In an astronomical telescope in normal adjustment a straight black line of length L is draw on inside part of objective lens. The eye.piece forms a real image of this line. The length of this image is I. The magnification of the telescope is

Options

A. `L/l+1`
B. `L/l-1`
C. `(L+1)/(L-1)`
D. `L/l`

Answer

To solve the problem, we need to determine the magnification of an astronomical telescope in normal adjustment, given that a straight black line of length \( L \) is drawn on the inside part of the objective lens and the length of the real image formed by the eyepiece is \( l \).

### Step-by-Step Solution:

1. **Understand the Setup**:
- We have an astronomical telescope consisting of an objective lens and an eyepiece.
- A black line of length \( L \) is drawn on the objective lens, and the eyepiece forms a real image of this line with length \( l \).

2. **Magnification of the Eyepiece**:
- The magnification \( M_e \) of the eyepiece can be expressed as:
\[ M_e = -\frac{l}{L} \]
- Here, \( l \) is the length of the image and \( L \) is the length of the object (the black line). The negative sign indicates that the image is inverted.

3. **Total Magnification of the Telescope**:
- The total magnification \( M \) of the telescope is given by the product of the magnifications of the objective and the eyepiece:
\[ M = M_o \times M_e \]
- Where \( M_o \) is the magnification of the objective lens.

4. **Magnification of the Objective Lens**:
- For the objective lens, the magnification can be expressed in terms of the focal lengths of the lenses. However, since we are focusing on the image formed by the eyepiece, we can relate the total magnification directly to the lengths:
\[ M = \frac{L}{l} \]

5. **Final Expression**:
- Therefore, the magnification of the telescope in normal adjustment is:
\[ M = \frac{L}{l} \]

### Conclusion:
The magnification of the telescope is given by the ratio of the length of the object (the black line) to the length of the image formed by the eyepiece:
\[ \text{Magnification} = \frac{L}{l} \]