निम्नलिखित कथनों पर विचार कीजिए :
- 1.भूकम्प-लेखी (सीस्मोग्राफ) में S तरंगों से पूर्व P तरंगें अभिलिखित की जाती हैं।
- 2.P तरंगों में, अलग-अलग कण तरंग प्रसार की दिशा में आगे-पीछे कंपन करते हैं, जबकि S तरंगों में कण तरंग प्रसार की दिशा के समकोणीय ऊपर-नीचे कंपन करते हैं।
उपर्युक्त कथनों में से कौन-सा/से सही है/हैं?
Read this question in English
Consider the following statements:
1. In a seismograph, P waves are recorded earlier than S waves.
2. In P waves, the individual particles vibrate to and fro in the direction of wave propagation whereas in S waves, the particles vibrate up and down at right angles to the direction of wave propagation.
Which of the statements given above is/are correct?
(a) 1 only · (b) 2 only · (c) Both 1 and 2 · (d) Neither 1 nor 2
उत्तर और व्याख्या · explanation in English
Answer: (c) 1 और 2 दोनों
Both statements are correct. P-waves travel faster, so a seismograph records them first, and they vibrate along the direction of travel. S-waves arrive later and vibrate at right angles to that direction.
- ✓ 1. P-waves are the primary waves: they move faster and are the first to reach the surface. S-waves, the secondary waves, arrive after a time lag, so they are recorded later.
- ✓ 2. P-waves vibrate parallel to the direction of the wave, pushing and squeezing the material to and fro. S-waves vibrate perpendicular to the wave direction in the vertical plane, so they create troughs and crests.
Remember · P-waves: primary, fastest, particles move to and fro along the path, pass through solids, liquids and gases. S-waves: secondary, slower, particles move up and down, only through solids.
📘 Read it in NCERT: Class 11 Fundamentals of Physical Geography, Ch 3 (practise this chapter) · Class 11 Fundamentals of Physical Geography, Ch 3 (practise this chapter)
Sources
- NCERT Class 11 · Fundamentals of Physical Geography, Chapter 3 “P-waves move faster and are the first to arrive at the surface. These are also called 'primary waves'.”
- NCERT Class 11 · Fundamentals of Physical Geography, Chapter 3 “The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane. Hence, they create troughs and crests in the material through which they pass.”
Question and answer: UPSC's official GS Paper I (2023, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). ·