Problem 16 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The Tyndall effect is observed only when following conditions are satisfied :(i) The diameter of the dispersed particles is much smaller than the wavelength of the light used. (ii) The diameter of the dispersed particle is not much smaller than the wavelength of the light used. (iii) The refractive indices of the dispersed phase and dispersion medium are almost similar in magnitude. (iv) The refractive indices of the dispersed phase and dispersion medium differ greatly in magnitude. (a) (i) and (iv) (b) (ii) and (iv) (c) (i) and (iii) (d) (ii) and (iii)

Step-by-Step Solution

Answer

The correct answer is (b) (ii) and (iv).

Step 1: Understand the Tyndall Effect

The Tyndall Effect occurs when light scatters due to small particles in a colloid. This phenomenon is used to differentiate between a true solution and a colloid.

Step 2: Identify Necessary Conditions

To observe the Tyndall Effect, two main conditions must be met: (i) the particle size should be comparable to the wavelength of light, e.g., not much smaller than the wavelength; (ii) there should be a significant difference in the refractive indices between the dispersed phase and the dispersion medium.

Step 3: Analyze Option (a)

Option (a) states (i) and (iv). (i) contradicts our understanding as it suggests particles are much smaller than the wavelength. (iv) aligns with the needed refractive index difference.

Step 4: Analyze Option (b)

Option (b) states (ii) and (iv). (ii) matches the requirement that particles not be significantly smaller than the wavelength. (iv) also matches the need for different refractive indices.

Step 5: Analyze Option (c)

Option (c) states (i) and (iii). Both conditions contradict the requirements as (i) mentions a much smaller particle size, and (iii) suggests similar refractive indices.

Step 6: Analyze Option (d)

Option (d) states (ii) and (iii). (ii) matches the particle size condition but (iii) contradicts the need for significantly different refractive indices.

Step 7: Select Best Option

Based on the analysis, the conditions in option (b) best satisfy the Tyndall Effect criteria: particles comparable to the wavelength and differing refractive indices.

Key Concepts

Colloids

Colloids are mixtures where one substance of microscopically dispersed insoluble particles is suspended throughout another substance. For example, milk is a colloid consisting of liquid fat globules dispersed in water. The dispersed phase is the particles, while the dispersion medium is the substance they are distributed in. This medium can be a gas, liquid, or solid.
Colloids are unique because the particle sizes are intermediate between those in true solutions and suspensions. They are too small to settle out, but they are large enough to scatter light. This ability to scatter light is what makes the Tyndall Effect possible. A colloid's particles create a visible path of light, helping distinguish it from a true solution.
Understanding colloids is essential in various fields, including food science, medicine, and cosmetics, because they affect texture, appearance, and stability of materials.

Light Scattering

Light scattering occurs when light encounters small particles, causing the light to deviate from its original path. This deviation occurs because the particles obstruct the light, causing it to spread out in different directions. In the context of the Tyndall Effect, light scattering helps differentiate colloids from true solutions.
This phenomenon is significant for several reasons:

• It helps reveal the presence of colloidal particles with a visible light beam.
• It informs about the interaction between light and small particles.
• It aids in understanding atmospheric phenomena, like why the sky is blue.

The extent of light scattering depends on factors like particle size and the difference in refractive indices between the colloid particles and the medium. This interaction indicates that even invisible particles can be detected via their effect on light.

Refractive Index

The refractive index is a dimensionless number that describes how light propagates through a medium. It indicates how much the speed of light is reduced inside a material compared to its speed in a vacuum. A higher refractive index means light travels more slowly through the medium.
In the Tyndall Effect, it's crucial that the refractive indices of the dispersed phase (colloid particles) and the dispersing medium (where they are suspended) significantly differ. This difference in refractive index causes light to bend, or refract, at the boundary of the particles, enhancing visibility through scattering.
Understanding the refractive index difference helps scientists and engineers design materials with specific optical properties, manipulate light paths in lenses, and even create invisible materials through clever engineering.

Particle Size

Particle size is a fundamental factor in the Tyndall Effect. It refers to the diameter of particles in a colloid, which should be similar to the wavelength of light used for scattering. If the particles are too small, they might not scatter light effectively. Conversely, if they're too large, they might settle out of the mixture, resembling a suspension instead of a colloid.
Micro and nanotechnology have allowed us to measure and manipulate particle sizes accurately in different materials. By controlling particle size, we can influence a material's behavior, from its reactivity to its optical properties. In the Tyndall Effect, correctly sizing particles ensures effective visible light scattering, which is a hallmark of colloidal solutions.
In practice, observing the Tyndall Effect in a solution can help determine if a sample is truly a colloid or a homogenous solution, highlighting the importance of understanding particle size in practical applications.