Q.The objective of an astronomical telescope has a diameter of $D=5\ \text{m}$. Taking the wavelength of light used as $\lambda=5000\ \text{\AA}$, calculate the limit of resolution (the smallest angular separation the telescope can resolve).

Given: $D_{ap}=5\ \text{m}$, $\lambda=5000\ \text{\AA}=5\times10^{-7}\ \text{m}$.

$d\theta=\frac{1.22\,\lambda}{D_{ap}}=\frac{1.22\times5\times10^{-7}}{5}=1.22\times10^{-7}\ \text{rad}$ …

Substitute directly into $d\theta=1.22\lambda/D_{ap}$; convert t …

• Confusing the limit of resolution (an angle) with a linear distance, and trying to divide by a screen distance that is not given (none is needed for a telescope). …