What is the frequency of the light emitted from a sodium lamp?

$A$ laser beam is used for carrying out surgery because it

$A$ source emits light of wavelengths $4700 \ \mathring{A}$,$5400 \ \mathring{A}$,and $6500 \ \mathring{A}$. The light passes through red glass before being tested by a spectrometer. Which wavelength is seen in the spectrum?

Statement $(S)$: Using Huygen's eye-piece,measurements can be taken but are not correct.
Reason $(R)$: The cross wires,scale,and final image are not magnified proportionately because the image of the object is magnified by two lenses,whereas the cross-wire scale is magnified by one lens only.
Identify the correct option from the following:

In the given figure,light of intensity $I$ is incident on a glass slab at point $A$ at an angle of incidence $\theta_1$. It undergoes partial reflection and partial refraction. At each reflection,$25\%$ of the incident energy is reflected. If interference occurs between the rays $AB$ and $A'B'$,then $I_{max}/I_{min} = \dots$ (in $: 1$)

Light from a point source falls on a small area placed perpendicular to the incident light. If the area is rotated about the incident light by an angle of $60^o$,by what fraction will the illuminance change?