The concave mirrors are used in

Options

A. reflecting telescopes only
B. magic-lanterns only
C. cinema projectors only
D. all the above

Answer

**Step-by-Step Solution:**

1. **Understanding Concave Mirrors**: Concave mirrors are curved mirrors that can converge light rays to a focal point. This property makes them useful in various optical devices.

2. **Application in Reflecting Telescopes**: One of the primary uses of concave mirrors is in reflecting telescopes. In these telescopes, a concave mirror gathers light from distant stars and focuses it to form an image. The structure typically includes a tube where light enters, and the concave mirror is positioned at the bottom to reflect the light towards a smaller secondary mirror, which then directs the light to the eyepiece for viewing.

3. **Use in Magic Lanterns**: Concave mirrors are also used in magic lanterns, which are early forms of projectors. In these devices, a concave mirror helps to project images formed by lanterns through a hole in the box. The mirror reflects and focuses the light to create a clear image on a surface.

4. **In Cinema Projectors**: Cinema projectors utilize concave mirrors to project films onto large screens. The concave mirror helps to focus the light from the film onto the screen, allowing for a bright and clear image.

5. **Conclusion**: Therefore, concave mirrors are used in various applications such as reflecting telescopes, magic lanterns, and cinema projectors. The correct answer to the question is that concave mirrors are used in all of the above-mentioned devices.

**Final Answer**: Concave mirrors are used in reflecting telescopes, magic lanterns, and cinema projectors.

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