A clinician designs a targeted infrared therapy system for deep-tissue treatment. The system delivers a uniform intensity of 420 W m−2420\text{ W m}^{-2}420 W m−2 of mid-infrared light at a wavelength of 2.8 μm2.8\ \mu\text{m}2.8 μm.
Calculate the energy of a single photon of this therapy light.
Estimate the number of such photons incident per second on a circular treatment spot on the skin with an area of 15 cm215\text{ cm}^215 cm2. Assume all incident photons have this wavelength.
To evaluate safety, the clinician estimates the rate of heating of the skin surface. Estimate the maximum initial rise in temperature per second of a cylindrical volume of skin tissue with a depth of 1.2 mm1.2\text{ mm}1.2 mm, assuming all radiation incident on the flat circular top of this volume is fully absorbed. (Density of tissue =1000 kg m−3= 1000\text{ kg m}^{-3}=1000 kg m−3, specific heat capacity of tissue =4200 J kg−1 K−1= 4200\text{ J kg}^{-1}\text{ K}^{-1}=4200 J kg−1 K−1)
26 exam-style questions on OCR A A Level Physics Quantum physics, covering Photons, The photoelectric effect, and Wave–particle duality. Each one has a worked solution and a mark scheme showing where the marks go.