When you turn on a torch (flashlight) in a dark room, the light emitted by the torch illuminates the surrounding area, allowing you to see objects within its range. However, beyond a certain distance from the torch, the light becomes less and less noticeable until it eventually appears to disappear. This phenomenon occurs due to the nature of light and its interaction with the environment. Here are a couple of reasons why the light from a torch may appear to vanish:
Inverse square law: Light spreads out in all directions from its source. According to the inverse square law, the intensity of light decreases as the square of the distance from the source increases. In simpler terms, the light becomes more spread out and less concentrated as it travels farther from the torch. The human eye is less sensitive to dimmer light, so as the light spreads out, it becomes progressively fainter, and eventually, it falls below the threshold of visibility.
Absorption and scattering: As light travels through space or interacts with objects in its path, it can be absorbed or scattered. Absorption occurs when the materials in the environment absorb some or all of the light energy, converting it into other forms of energy (such as heat). Scattering occurs when light interacts with particles or irregularities in the atmosphere or objects, causing it to change direction. This scattering can disperse the light in various directions, reducing its intensity and making it less visible.
In the case of a dark room, the walls, ceiling, and other surfaces may absorb some of the light emitted by the torch. The absorption reduces the amount of light that reflects back to your eyes, making the illuminated area appear smaller. Additionally, the air molecules in the room can scatter the light, causing it to be redirected away from your line of sight.
Therefore, as the light from the torch spreads out, becomes absorbed by surfaces, and scatters in different directions, it gradually diminishes in intensity and eventually reaches a point where it becomes imperceptible to the human eye. This is why you see the boundary where the torch light ends, as the surrounding area beyond that boundary receives insufficient light to be visible.