Fill in the following blanks with suitable words :
An image formed by a concave _____________ can never be obtained on a screen.

Answer

To fill in the blank in the statement "An image formed by a concave _____________ can never be obtained on a screen," we need to analyze the types of concave optical devices.

### Step-by-Step Solution:

1. **Identify the Types of Concave Devices**:
- There are two main types of concave devices: concave lenses and concave mirrors.

2. **Understand Image Formation**:
- A **concave lens** always forms a virtual image, which means the image cannot be projected onto a screen.
- A **concave mirror** can form both real and virtual images depending on the position of the object. However, when the object is placed between the focal point and the mirror, it forms a virtual image.

3. **Determine the Correct Answer**:
- Since the question specifically asks for an image that "can never be obtained on a screen," the most appropriate answer is **"lens"** because a concave lens always produces a virtual image.

4. **Fill in the Blank**:
- Thus, the complete statement is: "An image formed by a concave **lens** can never be obtained on a screen."

### Final Answer:
An image formed by a concave **lens** can never be obtained on a screen.

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