Why is Tyndall effect shown by colloidal particles? State four instances of observing the Tyndall effect.

Q: Why is Tyndall effect shown by colloidal particles? State four instances of observing the Tyndall effect.

### Difference Between a Glass Slab and a Glass Prism | Feature | Glass Slab | Glass Prism | | :--- | :--- | :--- | | **Refracting Surfaces** | The refracting surfaces are parallel to each other. | The refracting surfaces are inclined to each other at an angle (called the angle of the prism). | | **Emergent Ray** | The emergent ray is parallel to the incident ray but laterally displaced. | The emergent ray is not parallel to the incident ray; it bends at an angle called the angle of deviation ($\angle D$). | | **Dispersion** | Does not produce a net dispersion of white light (colors recombine before emerging). | Produces distinct dispersion, splitting white light into its seven constituent colours. | --- ### Effects of Light Passing Through #### (a) Through a Glass Slab: * **(i) Monochromatic Light:** Undergoes refraction at the first surface, travels through the slab, and refracts again at the second parallel surface. It emerges parallel to the incident ray, showing only **lateral displacement**. * **(ii) White Light:** Undergoes internal dispersion at the first surface, but the constituent colours recombine at the second parallel surface. It emerges as a single parallel beam of white light, showing only **lateral displacement** without any net dispersion. #### (b) Through a Glass Prism: * **(i) Monochromatic Light:** Undergoes refraction at both inclined surfaces, bending towards the base of the prism. It emerges deviated from its original path by an **angle of deviation ($\angle D$)**, but does not split into different colours. * **(ii) White Light:** Undergoes **dispersion** because different colours of light bend through different angles with respect to the incident ray (red bends the least, violet the most). It emerges as a beautiful band of seven colours (VIBGYOR spectrum) on a screen.

Refraction of Light through a Prism

Official Answer

Difference Between a Glass Slab and a Glass Prism

Feature	Glass Slab	Glass Prism
Refracting Surfaces	The refracting surfaces are parallel to each other.	The refracting surfaces are inclined to each other at an angle (called the angle of the prism).
Emergent Ray	The emergent ray is parallel to the incident ray but laterally displaced.	The emergent ray is not parallel to the incident ray; it bends at an angle called the angle of deviation ( $\angle D$ ).
Dispersion	Does not produce a net dispersion of white light (colors recombine before emerging).	Produces distinct dispersion, splitting white light into its seven constituent colours.

Effects of Light Passing Through

#### (a) Through a Glass Slab:

(i) Monochromatic Light: Undergoes refraction at the first surface, travels through the slab, and refracts again at the second parallel surface. It emerges parallel to the incident ray, showing only lateral displacement.
(ii) White Light: Undergoes internal dispersion at the first surface, but the constituent colours recombine at the second parallel surface. It emerges as a single parallel beam of white light, showing only lateral displacement without any net dispersion.

#### (b) Through a Glass Prism:

(i) Monochromatic Light: Undergoes refraction at both inclined surfaces, bending towards the base of the prism. It emerges deviated from its original path by an angle of deviation ( $\angle D$ ), but does not split into different colours.
(ii) White Light: Undergoes dispersion because different colours of light bend through different angles with respect to the incident ray (red bends the least, violet the most). It emerges as a beautiful band of seven colours (VIBGYOR spectrum) on a screen.

parallel refracting surfacesinclined surfaceslateral displacementangle of deviationdispersionmonochromatic lightrecombination

Marking Scheme

1Key differences between a glass slab and a glass prism: 1 Mark
2Correct description of monochromatic and white light passing through a glass slab: 1 Mark
3Correct description of monochromatic and white light passing through a glass prism: 1 Mark

Hint

Remember that parallel surfaces cause lateral displacement without net angular deviation, whereas inclined surfaces cause angular deviation and separate different wavelengths of light.

Quick Oral Answer

A glass slab has parallel refracting faces, so the emergent ray is parallel to the incident ray. A glass prism has inclined faces, so the emergent ray is deviated. White light disperses in a prism because different colours bend by different angles, whereas in a slab, they recombine and emerge parallel.

Analysis & Explanation

In a glass slab, the two refracting faces are parallel. The refraction at the first face is exactly equal and opposite to the refraction at the second face, meaning the angular deviation of each component colour is cancelled out, resulting in recombination and parallel emergence. In a glass prism, the refracting faces are inclined. The deviations at both faces add up rather than cancel out. Since different wavelengths (colours) of light have different refractive indices in glass, they deviate by different angles, leading to angular separation (dispersion).

Common Mistakes

1Stating that white light disperses into a spectrum when emerging from a glass slab.
2Thinking that monochromatic light splits into multiple colours when passing through a prism.
3Confusing lateral displacement with angular deviation.

Interesting Facts

Sir Isaac Newton used a second inverted prism to show that the dispersion of white light by a prism is a physical separation of existing colours, rather than the prism 'coloring' the light itself.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2020?

This question carries 3 marks in the CBSE Class 10 Science 2020 examination.

What topic does this question cover in Science?

This question covers the topic "Refraction of Light through a Prism" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2020 paper?

This is a Short Answer question from Section B in the CBSE Class 10 Science 2020 paper.