An astronomical refractive telescope has an objective of focal length 20 m and an eyepiece of focal length 2 cm. Then

Options

A. the magnification is 1000
B. the length of the telescope tube is 20.02 m
C. the image formed is inverted
D. all of these.

Explanation

To solve the problem regarding the astronomical refractive telescope, we will follow these steps:

### Step 1: Identify the given values
- Focal length of the objective lens (F₀) = 20 m
- Focal length of the eyepiece lens (Fₑ) = 2 cm = 0.02 m (conversion from cm to m)

### Step 2: Calculate the magnification (M)
The magnification of a telescope is given by the formula:
\[ M = \frac{F₀}{Fₑ} \]

Substituting the values:
\[ M = \frac{20 \, \text{m}}{0.02 \, \text{m}} \]
\[ M = 1000 \]

### Step 3: Determine the length of the telescope (L)
The length of the telescope is given by the sum of the focal lengths of the objective and the eyepiece:
\[ L = F₀ + Fₑ \]

Substituting the values:
\[ L = 20 \, \text{m} + 0.02 \, \text{m} \]
\[ L = 20.02 \, \text{m} \]

### Step 4: Determine the nature of the image formed
In an astronomical telescope, the image formed is always inverted. Therefore, this statement is correct.

### Conclusion
- Magnification (M) = 1000 (Option A is correct)
- Length of the telescope (L) = 20.02 m (Option B is correct)
- The image formed is inverted (Option C is correct)

Thus, all statements are true, leading us to conclude that the answer is:
**Option D: All of these.**

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