Dr. Christopher S. Baird

# Why does my room get dark when I turn the lights off even if my window is shut?

Category: Physics      Published: January 23, 2014

All of the light from a light bulb is converted to heat and other forms of energy within microseconds. Public Domain Image, source: Christopher S. Baird.

Light does not go away by leaking out through doors and windows. Light goes away by being quickly absorbed by materials and converted to heat and other forms of energy. In contrast to wind or smoke, light has no mass and is not composed of atoms. Because of this, light travels quickly in straight lines until it hits an object. It does not float around, billow with the wind, or leak out like a gas.

When light hits an object, part of the light gets absorbed, part of it is transmitted trough the object, and part of it is reflected/scattered. Exactly how much gets absorbed, transmitted, or reflected depends on the material, shape, and thickness of the object. Thick metal objects like cooking pots reflect most of the light that hits them, absorb a little of the light, and transmit almost none of the light. In contrast, clear glass transmits most of the light, reflects a little of the light, and absorbs almost none. Coal absorbs most the light that hits it, and reflects and transmits almost none. Understanding and predicting exactly how much light of a certain color that a certain object absorbs, transmits, and scatters involves a complex field of study that involves many different effects not worth examining here. However, we can summarize the basic principles behind absorption.

Absorption of light is the complete destruction of a particle of light (a photon) and the conversion of its energy to some other form. Light is composed of quantized bits of fluctuations in the electromagnetic field. The electromagnetic field of the light exerts a force on the negative electric charge of the electrons in the object and also on the positive electric charge of the atomic nuclei in the object. As a result, the electrons and molecules absorb some of the energy of the light and transition to higher-energy states. The light's energy therefore gets converted upon absorption to the potential/kinetic energy of the particles in excited states. There are five basic ways that light can excite electrons and molecules to higher-energy states:

1. Electronic. An electron can be elevated to a higher-energy state, where its wavefunction is more wavy (has more peaks and valleys).
2. Vibrational. The atoms in a molecule can be made to vibrate stronger relative to each other and thus be elevated to a higher-energy vibrational state. In a crystalline solid (which includes most solids), the entire crystalline structure is one giant molecule, so vibrations stretch through the entire object.
3. Rotational. The molecule can be made to rotate faster and thus be elevated to a higher-energy rotational state.
4. Nuclear. The nuclei of the atoms can be excited into higher-energy wavefunction states. However, it takes a large amount of energy to excite a nucleus. As a result, only high-energy gamma rays can induce nuclear transitions, whereas visible light cannot.
5. Translational. The molecule can be made to move faster linearly through space, increasing its kinetic energy. Note that atoms in a solid are not free to move about. Therefore, solids do not significantly experience molecules being excited to higher-energy translational states (unless, of course, there is enough energy present to explode the solid into little bits).

When light gets absorbed by an object, it is destroyed and its energy is converted to potential/kinetic energy in one of the forms listed above. Note that, strictly speaking, none of the excited states listed above constitutes heat when brought about by direct light absorption. The energy that was in the light is therefore not directly converted to heat. Heat consists of random motion, and none of the excited states listed above is truly random when brought about by direct light absorption. The excited states listed depend on the direction that the light was going, on its waveform, polarization, and color. The excited states in the object are therefore somewhat ordered.

Typically, these ordered, excited electrons and molecules in the object de-excite quickly in a somewhat random fashion. This random de-excitation therefore converts the ordered potential/kinetic energy to heat. What happens is this: An excited electron or molecule knocks into a neighboring electron or molecule and transfers its energy. Such collisions force the excited particle to relax back down to its normal state (e.g. stop vibrating or rotating so violently). The energy that the particle loses when transitioning down to a lower state is given to the particle that it collides with. Because the collisions are random, the resulting electronic, atomic, and molecular motions are random, therefore constituting heat.

Heat is the most common product of de-exciting particles. But it is not the only product. Electrons and molecules can also de-excite by emitting a bit of light. A good example of this is glow-in-the-dark stickers. In these stickers, some of the original light ends up again as light and not as heat. Electrons and molecules can also de-excite by enabling a chemical reaction (semi-permanently re-arranging bonds between atoms), so that the light energy ends up as chemical potential energy and not heat. A good example of this is the photosynthesis in plants. Also, excited electrons can be channeled away to form an electrical current such as in a solar cell. In this case, light energy ends up as electrical energy and not heat. Despite all these alternatives, light being converted to heat upon striking an object is the most common result.

If light from your light bulb is constantly being converted to heat in all the objects it strikes, why don't the objects heat up? They do heat up! Often, their rise in temperature is so small that you don't notice it. Occasionally, the heating of objects near a light source is very apparent. Touch the glass cover on a lighting fixture that has been on for a long time and you will feel that it is indeed hot.

Note that not all the light that hits an object is absorbed. Some of the light is reflected back and continues through the air until it hits another object. This brings us to the next question: Why does your room get dark at night right when you turn off the light even though some of the light bounces around? The reason for this darkness is that even the reflected light is very quickly absorbed. No surface is perfectly reflective. This means that with every reflection, some of the light is absorbed. After a few reflections, the last remaining bits of light are absorbed. Even if your room is constructed out of highly reflective materials, such as pure silver or aluminum mirrors, the light grows dimmer with every reflection until it is completely gone after a few hundred reflections. A few hundred reflections may sound like a lot, but the speed of light is so fast that light can reflect hundreds of times before you can finish blinking your eye. For instance, consider your room to be 5 meters (16 ft) long and your walls to be covered with excellent mirrors that reflect 97% of the light. After, bouncing once, only 97% of the photons are reflected. After the second bounce, only 97% times 97%, or 94% of the original light remains. After the third bounce, only 91% of the light remains. After bouncing about 2oo times between the mirrors on your walls and traveling 1000 meters total back and forth, only 0.2% of the original light has not been absorbed and converted to heat. Traveling at 3×108 meters per second, the light is therefore almost completely absorbed 4 microseconds after you turn off the light bulb, even if your walls are high-quality mirrors. And since darkness is the absence of light, and therefore travels at the speed of light right behind the rays the light, darkness fills your room just as quickly as the light disappears.