If `f_o` and `f_e` are the focal lengths of the objective lens and eye-piece lens respectively for a telescope, the length of telescope in normal adjustment is

Options

A. `|f_o|-|f_e|`
B. `2|f_e|-|f_o|`
C. `|f_o|+|f_e|`
D. `2sqrt(|f_o||f_e|)`

Answer

To find the length of a telescope in normal adjustment, we need to consider the focal lengths of the objective lens and the eyepiece lens.

### Step-by-Step Solution:

1. **Identify the Focal Lengths**:
- Let \( f_o \) be the focal length of the objective lens.
- Let \( f_e \) be the focal length of the eyepiece lens.

2. **Understanding Normal Adjustment**:
- In normal adjustment of a telescope, the image formed by the objective lens is at the focal point of the eyepiece lens. This allows the eyepiece to view the image as if it is coming from an infinite distance.

3. **Calculate the Length of the Telescope**:
- The length of the telescope in normal adjustment is the sum of the focal lengths of the objective lens and the eyepiece lens. Therefore, the formula for the length \( L \) of the telescope is:
\[ L = f_o + f_e \]

4. **Conclusion**:
- Thus, the length of the telescope in normal adjustment is given by:
\[ L = f_o + f_e \]