Physics - Kinetic model of matter

bonkysleuth SG

when the brownian motion of smoke particles in air is observed with a microscope, moving points of light are seen. these points of light are reflections from

a. air particles only, moving randomly

b. smoke particles only, moving randomly

c. smoke particles only, virbrating

d. both smoke and air particles, moving randomly.

my guess is A... or is it d? but it's the motion of air particles colliding with smoke molecules that give rise to smoke molecules moving. although we cant see air particles.

ditzy SG

Brownian motion occurs in liquids and gases because of the random motion of the molecules. In gases, Brownian motion is best observed by illuminating from the side under a microscope a shallow box containing smoke. A dark background is put behind the box. The illuminated smoke particles seen as bright spots of light execute a zigzag walk against the dark background. The smoke particles have smaller diameters than the wavelength of light but they can easily be seen as they scatter light into a diffraction halo.

Hence, the answer is b. The same question came out in 1999 o levels physics paper 5054/4 in june. See question 11.

Uncertain SG

ya i agree with ditzy.

The answer is indeed (b) cos

these points of light are reflections from smoke particles moving around randomly

bonkysleuth SG

Hence, the answer is b. The same question came out in 1999 o levels physics paper 5054/4 in june. See question 11.

So to put it in a more "shallow" explanation, does this mean the air molecules cannot be seen because they are too small in the first place and thus cannot cast shadows or reflect light? Whereas the smoke particles CAN be seen moving(due to collisions with air molecules) and so light can be reflected from it?

FrozernFire SG

if air particles can be seen in such a way, our atmosphere won't be as transparent as you know it

eagle SG

Even if they are too small, they can still reflect light.

The question is, can you see the reflection of the very little amount of light reflected. (You can see things because they reflect light.)

A normaly microscope is insufficient to see air particles. I believe it is possible with an electron microscope though...

UltimaOnline SG

There's some Chemistry involved in this thread, so here I am with my contribution :

Smoke particles are actually a collection of airborne solid and liquid particulates and gases emitted when a material undergoes combustion (particularly incomplete combustion), and hence the visible portion or particles of smoke, are mostly large solid molecules that consist of hundreds or thousands of atoms bonded (including induced dipole-dipole van der Waals forces, not necessarily covalent or ionic) together, and hence obviously visible. Put in another way, solid particles and liquid droplets that are close to the ideal range of sizes for the scattering of visible light.

Air particles, on the other hand, (are almost entirely) consist(ing) of gaseous nitrogen, oxygen, carbon dioxide, and noble gases, all of which are :

a) Simple molecular and mono, di, tri or oligo atomic, eg. N2, O2, CO2 He, etc. Hence only a tiny fraction of the size of smoke particles.

b) Spread out VERY far apart from each other. This is a basic tenet of the Ideal Gas Theory (Chemistry), that specifies : gas molecules can be considered to be "points"; ie. they possess mass but have negligible volume.

c) Colourless. The main gaseous constituents of air are all colourless, unlike coloured gases like NO2, Cl2, Br2, etc. (Noble gases emit light when excited by electromagnetic fields, but are also colourless under normal non-excitatory conditions).

For the 3 reasons above, air particles are not visible, in contrast with smoke particles, which again, are solid particles and liquid droplets that are close to the ideal range of sizes for the scattering of visible light.