The correct option is **D. erection of the image**. ### Detailed Explanation: 1. **Understanding Telescopes**: - A telescope is an optical instrument that gathers and magnifies light to observe distant objects. There are two main types of telescopes: astronomical telescopes and terrestrial telescopes. - **Astronomical telescopes** are designed primarily for viewing celestial objects, such as stars and planets. They typically produce an inverted image, which is acceptable for astronomical observations. - **Terrestrial telescopes**, on the other hand, are designed for viewing objects on Earth, such as landscapes or wildlife. For these observations, an upright image is preferred. 2. **Components of the Telescopes**: - Both types of telescopes consist of two main lenses: the objective lens and the eyepiece lens. - The **objective lens** is responsible for gathering light and forming an image of the distant object. - The **eyepiece lens** magnifies this image for the viewer. 3. **The Extra Lens in Terrestrial Telescopes**: - The terrestrial telescope includes an additional lens, often referred to as an **erecting lens** or **erector**. This lens is placed between the objective lens and the eyepiece lens. - The purpose of this extra lens is to invert the inverted image produced by the objective lens, resulting in an upright image for the viewer. This is crucial for terrestrial observations, where an inverted image would be confusing and impractical. 4. **Why Option D is Correct**: - The extra lens in a terrestrial telescope is specifically designed to **erect the image**. This means it takes the inverted image created by the objective lens and flips it back to an upright position, making it easier for the observer to interpret what they are seeing. ### Analysis of Other Options: - **Option A: Improving the sharpness**: - While sharpness is important in any optical system, the extra lens in a terrestrial telescope does not primarily serve to improve sharpness. Sharpness is more related to the quality of the lenses and their alignment rather than the presence of an additional lens. - **Option B: Creating an inverted image**: - This option is incorrect because the purpose of the extra lens is to **not** create an inverted image. In fact, it is designed to correct the inversion caused by the objective lens. - **Option C: Magnification of the image**: - Although magnification is a key function of telescopes, the extra lens in a terrestrial telescope does not primarily serve to increase magnification. The magnification is mainly determined by the focal lengths of the objective and eyepiece lenses. The erecting lens does not contribute to magnification but rather to the orientation of the image. ### Summary of Key Points: - The terrestrial telescope has an extra lens (erecting lens) to produce an upright image for terrestrial viewing. - The extra lens corrects the inversion caused by the objective lens, making it suitable for observing objects on Earth. - Other options (improving sharpness, creating an inverted image, and magnification) do not accurately describe the function of the extra lens in a terrestrial telescope. This understanding is crucial for anyone studying optics and the design of telescopes, as it highlights the specific adaptations made for different observational needs.