Resolving power of a telescope increases with:

Hint: In this question, we will first know what resolving power is. Then we will use the expression for resolving power of a telescope, this will directly give us the required result. Further, we will study the basics of mirror and its type, for our better understanding.

Formula used:
$R.P = \dfrac{a}{{1.22\lambda }}$

Complete answer:
As we know resolving power of a telescope can be termed as the reciprocal of the smallest angular separation between two distant objects whose images can be seen separately.

Now, resolving power of telescope is given by:
$R.P = \dfrac{a}{{1.22\lambda }}$
Here, a is the diameter of the objective lens of the telescope.

Above equation can also be written as:
$R.P \propto a$

So, we can conclude that the resolving power of the telescope can be increased by increasing the diameter of the objective lens of the telescope.

Therefore, we get the required answer.

Additional information:
As we know, a telescope can be defined as an optical instrument which uses lenses, some curved mirrors, or a combination of both lens and mirror. Telescope is used to observe distant objects, by its emission, absorption, or reflection of electromagnetic radiation or light.
As we know a mirror is defined as a device that contains a surface, which is mainly used to reflect light. As mirrors do not usually absorb much light, most of the light is redirected. The mirror and the object's overall quality is generally preserved in the new image.
In the given figure, the shaded portion is not used to reflect light.

convex mirror.


concave mirror.

Now, as we know that a lens is defined as a transmissive optical device that is used to focus or disperse beams of light by the means of refraction. For example- a lens can focus light to form an image, unlike a prism, which refracts light without focusing.

Note:
We should remember that a mirror will have only one focal point, which is in front of the mirror. We should note that compound lenses are constructed from a combination of different simple lenses. The lens which is used to build a compound lens may have different refractive indices and other properties.