Which of the following phenomena produces the colour in a soap bubble?

Imperfections in optical lenses can be observed with the help of

Which of the following produces coherent waves?

If Fresnel's biprism experiment is performed in water instead of air,what will be the effect on the fringe width?

In a Fresnel's biprism experiment,the fringe width is $1 \,mm$. The distance of the nearest dark fringe from the central bright fringe is .......$mm$.

In Fresnel's biprism $(\mu = 1.5)$ experiment,the distance between the source and the biprism is $0.3\, m$,the distance between the biprism and the screen is $0.7\, m$,and the angle of the prism is $1^\circ$. The fringe width with light of wavelength $6000\ \mathring{A}$ will be: (in $, cm$)