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

Understanding Light Absorption and Reflection

When you turn off the lights in your room, it quickly becomes dark, even if your window is shut. This phenomenon can be understood through the principles of light absorption, reflection, and the nature of light itself.

Light emitted from a light bulb is not merely a continuous stream that escapes into the environment. Instead, all of the light is rapidly absorbed by objects in the room, converting its energy into heat and other forms of energy within microseconds after the light is turned off.

The Nature of Light

Light is composed of photons, which are quantized bits of electromagnetic energy. Unlike materials such as wind or smoke, light has no mass and does not consist of atoms. Therefore, when light travels, it moves in straight lines until it encounters an object. Unlike gases, light does not leak out or diffuse; it interacts with surfaces it encounters.

Interaction of Light with Objects

When light strikes an object, several outcomes can occur:

1. Absorption: Some of the light energy is absorbed by the object, converting it into other forms of energy, primarily kinetic and potential energy in atoms and molecules.
2. Reflection: Part of the light is reflected back into the environment.
3. Transmission: Some light may pass through the object.

The specific proportions of absorption, reflection, and transmission are determined by the material properties of the object, including its shape, thickness, and color.

For instance, consider three different materials:

• Thick metal (like a cooking pot): Reflects most of the light, absorbs a small amount, and transmits almost none.
• Clear glass: Transmits most of the light, reflects a little, and absorbs very little.
• Coal: Absorbs most of the light, reflecting and transmitting almost none.

The Process of Absorption

The process of absorption involves the complete destruction of a photon. When light is absorbed by an object, its energy is converted into the potential or kinetic energy of electrons and molecules in the object. This energy transfer can occur in several ways:

• Electronic excitation: Electrons are elevated to higher energy states.
• Vibrational excitation: Atoms in a molecule vibrate more vigorously.
• Rotational excitation: Molecules rotate at increased speeds.
• Translational excitation: Molecules move faster through space.
• Nuclear excitation: Though less common for visible light, this involves exciting atomic nuclei.

Why the Room Gets Dark

When you turn off the light, the light that has been bouncing around the room is quickly absorbed by the surfaces it encounters. No surface is perfectly reflective, meaning that with each reflection, a fraction of the light is absorbed. For example, if your walls are made of highly reflective materials, like pure silver, they might reflect about 97% of the light.

To illustrate this mathematically, consider a room that is $5$ meters (about $16$ feet) long with walls that reflect $97\%$ of the light. After one reflection, $97\%$ of the original light remains. After two reflections, the remaining light is:

Continuing this process, after three reflections, the remaining light is:

As this process continues, after around $200$ reflections, the light can be reduced to $0.2\%$ of the original intensity. Given that light travels at approximately $3 \times 10^8$ meters per second, this absorption occurs almost instantaneously—roughly $4$ microseconds after you turn off the light.

Conclusion

Consequently, once the light is turned off, darkness fills the room just as quickly as the light disappears. Darkness can be understood as the absence of light, which travels at the same speed, creating an immediate transition to darkness when the light source is removed. Thus, even with your window shut, your room gets dark almost instantaneously after switching off the lights due to the rapid absorption of light by surrounding materials.