$2.$ Two sources are $5\mathrm{.}4$ cm apart and vibrate in phase at $3\mathrm{.}0$ Hz A point on the third nodal line is $20\mathrm{.}0$ cm from one source and $26$ cm from the other.

a$)$ Calculate the wavelength of the waves. $(2$ marks$)$

b$)$ Calculate the speed of the waves. $(1$ marks$)$

$3.$ Two point$-$sources are creating circular waves in a pool. You have a little remote$-$controlled motor boat and gently move it to a location where it does not move up or down. The boat is $13\mathrm{.}7$ m from one source and $15\mathrm{.}2$ m from the other source. Very slowly, you now move the boat across a section where it experiences lots of wave action, and settle it into the next quiet spot. At this time, the boat is $12\mathrm{.}1$ m from one source and $13\mathrm{.}0$ m from the other source. Determine the wavelength of the water waves. $(3$ marks$)$

$4.$ Light is incident on two slits that are separated by $0\mathrm{.}35$ mm $.$ The figure shows the resulting interference pattern observed on a screen $1\mathrm{.}5$ m from the slits. Determine the wavelength of light used in this experiment. $(2$ marks$)$