Choose the correct alternative (a), (b), (c) or (d) for each of the questions given below :
In an Astronomical Telescope of Refracting type :

Options

A. Objective should have small focal length
B. Objective should have large focal length
C. Eyepiece should have large focal length
D. Both objective and eyepiece should have large focal length

Answer

To solve the question regarding the characteristics of an astronomical telescope of the refracting type, we need to analyze the options provided and understand the fundamental principles behind the design of such telescopes.

### Step-by-Step Solution:

1. **Understanding the Components of an Astronomical Telescope**:
- An astronomical telescope consists of two main lenses: the objective lens and the eyepiece (or ocular lens).
- The objective lens is responsible for gathering light from distant objects and forming a real image.
- The eyepiece lens magnifies this image for viewing.

2. **Focal Length of the Objective Lens**:
- The objective lens should have a **large focal length**. This is because a longer focal length allows the telescope to collect light from distant celestial objects more effectively, resulting in clearer and brighter images.
- A larger focal length also helps in reducing the aberrations and providing a better field of view.

3. **Focal Length of the Eyepiece**:
- The eyepiece lens typically has a **shorter focal length** compared to the objective lens. This is because a shorter focal length eyepiece provides higher magnification of the image formed by the objective lens.

4. **Evaluating the Options**:
- **Option (a)**: Objective should have small focal length - Incorrect.
- **Option (b)**: Objective should have large focal length - Correct.
- **Option (c)**: Eyepiece should have larger focal length - Incorrect.
- **Option (d)**: Both objective and eyepiece should have larger focal length - Incorrect.

5. **Conclusion**:
- The correct answer is **Option (b)**: The objective should have a large focal length.