Question

why diffraction is more when the wave length of the wave is equal to the gap through which it passes?

Answer 1

Because, the wider the gap, the more chance there is for sideways vibrations to cancel each other out, leaving only the wave in the initial direction to be transmitted.

Answer 2

how it is connected to the wavelength sir?

Answer 3

Because the "cancelling out" I mentioned before depends on the wavelength. Two waves will cancel out in a direction where one lags half a wavelength behind the other.