"If you illuminate two pinholes using two lamps, the interference pattern will not b observed" - Explain.

Explanation

Make two small holes close to each other on th cardboard and illuminated the holes with tw sodium lamps and placing the screen behind i we will not see the bright and dark points interference. This is shown in the figure.

If two sodium lamps in the figure, illuminate the two pin holes `S_(1)` and `S_(2)` then the intensity i added and the interference points will not be seen on the screen.
Because, the light wave emitted from a sodiun lamp undergoes abrupt phase change in time of the order of `10^(-9)` seconds. Thus the ligh waves coming out from two independen sources of light will not have any fixed phase relationship. So this source is incoherent.
And from the incoherent source, the screen intensity will be added to each other so the screen will only appear bright.