When light is passed through a collidal solution, due to Tyndall scattering, the path of light appears to be a bright cone. Why does it have to form only a conical shape?

The conditions for Tyndall effect are that the diameter of dispersed particles should not be much smaller than wavelength of light used and refractive index of dispersed phase and dispersed medium should differ greatly. This is perhaps because colloidal particles scatter light in all directions. Normal refraction shouldn't cause such a shape.

Is there any logical reson for the conical shape?

Tyndall Cone

Source: NCERT class 12

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    $\begingroup$ Any chance you could post a photo or reasonable figure to illustrate the effect? $\endgroup$ – Ed V Mar 5 '20 at 15:54
  • $\begingroup$ I have added a picture. $\endgroup$ – user600016 Mar 5 '20 at 16:24
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    $\begingroup$ The answer is fine, but the reason your figure shows a conical shape is that the double convex lens focusses the beam in that fashion so that the incident light intensity is high in the region being observed with the microscope. This just increases the Tyndall scattering light intensity. $\endgroup$ – Ed V Mar 5 '20 at 17:51
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    $\begingroup$ One last thing: the illustration should not have mistakenly labelled the light cone as “Tyndall cone”. That is just nonsense. $\endgroup$ – Ed V Mar 5 '20 at 18:17

It is simple. Light always travels in a straight line and if a lens is present it would diverge or converge ( in a straight line ) accordingly.

The question is why no other shape like maybe a hemisphere or a paraboloid ?

Now let me ask the question, have you seen such thing? Check out this diagram to make everything clear.

If the beam of light increases in width ( diverging beam) linearly with distance. It is because all the particles scatter equally in all directions and some of them reach your eyes and this reveals the shape of the beam. The adjacent particles does the same and so on. Therefore you can also see the light getting more and more diffused linearly as more and more light is scattered.

It also depends on the refractive index of the medium where the incident light ray will either move closer to the normal or move further. Again this would lead to a conically diverging beam.


This is to be noted that the colloidal solution does not affect the shape of the beam of light ( converging or diverging ).

This is also evident from your diagram.

From your question I feel that you think that the colloidal solution is responsible for the cone shape, it is not so. It is simply the geometry of the light rays

What the solution does is scatter the light in all directions and makes the "beam" visible. A straight beam of light in perfect vacuum would not be visible as a stalk of light ( that we see when light enters a dark room thought a window )

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    $\begingroup$ Check the Edit @user600016 $\endgroup$ – Navoneel Karmakar Mar 5 '20 at 17:49

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