$A$ laser beam can be focused on an area equal to the square of its wavelength. $A$ $He-Ne$ laser radiates energy at the rate of $1 \, mW$ and its wavelength is $632.8 \, nm$. The intensity of the focused beam will be:

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

The difference in the number of waves when yellow light propagates through air and vacuum columns of the same thickness is $1$. The thickness of the air column is $....\,mm$. [Refractive index of air $= 1.0003$,wavelength of yellow light in vacuum $= 6000\,\mathring{A}$]

Red light differs from blue light as they have :

Answer the following questions:
$(a)$ When a low flying aircraft passes overhead,we sometimes notice a slight shaking of the picture on our $TV$ screen. Suggest a possible explanation.
$(b)$ As you have learnt in the text,the principle of linear superposition of wave displacement is basic to understanding intensity distributions in diffraction and interference patterns. What is the justification of this principle?

The difference in the number of wavelengths,when yellow light propagates through air and vacuum columns of the same thickness,is one. Find the thickness of the air column. Given: Refractive index of air $\mu_a = 1.0003$,Wavelength of yellow light in vacuum $\lambda_0 = 6000 \text{ Å}$.