Tyndall Effect
When a strong converging beam of light is passed through a colloidal solution placed in a dark room, the path of beam gets illuminated with a bluish light when viewed at right angles to the direction of the passage of light. This phenomenon is known as Tyndall effect as shown in figure. The Tyndall effect is due to scattering of light by colloidal particles.
Tyndall effect is observed only when the following two conditions are satisfied:
- The diameter of the dispersed particles is not much smaller than the wavelength of the light used
- The refractive indices of the dispersed phase and the dispersion medium differ greatly in magnitude.
Tyndall effect is used to distinguish between a colloidal and true solution.
Examples
- Sunlight streaming through a window and illuminating dust particles.
- The visible beam of headlights in fog.
- The bluish appearance of smoke from motorcycles or burning matter.
- Milk, starch solution, jelly, and fog—all colloidal systems—show this effect
Mechanism
When light passes through a colloid, the particles (40 to 900 nanometers) scatter the light in all directions. This scattering happens because the particle size is comparable to the wavelength of visible light. As a result, the actual beam of light becomes visible against the background, such as a shaft of sunlight made visible by dust particles in a dark room or the beam from a flashlight in fog.
Color Dependence
The Tyndall effect shows a stronger scattering of shorter wavelengths of light (like blue light) compared to longer wavelengths (like red light). This makes some phenomena, like smoke or fog, appear bluish when illuminated.
⮞ Test Your Knowledge
1. What is the primary phenomenon that defines the Tyndall effect?
A. Absorption of light by particles in a solution
B. Reflection of light from a smooth surface
C. Scattering of light by particles in a colloid or fine suspension
D. Bending of light as it passes from one medium to another
View Answer
Answer: C. Scattering of light by particles in a colloid or fine suspension.
The Tyndall effect is the scattering of a light beam by the dispersed particles of a colloid, making the path of the light visible.
2. Which of the following mixtures will exhibit the Tyndall effect?
A. Saltwater solution
B. Mixture of sand and water
C. Sugar dissolved in water
D. Milk and water mixture
View Answer
Answer: D. Milk and water mixture.
Milk is a classic example of a colloid, as its fat and protein particles are the right size to scatter light, thus exhibiting the Tyndall effect.
3. The blue color of the sky is often cited as an example of light scattering. Is this due to the Tyndall effect?
A. No, it is due to Mie scattering by large particles.
B. Yes, because the Tyndall effect causes shorter wavelengths to be scattered more.
C. No, it is due to Rayleigh scattering by air molecules.
D. Yes, because the Tyndall effect is what makes light visible in a medium.
View Answer
Answer: C. No, it is due to Rayleigh scattering by air molecules.
The blue color of a clear sky is caused by Rayleigh scattering, which involves particles much smaller than the wavelength of light, not colloidal particles.
4. Why is the Tyndall effect not observed in a true solution?
A. The particles are too large to scatter light.
B. The refractive indices of the solute and solvent are too different.
C. The particles are too small to scatter the light.
D. The true solution is not a homogeneous mixture.
View Answer
Answer: C. The particles are too small to scatter the light.
In a true solution, the solute particles are dissolved at the molecular level and are too small to interact with and scatter visible light.
5. Which of the following conditions is necessary for the Tyndall effect to be observed?
A. The dispersed particles must be smaller than the wavelength of light.
B. The refractive indices of the dispersed phase and the medium must be identical.
C. The dispersed particles must be dissolved in the medium.
D. The diameter of the dispersed particles is not much smaller than the wavelength of light.
View Answer
Answer: D. The diameter of the dispersed particles is not much smaller than the wavelength of light.
For light to be scattered, the particles must be large enough to interact with the light waves.
6. What visual observation indicates that a mixture is a colloid and not a true solution?
A. The mixture appears perfectly clear.
B. The particles in the mixture settle to the bottom over time.
C. The path of a light beam is visible when shone through it.
D. The mixture has a constant boiling point.
View Answer
Answer: C. The path of a light beam is visible when shone through it.
The visibility of the light path is the defining characteristic of the Tyndall effect, which occurs in colloids and fine suspensions.
7. When a beam of light is passed through fog, the path of the light becomes visible. This is an example of the Tyndall effect because:
A. Fog is a true solution.
B. The water droplets in fog are large enough to reflect light.
C. The water droplets in fog are large enough to absorb light.
D. The water droplets in fog are colloidal particles that scatter the light.
View Answer
Answer: D. The water droplets in fog are colloidal particles that scatter the light.
Fog is a colloid of water droplets in air. These dispersed droplets are the correct size to scatter the light, making the light beam visible.
8. How does the wavelength of light affect the scattering in the Tyndall effect?
A. All wavelengths are scattered equally.
B. Longer wavelengths are scattered more than shorter wavelengths.
C. Scattering intensity is independent of wavelength.
D. Shorter wavelengths are scattered more than longer wavelengths.
View Answer
Answer: D. Shorter wavelengths are scattered more than longer wavelengths.
The intensity of scattered light is inversely proportional to the fourth power of the wavelength, so blue light (shorter wavelength) is scattered more strongly than red light.
9. The Tyndall effect can be used to distinguish between:
A. A compound and an element
B. An acid and a base
C. A true solution and a colloid
D. A physical change and a chemical change
View Answer
Answer: C. A true solution and a colloid.
The Tyndall effect is visible in colloids but absent in true solutions, making it a simple test to differentiate between them.
10. What is the key difference between the Tyndall effect and Rayleigh scattering?
A. Tyndall scattering occurs in gases, while Rayleigh scattering occurs in liquids.
B. Rayleigh scattering involves reflection, while Tyndall scattering involves absorption.
C. Tyndall scattering involves particles comparable in size to the wavelength of light, while Rayleigh scattering involves much smaller particles.
D. Rayleigh scattering makes the light path visible, while the Tyndall effect does not.
View Answer
Answer: C. Tyndall scattering involves particles comparable in size to the wavelength of light, while Rayleigh scattering involves much smaller particles.
This size difference is the fundamental distinction between the two scattering phenomena.