= Solution
Treat the magnetic <electron lens> as a paraxial <thin lens>. A point in the specimen sends rays through different lens positions; the lens brings these to a conjugate point in the image plane, with $1/u+1/v=1/f$ and transverse <magnification> $-v/u$. Conversely, waves leaving different specimen points at the same angle are focused to the same point in the <back focal plane>. This plane therefore contains the <electron diffraction> pattern, with radius $r\simeq f\theta$ for small scattering angle $\theta$. The specimen is just beyond the front focal plane, giving a magnified real image farther downstream.
\Image[/past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2020/paper-1-tem-planes.png]
{title=Distinct real-space image and angular diffraction planes of a TEM objective}
{source=paper-1-tem-planes.py}
The sketch assumes a thin specimen, coherent approximately parallel illumination, small angles and an ideal lens without aberrations. Quantitative diffraction intensities may require dynamical scattering; the ray sketch only establishes the plane conjugacies.<Objective lens by JEOL>[Objective image formation]{ref}
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