Question

A cylindrical capillary glass tube, open at both ends, is vertically inserted in a pool of water; the water level in the tube rises 7 cm relative to the level outside. Surface tension of water is 0.07 N $m⁻¹,$ contact angle $0°,$ density 1000 kg $m⁻³,$ g = 10 m $s⁻².$ The diameter of the capillary tube (in $μm)$ is __________ (rounded off to the nearest integer).

Official answer: 400