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

$A$ 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?

The figure shows a schematic diagram of Young's Double Slit Experiment. If the distance $D$ between the slits and the screen is varied,choose the correct statement$(s)$.

Fraunhofer lines are produced by the absorption of light in

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?

$A$ white screen illuminated by green and red light appears to be