Can the light of a flashlight shine on the moon?

As for infrared rays, they are invisible. They are the part of the spectrum other than red light. Infrared rays are a kind of electromagnetic wave that travel at the speed of light in a vacuum at 300,000 kilometers per second. The speed in the medium is also different. It will be slower in the air. Satellite relay and communication satellites rely on electromagnetic waves, so electromagnetic waves can reach the moon.

The distance between the moon and the earth is about 380,000 kilometers. Divide the distance by the speed to get the time of about 1.281336 seconds. Theoretically, the moon can be illuminated. The light from the flashlight is emitted in one direction (a light condensing device is installed) but is not parallel. The divergence of the light beam is extremely large, and there is a lot of loss during the propagation of light. Secondly, the flashlight does not emit light in a directional manner, and a large number of photons are not concentrated in a very small space. The energy density is naturally not high, and the brightness of the light emitted is not high, so it cannot illuminate distant objects (you can Experiment), then the illumination produced by the emitted light beam on the moon is only about one trillionth of a lux (unit of illumination), which is completely undetectable by the human eye. Furthermore, if the frequency, vibration direction and phase of the light emitted by the light source are highly consistent, when the light waves overlap in space, the light intensity distribution in the overlapping area will show a stable alternating phenomenon of strong and weak. This phenomenon is called interference of light. Ordinary light sources such as flashlights The emitted light has inconsistent frequency, vibration direction, and phase, which is called incoherent light. It has poor coherence and dispersed energy.