A virtual image is formed by:

Options

A. a slide projector in a cinema hall
B. the ordinary camera
C. a simple microscope
D. telescope

Explanation

To answer the question "A virtual image is formed by:", we can analyze the information provided in the video transcript step by step.

### Step-by-Step Solution:

1. **Understanding Virtual Images**:
- A virtual image is an image that cannot be projected onto a screen because the light rays do not actually converge at the location of the image. Instead, they appear to diverge from a point behind the mirror or lens.

2. **Examples of Formation**:
- The video mentions various optical devices such as a slide projector, ordinary camera, simple microscope, and telescope.
- It is important to note that a slide projector and an ordinary camera form real images, while a simple microscope and a telescope can form virtual images.

3. **Simple Microscope**:
- In the case of a simple microscope, the object is placed at the focal point of a concave lens. This setup causes the light rays to diverge, making the image appear upright and larger than the object. Hence, the image formed is virtual.

4. **Conclusion**:
- Based on the analysis, we conclude that a virtual image is formed by devices like a simple microscope, where the object is positioned in a way that the light rays diverge, creating an image that appears to be behind the lens.

### Final Answer:
A virtual image is formed by a simple microscope.