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When sunlight passes through our home’s window, the dust particles move in a light path of the light visible. Why the dust particles move in light rays?
Sara Asked good question. Let me answer it.
The phenomenon is well-known: even though you cannot see dust particles in the beam of an ordinary flashlight, it is quite easy to see dust in the beam of a powerful flashlight – or in the beam of the Sun. This is because dust particles are much smaller than the wavelength of light, so the scattering process is Rayleigh scattering, which is proportional to the inverse fourth power of the wavelength. This means that to scatter more light, you need particles that are much smaller than the wavelength of the light. Since the wavelength of visible light is a couple of hundred nanometers or so, the particles have to be about a tenth of a micrometer or so to look visibly “dusty” in the beam of a flashlight. For daylight the wavelength is larger – around 500 nanometers – so it is much harder to make visible dust particles.
When the rays of light fall on a particle, the light is either reflected/refracted or absorbed by the particle. This can happen in all three media (vacuum, air and water). When the light falls on a water droplet, it is either reflected, refracted or absorbed by the water droplet. Since, there is no medium between the dust particle and the water droplet (air) the dust particle absorbs the sunlight.
If you have ever watched a sunrise or sunset, you know that sunlight comes in all kinds of colors, including dark red and orange. The dust particles in the atmosphere scatter the light, and that’s the reason we see all those glorious colors. The same is true for when the sun is low on the horizon.
Hello Sara,
The reason why you see dust particles as shadowy dots is due to the fact that the light rays coming from the sun are scattered by the particles. This is called ‘Rayleigh scattering‘. It is named after Lord Rayleigh who discovered this phenomenon in 1871. This is the same phenomenon that causes the sky to appear blue, the sun to take on a reddish color in the sunset etc. It is in fact a scattering of light which is the result of the collisions between the molecules of air. The reason why the light is scattered is because the wavelengths of the light are very tiny (of the order of a micrometer – a millionth of a meter) and shorter wavelength means higher energy (and hence higher frequency). The collisions between the air molecules can cause the light to lose some of its energy and hence some of its frequency (and hence some of its wavelength). This is the reason why the light appears to come in a slightly different color.
just a follow-up question on this.
what will be the velocity of the dust particle on an average?
how does these particle moves in space, is it completely random or is it related to the temperature gradient inside the space?
thanks in advance for your answers.