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GATE 2020 PH – Question 13

Quantum Mechanics · One dimensional potentials: step potential, finite rectangular well, tunnelling from a potential barrier, particle in 1,2,3-dimensional box, particle in single and double delta function potentials, 1,2,3 dimensional harmonic oscillator: concept of degeneracy · 1 mark · Multiple choice

A quantum particle has $V(x)=0$ for $-a/2<x<a/2$ and $V(x)=\infty$ outside, including the walls. Its ground-state wave function inside the well is proportional to

  1. $\sin(\pi x/(2a))$
  2. $\sin(\pi x/a)$
  3. $\cos(\pi x/(2a))$
  4. $\cos(\pi x/a)$

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Correct answer: (D) $\cos(\pi x/a)$

Explanation

The ground state is even, has no interior node and must vanish at $x=\pm a/2$. The function $\cos(\pi x/a)$ has all these properties. The cosine with denominator $2a$ does not vanish at the walls, while the sine alternatives have a node at the centre or fail the boundary condition. Thus D.

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