Which of the following is conserved when light waves interfere?

Four light sources produce the following four waves:
$(i)$ $y_1 = a \sin(\omega t + \phi_1)$
(ii) $y_2 = a \sin(2\omega t)$
(iii) $y_3 = d' \sin(\omega t + \phi_2)$
(iv) $y_4 = d' \sin(3\omega t + \phi)$
Superposition of which two waves gives rise to interference?

Two point sources $S_1$ and $S_2$ separated by a distance $10 \mu m$ emit light waves of wavelength $4 \mu m$ in phase. $A$ circular wire of radius $40 \mu m$ is placed around the sources as shown in the figure,where $O$ is the centre of the circle and $OS_1 = OS_2$. Then:

The optical path difference between two identical light waves arriving at a point is $31.5 \lambda$,where $\lambda$ is the wavelength of light used. The point is [Given: Two light sources are coherent]

As a result of interference of two coherent sources of light,energy is

Write down the principle of superposition and what is interference? Write down its types.