Solar cells (photovoltaic effect): Photons with energy hν > band gap create electron-hole pairs in silicon. $Current \propto photon$ flux (intensity). Based directly on photon absorption — the photoelectric effect in semiconductor form.

CCD and CMOS image sensors: Digital camera pixels each collect photoelectrons created by incoming photons. Charge accumulated per $pixel \propto photon$ $count \propto light$ intensity. The entire digital photography industry rests on this principle.

Photodiodes and phototransistors: Reverse-biased p-n junctions generate photocurrent when illuminated. Used in optical fiber receivers, barcode scanners, TV remote sensors. Photocurrent $I \propto intensity$; response time = nanoseconds (near-instantaneous).

Photomultiplier tubes (PMT): Ultra-sensitive photon detectors using cascade photoelectric emission + secondary electron multiplication. Used in PET scanners, gamma cameras, particle physics detectors, astronomical telescopes. Can detect single photons.

X-ray Photoelectron Spectroscopy (XPS): X-ray photons (high hν) eject core electrons; their measured $KE_{max}$ reveals binding energies of specific elements and chemical bonds. Crucial analytical tool in materials science and pharmaceuticals.

Burglar alarms and photoelectric sensors: An IR beam creates photocurrent; interrupting the beam stops the current and triggers the alarm. Direct application of photocurrent-intensity relationship.

Electron microscope $\frac{TEM}{SEM}$: Accelerated electrons with λ ≈ 0.004 nm (at 100 kV) are used instead of visible light (λ ≈ 500 nm). Resolution improved by factor ~125,000. Applications: virology (COVID-19 imaging), semiconductor chip inspection, materials characterisation.

Electron diffraction $\frac{LEED}{RHEED}$: Low-energy electron diffraction uses de Broglie wavelength matching crystal lattice spacing to reveal surface atomic structure. Standard technique for thin film and 2D material characterisation.

Neutron diffraction: Thermal neutrons (from nuclear reactors) have λ ≈ 1–2 Å, matching interatomic spacings. Used to determine crystal structures of complex molecules including proteins (complementary to X-ray diffraction).

Atom interferometry: Matter waves of laser-cooled atoms create interference patterns of extraordinary precision. Used in gravitational wave detection (proposed), navigation (gyroscopes), measuring fundamental constants $\frac{h}{m with ppb accuracy}$.