Add wavevector_from_wavelength

Author: jl-wynen

src/scippneutron/conversion/tof.py

@@ -312,6 +312,37 @@ def wavelength_from_energy(*, energy: Variable) -> Variable:
     return sc.sqrt(c / energy)
 
 
+def wavevector_from_wavelength(*, wavelength: Variable, beam: Variable) -> Variable:
+    r"""Compute a wavevector from a wavelength.
+
+    The result is
+
+    .. math::
+
+        Q = 2 \pi \frac{\hat{b}}{\lambda}
+
+    where :math:`\hat{b}` is the normalized beam vector.
+
+    Parameters
+    ----------
+    wavelength:
+        De Broglie wavelength :math:`\lambda`.
+    beam:
+        The direction the neutron travels :math:`\vec{b}`.
+        It does not have to be normalized.
+
+    Returns
+    -------
+    :
+        Wavevector :math:`k`.
+        Has unit 1/ångström.
+    """
+    c = sc.scalar(2 * np.pi).to(
+        unit=elem_unit(wavelength) / sc.units.angstrom,
+    )
+    return c * (beam / sc.norm(beam)) / wavelength
+
+
 def _wavelength_Q_conversions(x: Variable, two_theta: Variable) -> Variable:
     """Convert either from Q to wavelength or vice-versa."""
     c = as_float_type(4 * const.pi, x)

tests/conversion/tof_conversions_test.py

@@ -1,6 +1,5 @@
 # SPDX-License-Identifier: BSD-3-Clause
-# Copyright (c) 2023 Scipp contributors (https://github.com/scipp)
-# @author Jan-Lukas Wynen
+# Copyright (c) 2025 Scipp contributors (https://github.com/scipp)
 
 import numpy as np
 import pytest
@@ -69,6 +68,13 @@ def time_variables():
     )
 
 
+def wavelength_variables():
+    return simple_variables(
+        dims=st.sampled_from(('wavelength', 'lambda', 'tof')),
+        unit=st.sampled_from(('angstrom', 'mÅ', 'nm')),
+    )
+
+
 def space_variables():
     return simple_variables(
         dims=st.sampled_from(('position', 'spectrum', 'x')),
@@ -146,6 +152,17 @@ def test_wavelength_from_tof_single_precision(Ltotal_dtype):
     assert tof_conv.wavelength_from_tof(tof=tof, Ltotal=Ltotal).dtype == 'float32'
 
 
+@given(wavelength=wavelength_variables(), beam=vector_variables())
+@settings(**global_settings)
+def test_wavevector_from_wavelength(wavelength: sc.Variable, beam: sc.Variable) -> None:
+    wavevector = tof_conv.wavevector_from_wavelength(wavelength=wavelength, beam=beam)
+    naive = sc.to_unit(
+        2 * np.pi * beam / sc.norm(beam) / wavelength,
+        '1/angstrom',
+    )
+    sc.testing.assert_allclose(wavevector, naive)
+
+
 @given(
     tof=time_variables(),
     Ltotal_and_two_theta=n_space_variables(2),