Why does the light from a torch doesn't reach some 100 meters whereas the speed of light is 300000 km per second?

Answer 1

It makes no difference how fast it goes; you can't shine light through, for example, solid rock. The light from a torch (or flashlight) is not particularly strong, and it usually is not coherent or laser light. The scattered nature of the light causes a reduction in intensity over distance, and there are things in the atmosphere (dirt, moisture, several different gases, etc) that absorb some of the energy along the way. Any light energy that is absorbed stops where it is absorbed; it doesn't continue as light. It's not as if light, traveling so fast, can punch its way through barriers. Photons are massless and very easy to stop. It appears that you are asking why the light from a torch doesn't illuminate anything well at 100 meters. Here's why: the brightness of the light falls off inversely to the square of the distance from the source. That means that at 2 m it is 1/4 of it's brightness at the source; 4 m, 1/16 the brightness, 8 m, 1/64 the brightness, and so on. At 100 meters, the amount of light falling on the surface to be illuminated is 1/10,000 as bright as it was at the source. That small amount of light has to be reflected back, and the same rules apply to the reflected light. Even if you eliminate other attenuating circumstances like scattering, the light coming back to you is 1/40,000 as bright as when it left--perhaps below your ability to sense it--but in any event, not useful illumination. It has nothing to do with its speed. It has to do with the fact that the wave front is (ideally) an expanding sphere, and the area of the sphere grows by the square of the distance, while the amount of light remains the same. The number of photons leaving a square cm of the source is spread out over four square centimeters at 2 meters, 16 square cm at 4 meters, etc.

Answer 2

The speed of electromagnetic radiation, including light, depends on the electrical

characteristics of the medium it's in, specifically, the static permittivity and the

magnetic permeability. The speed of light is the reciprocal of the square root of

the product of those two numbers in any medium. In vacuum, that number is

299,792,458 meters per second.

Answer 3

The light from a torch spreads out in all directions, so the intensity decreases as you move further away. At 100 meters, the light is more spread out, and therefore less intense. Additionally, the atmosphere can scatter and absorb some of the light, further reducing its reach.