The type of lens present in human eye is

In the figure below, $P$ and $Q$ are two equally intense coherent sources emitting radiation of wavelength $20\mathrm{m}.$ The separation between $\mathrm{P}$ and $\mathrm{Q}$ is $5\mathrm{m}$ and the phase of $\mathrm{P}$ is ahead of that of $\mathrm{Q}$ by $90°.\mathrm{A},\mathrm{B}$ and C are three distinct point of observation, each equidistant from the midpoint of PQ. The intensities of radiation at $\mathrm{A},\mathrm{B},\mathrm{C}$ will be in the ratio :

A parallel beam of light is incident on a tank filled with water up to a height of $61.5 \mathrm{mm}$ as shown in the figure below. Ultrasonic waves of frequency $0.5 \mathrm{MHz}$ are sent along the length of the water column using a transducer placed at the top, and they form longitudinal standing waves in the water. Which of the schematic plots below best describes the intensity distribution of the light as seen on the screen? Take the speed of sound in water to be $1,500 \mathrm{m} {\mathrm{s}}^{-1}$.

If the focal length of a mirror is +15, then it is______: