To observe diffraction,the size of the obstacle

Due to which optical phenomenon do we not see the shadow of an opaque object clearly?

In a single-slit diffraction experiment,the first minimum for a wavelength $\lambda_1 = 660 \ nm$ coincides with the first maximum for a wavelength $\lambda_2$. Find the value of $\lambda_2$ in $nm$.

$A$ diffraction pattern is obtained by using a beam of red light. What will happen if the red light is replaced by blue light?

The box of a pinhole camera,of length $L$,has a hole of radius $a$. It is assumed that when the hole is illuminated by a parallel beam of light of wavelength $\lambda$,the spread of the spot (obtained on the opposite wall of the camera) is the sum of its geometrical spread and the spread due to diffraction. The spot would then have its minimum size (say $b_{min}$) when

The half angular width of the central maxima in the Fraunhofer diffraction due to a slit of width $\frac{1200}{\sqrt{2}} \ \mu m$ is $45^o$. Then the wavelength of the light is ...... $\mu m$.