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The condition for two distinct quantum mechanical states, represented by wave functions \(\psi_{1}(x)\) and \(\psi_{2}(x)\) respectively, to be orthogonal is :

  • A.\(\int_{-\infty}^{\infty} \psi_{1}^{*}(x)\psi_{2}(x)\, dx = 1\)
  • B.\(\int_{-\infty}^{\infty} \psi_{1}^{*}(x)\psi_{2}(x)\, dx = 0\)
  • C.\(\int_{-\infty}^{\infty} \psi_{1}^{*}(x)\psi_{2}(x)\, dx = \infty\)
  • D.\(\int_{-\infty}^{\infty} [\psi_{1}^{*}(x)\psi_{2}(x)\) \(+ \psi_{2}^{*}(x)\psi_{1}(x)]\, dx = 1\)

Answer: B

The states are orthogonal when \(\int_{-\infty}^{\infty}\psi_{1}^{*}(x)\psi_{2}(x)\,dx = 0\).

Orthogonality means the inner product of the two wave functions vanishes. For functions of x the inner product is the overlap integral of the complex conjugate of one with the other, so the condition is that this integral is zero. Eigenfunctions of a Hermitian operator belonging to different eigenvalues always satisfy it.

An integral equal to 1 is the normalization condition, which applies when the two functions are the same normalized state. An infinite overlap signals non-normalizable functions, and the symmetric sum set equal to 1 is not a condition of orthogonality at all.

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