Question

A lens of large focal length and large aperture is best suited as an objective of an astronomical telescope since

• A. a large aperture contributes to the quality and visibility of the images
• B. a large area of the objective ensures better light gathering power
• C. a large aperture provides a better resolution
• D. all of the above

Answer 1

The Correct Option is D

To understand why a lens with a large focal length and large aperture is best suited as an objective of an astronomical telescope, let's break down the key aspects involved:

1. Light Gathering Power:
   • A large aperture significantly increases the telescope's capacity to collect more light. This is crucial because celestial objects are often very dim due to their great distances from Earth.
   • The light-gathering power of a telescope is proportional to the area of the aperture, i.e., the square of the radius of the lens.
   • Therefore, a lens with a larger aperture will collect more light, ensuring better visibility and brighter images of distant astronomical objects. This reasoning supports option: "a large area of the objective ensures better light gathering power."
2. Resolution:
   • The resolution of a telescope, or its ability to distinguish between two closely spaced objects, is directly related to the size of the aperture. According to the Rayleigh criterion, the resolving power of a telescope improves with an increase in aperture size.
   • This gives a clearer and more detailed view of celestial objects, supporting option: "a large aperture provides a better resolution."
3. Quality and Visibility of Images:
   • A larger aperture not only improves the brightness and resolution but also reduces diffraction effects, leading to sharper images.
   • The enhanced quality and visibility of the images make it easier to observe faint details in astronomical objects, aligning with option: "a large aperture contributes to the quality and visibility of the images."

Thus, the combination of all these factors – improved light-gathering capability, increased resolution, and enhanced image quality – are achieved by using a lens with a large focal length and large aperture, which makes option: "all of the above" the correct answer.