundred-millionth of a second, the whole thing lasts only a tiny fraction of a blink, hundreds of thousands of times shorter. All you can see is that it "vanished in an instant."
While the room is lit, the same thing is going on all the time. A "bright room" is a balance: the lamp keeps supplying light and the walls keep soaking it up as heat. The switch only stops the supply. It doesn't collect the light.
Sound also bounces off walls and weakens as it is absorbed. This is called reverberation. Sound travels at about 340 metres per second, so one bounce takes a fraction of a second and the echo lasts long enough for your ears to catch. The same thing happens with light. It is just about 900,000 times faster, so the "echo" ends incredibly quickly.
Black cloth absorbs nearly all the light that hits it and bounces almost none back. With the same lamp, the room is darker because the light doesn't get more bounces. A white-walled room is bright not because the lamp is strong, but because the light is reused over and over.
Summary
The light didn't vanish. It was absorbed by the walls, floor and furniture and turned into a very small amount of heat. To us, the time between switching off and the room going dark is zero. But in that instant the light crossed the room dozens of times, and each time a bit more of it was replaced by heat.
The light in the room didn't go out of the room.
It became a tiny bit of warmth inside the walls.
The warmth you feel when you rub your hands together is also motion turned into heat. Read Why do your hands get warm when you rub them together? and you'll see the same idea: something that seems to disappear is really becoming heat. How your eyes adjust in a dark room is covered in Why can't you see anything for a while when you go from a bright place into a dark room?
- At night, compare the brightness of a room with white walls and a room with dark curtains drawn. With the same lamp, the room with fewer bounces feels darker.
- Hold a hand mirror facing a bathroom mirror and look at the images receding into the distance. The images get darker the deeper they go, because each bounce off a mirror absorbs a little more light.
- Shine a flashlight on a sheet of white paper and notice that the whole room becomes faintly bright. The light bounced off the paper is bouncing again off the walls and travelling around the room.
In the mirror experiment, the images fade out after four or five reflections. Count how many you see before they vanish, and you get a rough idea of how much light the mirror absorbs.
For those who want more ― terms, formulas and links to textbooksEach part is labelled by level, from middle-school science to university courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school physics
- High school+Advanced high-school material, or textbook sidebar content
- UniversityUniversity-level topics (optics, building environmental engineering) not taught in high school
- ResearchTopics researchers are still investigating, not yet settled even at university level
Middle schoolTerms: this phenomenon has names
- Absorption: when light hits a material and neither bounces off nor passes through, but is turned into another kind of energy, such as heat, inside it.
- Reflectance: the fraction of the light that bounces back. It is said to be about 0.5 for white wallpaper and about 0.05 for black cloth.
- Conservation of energy: the rule that energy is never created or destroyed, only changed in form. Light turning into heat is one example.
Middle schoolHigh schoolChecking with a formula: how many seconds until the room's light is used up?
Let's work out how long it takes for the room's brightness to fall to one thousandth after the switch is turned off. That time is what we want to find.
| In symbols | I = I0 × r^n ; t = (L ÷ c) × n |
| In words | Remaining brightness = starting brightness × the reflectance multiplied in once for each bounce. Elapsed time = distance per bounce ÷ speed of light × number of bounces. |
| Where it comes from | Conservation of energy. Light that hits a surface splits into a "bounced" part and an "absorbed" part, and together they always add up to the original amount. The absorbed part becomes heat, so the light left in the room shrinks by a fixed fraction at each bounce. |
| Symbol L: distance per bounce (average distance to a wall, in metres) | about 4 metres |
| Symbol c: speed of light (in metres per second) | about 300000000 metres per second |
| Symbol r: average reflectance of walls, floor and furniture (a fraction, so no unit) | about 0.5 |
| Symbol n: number of bounces for the brightness to drop to one thousandth (a count) | Multiplying 0.5 ten times gives about one thousandth, so n is taken as 10 |
| Time for one bounce (seconds) | 4 ÷ 300000000 ≒ 0.0000000133 |
| Time for 10 bounces (seconds) | 0.0000000133 × 10 ＝ 0.000000133 |
| How many times longer is a 0.1-second blink? | 0.1 ÷ 0.000000133 ≒ 751880 |
③ In everyday terms: within a single blink, the room's light could "go out" 750,000 times over. Human eyes and nerves can't react anywhere near that fast, so to us the room seems to go dark at the same moment as the switch.
High schoolHigh school+What does "halving each time" mean?
High schoolA decrease that multiplies by the same fraction over and over is called a geometric sequence. Multiplying 0.5 ten times gives about 0.00098, roughly one thousandth. This "shrinking by a constant fraction" pattern is called exponential decay. The half-life of radioactive material and the fading of an echo follow the same pattern.
High school+For an opaque wall, reflectance plus absorptance equals 1. When this reflectance is averaged over a whole object or planet instead of a single surface, it is called albedo. Earth's albedo is said to be about 0.3, meaning it sends 30% of the incoming sunlight back to space and takes in the rest as heat. What happens at a room's wall and the reasoning that sets Earth's temperature actually have the same form.
UniversityLight has a "reverberation time" too
In building environmental engineering, the time for sound to fall to a set level is called the reverberation time, and it is estimated from the room's volume and sound absorption using Sabine's formula. Exactly the same setup works for light: from the room's volume and the walls' absorptance you can estimate how long light "stays" in it. In optics this is called the photon lifetime of a resonator. Solving a room's brightness as an exchange of light between surfaces is called the radiosity method, and it is used in interior lighting design and computer graphics.
📖 Derivations and further reading: Japanese Wikipedia, "Reverberation" / Japanese Wikipedia, "Albedo"
ResearchWhat is still not well understood
- What the absorbed light shakes to become heat. Which vibrations the molecules pass the energy to, and how fast, differs from material to material, and measurements are still under way.
- Predicting a real room accurately. Wallpaper and floors vary in colour and texture, and reflectance changes with the colour of the light. Predicting brightness without measuring is still not easy.
- Filming light in flight. Researchers are developing technology that takes a picture every trillionth of a second to film light travelling through a room. The day when we can follow the bounces by eye is getting closer.
In other words, this article too is "an explanation of what is known today." The numbers vary greatly with the room, so please treat them only as a sense of scale.
Links to textbooks (by level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Middle school | Science: properties of light / energy and its transformations | Light bouncing, and being absorbed and turned into heat |
| High school | Physics / Mathematics B (sequences) | Reflection and absorption, the geometric sequence that halves each time |
| High school+ | Advanced physics / earth science (radiation balance) | Sum of reflectance and absorptance, albedo |
| University | Optics / building environmental engineering | Reverberation time, photon lifetime, interreflection calculations |
| Research | Molecular spectroscopy / computational imaging | Where absorbed energy goes, ultrafast imaging of light in flight |
| ― | Link to daily life | Why light-coloured walls and furniture make a room brighter under the same lamp |
- Illuminating Engineering Institute of Japan (照明学会), ed., Lighting Handbook (照明ハンドブック), on the reflectance of interior materials
- Architectural Institute of Japan (日本建築学会), ed., Teaching Materials for Building Environmental Engineering: Environment Volume (建築環境工学用教材 環境編), chapter on reverberation and sound absorption
- Japanese Wikipedia, "Reverberation" (残響)
- Japanese Wikipedia, "Albedo" (アルベド)
- Definition of the metre by the International Committee for Weights and Measures (based on the distance light travels in a vacuum)
Note: This article is a general-audience science explainer. The figures given are rough estimates to help you understand how things work. A room's brightness varies greatly with the colour and material of its interior.