NCERT Class 9 Science — Chapter 2: Is matter around us pure?

mcqeasy

The visibility of a light beam entering a dark room through a small opening is due to:

Official previous-year question

Verified 1 Aug 2026.

Options

AThe scattering of light by dust and smoke particles.
BThe reflection of light by the walls.
CThe refraction of light by air molecules.
DThe diffraction of light at the edges of the opening.

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Solution

Correct Option

The correct option is

The scattering of light by dust and smoke particles.

Explanation

The phenomenon responsible for making the path of a light beam visible in a medium containing suspended particles is the scattering of light. When this occurs in a colloidal dispersion or a fine suspension (like dust in air), it is specifically known as the Tyndall effect.

Option Analysis

The reflection of light by the walls. is Incorrect: Reflection of light by the walls contributes to the general illumination of the room but does not explain why the specific path of the light beam is visible in mid-air.
The scattering of light by dust and smoke particles. is Correct: The air in a dark room typically contains suspended particulate matter such as dust and smoke. When a beam of light strikes these particles, the light is scattered in all directions. This scattered light reaches the observer's eyes, rendering the path of the beam visible.
The refraction of light by air molecules. is Incorrect: Refraction refers to the bending of light as it passes from one medium to another with a different optical density. It determines the direction of the beam but does not make the path itself visible.
The diffraction of light at the edges of the opening. is Incorrect: Diffraction is the bending of light around the corners of an obstacle or aperture. While diffraction occurs at the edges of the small opening, it is not the primary mechanism that makes the beam's path visible through the air.

Key Takeaway: The visibility of a light beam's path through a mixture containing suspended particles (such as dust in air) is caused by the scattering of light, a phenomenon known as the Tyndall effect.

Your note

More from Chapter 2: Is matter around us pure?

With reference to 'Alloys', consider the following statements: 1. They are considered mixtures because they show the properties of their constituents. 2. They can be separated into their components by physical methods. 3. They have a fixed composition and cannot vary. Which of the statements given above is/are correct?

medium

Consider the following statements regarding the properties of metals: 1. They are generally brittle and non-ductile. 2. They are generally good conductors of heat and electricity. 3. They are sonorous. Which of the statements given above is/are correct?

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Consider the following statements about elements: 1. The majority of elements are solids. 2. Only two elements are in the gaseous state at room temperature. 3. Gallium and Cesium remain solid even at temperatures well above 303 K. Which of the statements given above is/are correct?

medium

In the classification of colloids, 'Cheese' and 'Butter' are examples of:

medium

Consider the following statements regarding the burning of wood: 1. It involves the conversion of wood into new substances like ash and smoke. 2. It is a physical change. 3. It is an irreversible process. Which of the statements given above is/are correct?

easy

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Correct Option

The correct option is

The scattering of light by dust and smoke particles.

Explanation

Option Analysis

The reflection of light by the walls. is Incorrect: Reflection of light by the walls contributes to the general illumination of the room but does not explain why the specific path of the light beam is visible in mid-air.
The scattering of light by dust and smoke particles. is Correct: The air in a dark room typically contains suspended particulate matter such as dust and smoke. When a beam of light strikes these particles, the light is scattered in all directions. This scattered light reaches the observer's eyes, rendering the path of the beam visible.
The refraction of light by air molecules. is Incorrect: Refraction refers to the bending of light as it passes from one medium to another with a different optical density. It determines the direction of the beam but does not make the path itself visible.
The diffraction of light at the edges of the opening. is Incorrect: Diffraction is the bending of light around the corners of an obstacle or aperture. While diffraction occurs at the edges of the small opening, it is not the primary mechanism that makes the beam's path visible through the air.