A student uses spectacles of focal length $-2.5 \text{ m}$ .

(a) Name the defect of vision he is suffering from.

(b) Which lens is used for the correction of this defect?

(c) List two main causes of developing this defect.

(d) Compute the power of this lens.

Q: A student uses spectacles of focal length $-2.5 \text{ m}$. (a) Name the defect of vision he is suffering from. (b) Which lens is used for the correction of this defect? (c) List two main causes of developing this defect. (d) Compute the power of this lens.

(a) **Red colour is selected for danger signals** because red light has a longer wavelength and is scattered the least by atmospheric particles like smoke, dust, or fog. Consequently, it can travel longer distances without losing intensity and remains clearly visible from far away. (b) **The sky appears dark in space** because there is no atmosphere in outer space to scatter sunlight. In the absence of atmospheric particles, no scattering of light occurs, and hence no light reaches the observer's eyes from other directions, making the sky appear black/dark. (c) **The time difference between actual sunset and apparent sunset is about 2 minutes** due to atmospheric refraction. As sunlight enters the Earth's atmosphere, it passes through layers of gradually increasing refractive index and continuously bends towards the normal. This causes the Sun to appear slightly higher than its actual position, making it visible to us even after it has actually crossed below the horizon.

The Human Eye and the Colourful World

Atmospheric Refraction and Scattering of Light

Official Answer

(a) Red colour is selected for danger signals because red light has a longer wavelength and is scattered the least by atmospheric particles like smoke, dust, or fog. Consequently, it can travel longer distances without losing intensity and remains clearly visible from far away.

(b) The sky appears dark in space because there is no atmosphere in outer space to scatter sunlight. In the absence of atmospheric particles, no scattering of light occurs, and hence no light reaches the observer's eyes from other directions, making the sky appear black/dark.

(c) The time difference between actual sunset and apparent sunset is about 2 minutes due to atmospheric refraction. As sunlight enters the Earth's atmosphere, it passes through layers of gradually increasing refractive index and continuously bends towards the normal. This causes the Sun to appear slightly higher than its actual position, making it visible to us even after it has actually crossed below the horizon.

red lightleast scatteredlonger wavelengthno atmosphereno scatteringatmospheric refractionhorizonbending towards normal

Marking Scheme

1Reason for red danger signals: least scattering due to longer wavelength (1 mark)
2Reason for dark sky in space: absence of atmosphere and no scattering of light (1 mark)
3Reason for 2-minute sunset difference: atmospheric refraction bending light towards the normal (1 mark)

Quick Oral Answer

Red is used for danger because it scatters the least and is visible from afar. The sky in space is dark because there is no atmosphere to scatter light. The 2-minute sunset delay is caused by atmospheric refraction bending light rays over the horizon.

Common Mistakes

1Attributing the dark sky in space to the absence of the Sun, rather than the absence of scattering.
2Confusing atmospheric refraction with atmospheric dispersion for the sunset time difference.
3Not mentioning 'wavelength' or 'scattering' when explaining the red danger signal.

Interesting Facts

Because of atmospheric refraction, we also get to see the Sun 2 minutes before the actual sunrise, meaning our day is effectively lengthened by about 4 minutes!

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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.

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 "Atmospheric Refraction and Scattering of Light" 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.