(a)[2]
Circle two of the options below that fit an ultrasound wave travelling in air: frequency $3.5\,\text{Hz}$, frequency $350\,\text{Hz}$, frequency $35000\,\text{Hz}$, longitudinal, transverse, speed $1.5\,\text{m s}^{-1}$, speed $1.5 \times 10^3\,\text{m s}^{-1}$, speed $1.5 \times 10^6\,\text{m s}^{-1}$.
(b)[3]
Calculate the wavelength in a vacuum of X-rays with frequency $1.3 \times 10^{17}\,\text{Hz}$.
(c)[2]
A dentist takes an X-ray photograph of a patient’s teeth. Explain why it is safe for the patient to be close to the source of X-rays, but the dentist must remain farther away from the source.
(d)[2]
State, with a reason, why microwave ovens are designed to operate only when the door is closed.