The correct option is **A. They are formed by two identical waves travelling in opposite directions.** ### Detailed Explanation: **Understanding Stationary Waves:** Stationary waves, also known as standing waves, are a type of wave pattern that occurs when two waves of the same frequency and amplitude travel in opposite directions and interfere with each other. This interference creates points of no displacement (nodes) and points of maximum displacement (antinodes). **Why Option A is Correct:** - **Formation of Stationary Waves:** Stationary waves are indeed formed by the superposition of two identical waves moving in opposite directions. When these waves meet, they interfere constructively and destructively at different points, leading to the characteristic pattern of nodes and antinodes. - **Example:** If you imagine two people on a rope, each shaking their end in opposite directions, the resulting wave pattern will show areas where the rope does not move (nodes) and areas where it moves the most (antinodes). ### Why the Other Options are Incorrect: **Option B: They can be transverse or longitudinal.** - **Explanation:** While stationary waves can indeed be either transverse (like waves on a string) or longitudinal (like sound waves), this statement is too broad and does not specifically define a characteristic of stationary waves. The key feature of stationary waves is their formation through interference, which is not captured in this option. **Option C: The distance between two successive nodes is one wavelength.** - **Explanation:** This statement is incorrect. The distance between two successive nodes is actually half a wavelength (λ/2). This is because a full wavelength consists of one complete cycle of a wave, which includes one node and one antinode. Therefore, the distance between two nodes (where there is no displacement) is half of that cycle. **Option D: The antinode is a point of minimum displacement.** - **Explanation:** This statement is fundamentally incorrect. An antinode is defined as a point of maximum displacement in a stationary wave. At the antinode, the amplitude of the wave is at its highest, while at the nodes, the displacement is zero (minimum). Thus, this option misrepresents the nature of antinodes. ### Summary of Key Points: - Stationary waves are formed by the interference of two identical waves traveling in opposite directions. - They exhibit nodes (points of no displacement) and antinodes (points of maximum displacement). - The distance between successive nodes is half a wavelength (λ/2), not a full wavelength. - Antinodes are points of maximum displacement, contrary to the claim in option D. ### Revision Summary: - Stationary waves result from the superposition of two identical waves moving in opposite directions. - They can be either transverse or longitudinal, but this does not define their key characteristics. - The distance between nodes is half a wavelength (λ/2). - Antinodes are points of maximum displacement, not minimum.