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Shine a torch through a glass of salt water and you see almost nothing. Shine it through water with a few drops of milk and the beam lights up like a rod. That difference is the Tyndall effect, and it is how your Class 9 science book tells a solution from a colloid. This post covers the physicist behind it, what he saw, how to use the effect in exam questions, and the mistakes students make.
Who was John Tyndall?
John Tyndall was an Irish physicist who lived from 1820 to 1893, as his Oxford Dictionary of National Biography entry records. By 1861 he was Professor of Natural Philosophy at the Royal Institution in London, as the byline of his 1861 Bakerian Lecture shows.
That lecture began with glaciers. Tyndall wrote that his research on glaciers led him to ask how heat from the Sun and the Earth passes through the atmosphere, and so to measure how different gases absorb radiant heat. He found that water vapour in ordinary laboratory air absorbed far more than dry air, and wrote that any change in the amount of water vapour, or of carbonic acid (carbon dioxide), in the air would produce a change of climate. That work is one reason his name comes up in climate science today.
What did Tyndall see in his light beam?
In a paper received by the Royal Society in December 1868 and published in 1869, On the blue colour of the sky, the polarization of skylight, and on the polarization of light by cloudy matter generally, Tyndall described his apparatus: a glass tube about a yard long and 2.5 to 3 inches across, filled with a thin vapour, with the condensed beam of an electric lamp sent through it.
Light broke down the vapour into tiny particles. When the vapour was thin enough, the first thing to appear was what he called a "blue cloud". He stressed that it was "totally invisible in ordinary daylight" and could only be seen in darkness with a powerful beam. He believed some particles were a very small fraction of the wavelength of violet light. Looking through a polarising prism, he found the light coming sideways from the cloud was polarised, most strongly at right angles to the beam. He linked this to two puzzles he called the great standing enigmas of meteorology: why the sky is blue and why skylight is polarised.
How does the Tyndall effect appear in your syllabus?
In the new NCERT Class 9 science book Exploration, Chapter 5, Exploring Mixtures and their Separation, section 5.5 defines the Tyndall effect as the scattering of light by particles. The chapter gives these particle sizes:
Solution: particles smaller than 1 nm. The beam is not visible.
Colloid: particles 1 to 1000 nm. The beam is visible, and the particles do not settle.
Suspension: particles larger than 1000 nm. The beam is visible, and the particles settle over time.
In its Activity 5.1, a laser beam passes through salt water, chalk in water and a few drops of milk in water. The beam is invisible in the salt solution but visible in the other two. Class 10 returns to the effect in The Human Eye and the Colourful World, section 10.6.1, with sunlight through a dense forest canopy and the colour of the clear sky.
A worked example: classify these mixtures
Question. Which of these show the Tyndall effect: (a) air with dust, (b) copper sulphate in water, (c) starch in water, (d) acetone in water?
Air with dust: dust particles are large enough to scatter light, which is why you see sunbeams in a dusty room. Shows the effect.
Copper sulphate in water: a true solution, with particles smaller than 1 nm. The solution is blue, but a beam passing through it is not visible from the side. No effect.
Starch in water: a colloid. Shows the effect.
Acetone in water: they mix completely to form a solution. No effect.
So the answer is (a) and (c). This is one of the end-of-chapter questions in the NCERT chapter.
Common mistakes students make
Calling coloured liquids colloids. Colour comes from absorbing light. Scattering is what makes the beam visible from the side. Copper sulphate solution is coloured but is not a colloid.
Thinking only colloids scatter light. NCERT's activity shows the chalk suspension scattering the beam too. What sets colloids apart is that they scatter light and do not settle.
Getting the reason backwards. Solutions do not show the effect because their particles are too small, under 1 nm, not too large.
Mixing up scattering and reflection. You see the beam from the side because particles send light in many directions.
C. V. Raman took the study of scattered light much further, as our post on Raman and the scattering of light explains. To try the effect at home, see our kitchen science experiments, and use a torch rather than a laser pointer, which should never be pointed near anyone's eyes. If you would rather see scattering before you read about it, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than to read.
What can a student take from Tyndall's way of working?
Make the invisible visible. His blue cloud could not be seen in daylight. He built conditions, darkness and a strong beam, in which it could.
Follow a question where it leads. Glaciers led him to heat in the atmosphere, and vapours in a tube led him to the colour of the sky.
Be careful with words. He took a paragraph to define exactly what he meant by "cloud" before using it. Defining your terms in an answer earns marks.
Tonight, shine a torch through salt water, sugar water and very dilute milk in a dark room, and write down which beams you can see from the side.
