Some currency notes (to avoid counterfeits) change their colour as you tilt them. This is due to

In a biprism experiment,if the $5^{\text{th}}$ bright band with wavelength $\lambda_1$ coincides with the $6^{\text{th}}$ dark band with wavelength $\lambda_2$,then the ratio $(\lambda_1 / \lambda_2)$ is:

Oil floating on water looks coloured due to interference of light. What should be the order of magnitude of the thickness of the oil layer in order that this effect may be observed? (in $m$)

An oil film flowing on water appears coloured due to interference. For observing this effect,the approximate thickness of the oil film should be:

In a biprism experiment,the $4^{\text{th}}$ dark band is formed opposite to one of the slits. The wavelength of light used is ($D=$ distance between source and screen,$d=$ distance between the slits).

In a biprism experiment,the fringe width is $0.6 \ mm$. The distance between the $6^{th}$ dark fringe and the $8^{th}$ bright fringe on the same side of the central bright fringe is: (in $mm$)