Optical Instruments

Two different lenses are used in a telescope to increase the effective focal length of the system while maintaining a reasonable level of image quality. The objective lens, or primary lens, is typically a large lens with a long focal length that captures light from a distant object and forms a real image. The eyepiece lens, or secondary lens, is a smaller lens placed near the focal point of the objective lens and is used to magnify the real image formed by the objective lens.
By using two lenses, the effective focal length of the telescope is increased, allowing for greater magnification of distant objects. This also enables the observer to see finer details of the object being observed. Additionally, using two lenses reduces spherical aberration, which can occur when using a single lens with a large aperture, leading to a clearer and more detailed image.
Answer. A

The resolving power ( ) of a microscope is given by: where: is the refractive index of the medium, \item is the wavelength of light.
To improve resolving power: Increasing (refractive index) improves , Decreasing also increases .