$A$ zone plate of focal length $60\,cm$ behaves as a convex lens. If the wavelength of incident light is $6000\,\mathring{A},$ then the radius of the first half-period zone will be:

Laser beams are used to measure long distances because

Which of the following phenomena support the wave theory of light?
$(1)$ Scattering
$(2)$ Interference
$(3)$ Diffraction
$(4)$ Velocity of light in a denser medium is less than the velocity of light in a rarer medium

Assertion : Different colours travel with different speed in vacuum.
Reason : Wavelength of light depends on refractive index of medium.

Statement $(S)$: Using Huygen's eye-piece,measurements can be taken,but they 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 one of the following:

Which colour of the light has the longest wavelength?