### Correct Option: D. Final image is at infinity ### Detailed Explanation: An astronomical telescope is designed to magnify distant objects, such as stars and planets, allowing us to observe them more clearly. The telescope consists of two main lenses: the objective lens and the eyepiece (or ocular lens). 1. **Understanding Normal Adjustment**: - A telescope is said to be in "normal adjustment" when it is set up in such a way that the final image produced by the telescope is at infinity. This means that the light rays coming from the object being observed are parallel when they enter the eyepiece. 2. **Why the Final Image is at Infinity**: - When the final image is at infinity, the eye can comfortably focus on it without straining. This is because the eye is naturally adapted to focus on distant objects, which is crucial for long-term viewing, such as stargazing. - In this configuration, the objective lens collects light from a distant object and brings it to a focus at its focal point. The eyepiece then takes this focused light and allows the observer to view it as if it were coming from a much larger object at infinity. 3. **The Role of the Lenses**: - The **objective lens** has a longer focal length and is responsible for gathering light and forming a real image of the distant object. - The **eyepiece** has a shorter focal length and magnifies this image for the observer. 4. **Accommodation of the Eye**: - When the final image is at infinity, the eye does not need to accommodate (change its shape) to focus on the image. This is important for comfort during prolonged observations. ### Why Other Options are Incorrect: - **Option A: Eye is accommodated**: - This option is incorrect because, in normal adjustment, the eye does not need to accommodate. If the final image is at infinity, the eye is relaxed and does not need to change its shape to focus. - **Option B: Focal length of objective lens is longer than that of eyepiece**: - While it is true that the focal length of the objective lens is typically longer than that of the eyepiece, this statement does not define the condition of normal adjustment. The relationship between the focal lengths does not directly relate to the position of the final image. - **Option C: Final image is at the near point of eye**: - This option is incorrect because the near point of the eye (usually around 25 cm for a normal human eye) is not the condition for normal adjustment. If the final image were at the near point, the eye would have to accommodate, which contradicts the definition of normal adjustment. ### Summary of Key Points: - An astronomical telescope is in normal adjustment when the final image is at infinity. - This allows the eye to view the image without straining, as it can focus on distant objects naturally. - The objective lens forms a real image, which the eyepiece magnifies for the observer. - The other options either misinterpret the conditions of normal adjustment or describe scenarios that require eye accommodation. By understanding these principles, you can better grasp how telescopes function and the importance of image positioning in optical instruments.