Give reasons for the following: (a) Danger signals installed at airports and at the top of tall buildings are of red colour. (b) The sky appears dark to the passengers flying at very high altitudes. (c) The path of a beam of light passing through a colloidal solution is visible.

Q: Give reasons for the following: (a) Danger signals installed at airports and at the top of tall buildings are of red colour. (b) The sky appears dark to the passengers flying at very high altitudes. (c) The path of a beam of light passing through a colloidal solution is visible.

### **(a) Why danger signals are red in colour:** Red light has a longer wavelength compared to other colours in the visible spectrum. According to the principles of light scattering, longer wavelengths are scattered the least by very fine particles (such as smoke, dust, and tiny water droplets) present in the atmosphere. Because it is scattered the least, red light can travel long distances through fog, dust, or smoke without losing its intensity, making it clearly visible from far away. ### **(b) Why the sky appears dark to passengers flying at very high altitudes:** At very high altitudes, the earth's atmosphere becomes extremely thin or is practically absent. Because there are no air molecules, dust, or water droplets to scatter sunlight, no scattering of light takes place. In the absence of scattered light reaching the passengers' eyes, the sky appears dark or black instead of blue. ### **(c) Why the path of a beam of light passing through a colloidal solution is visible:** The particles in a colloidal solution are relatively larger in size compared to the individual molecules in a true solution. When a beam of light passes through a colloidal solution, these larger colloidal particles scatter the light rays in all directions. This phenomenon is known as the **Tyndall effect**. The scattered light enters our eyes, making the path of the light beam clearly visible.

The Human Eye and the Colourful World

Scattering of Light

Official Answer

(a) Why danger signals are red in colour:

Red light has a longer wavelength compared to other colours in the visible spectrum. According to the principles of light scattering, longer wavelengths are scattered the least by very fine particles (such as smoke, dust, and tiny water droplets) present in the atmosphere. Because it is scattered the least, red light can travel long distances through fog, dust, or smoke without losing its intensity, making it clearly visible from far away.

(b) Why the sky appears dark to passengers flying at very high altitudes:

At very high altitudes, the earth's atmosphere becomes extremely thin or is practically absent. Because there are no air molecules, dust, or water droplets to scatter sunlight, no scattering of light takes place. In the absence of scattered light reaching the passengers' eyes, the sky appears dark or black instead of blue.

(c) Why the path of a beam of light passing through a colloidal solution is visible:

The particles in a colloidal solution are relatively larger in size compared to the individual molecules in a true solution. When a beam of light passes through a colloidal solution, these larger colloidal particles scatter the light rays in all directions. This phenomenon is known as the Tyndall effect. The scattered light enters our eyes, making the path of the light beam clearly visible.

Red lightLonger wavelengthScattered the leastHigh altitudesNo atmosphereNo scatteringColloidal solutionTyndall effectScattering of light

Marking Scheme

1Part (a): Mentioning red light has a longer wavelength and is scattered the least by smoke/fog particles (1 mark)
2Part (b): Explaining that at high altitudes there is no atmosphere/particles to cause scattering of light (1 mark)
3Part (c): Explaining that colloidal particles are large enough to scatter light (Tyndall effect), making the path visible (1 mark)

Hint

Think about how the presence or absence of atmospheric particles affects light, and how wavelength determines how much a color scatters.

Quick Oral Answer

The Tyndall effect is the scattering of light by colloidal particles, which makes the path of a light beam visible. A common example is a beam of sunlight entering a dusty room through a small hole.

Analysis & Explanation

This question tests the practical applications of the scattering of light and the Tyndall effect. The key physical principle is that the intensity of scattered light is inversely proportional to the fourth power of the wavelength (Rayleigh scattering) for particles smaller than the wavelength of light. Thus, shorter wavelengths (blue/violet) scatter much more than longer wavelengths (red). At high altitudes, the absence of a scattering medium (particles) prevents any light from being redirected, resulting in a black sky. In colloidal solutions, the larger particle size is sufficient to scatter light, demonstrating the Tyndall effect.

Common Mistakes

1In part (a), saying red light 'travels faster' instead of stating it has a longer wavelength and is scattered the least.
2In part (b), stating that there is no sunlight at high altitudes, which is incorrect; sunlight is present, but it is not scattered.
3In part (c), failing to mention the relative size of colloidal particles compared to true solutions.

Interesting Facts

If Earth had no atmosphere, the sky would look completely black even during the daytime, just as it does to astronauts on the Moon!

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2023?

This question carries 3 marks in the CBSE Class 10 Science 2023 examination.

Which chapter does this question come from in Science?

This question is from the chapter "The Human Eye and the Colourful World" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Scattering of Light" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2023 paper?

This is a Short Answer question from Section C in the CBSE Class 10 Science 2023 paper.