Which statement is correct for a zone plate and a lens?

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 $\lambda_0 = 6000 \text{ Å}$.

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

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

To prepare a print,the time taken is $5\, sec$ due to a lamp of $60\, W$ at a $0.25\, m$ distance. If the distance is increased to $40\, cm$,what is the time taken to prepare the similar print in seconds?

$A$ leaf which contains only green pigments is illuminated by a laser light of wavelength $0.6328 \,\mu m$. It would appear to be