An optical device has been given to a student and he determines its focal length by focusing the image of the sun on a screen placed 24 cm from the device on the same side as the sun. Select the correct statement about the device

Options

A. Convex mirror of focal length 12 cm
B. Convex lens of focal length 24 cm
C. Concave mirror of focal length 24 cm
D. Convex lens of focal length 12 cm

Explanation

To determine the correct statement about the optical device used by the student, we need to analyze the information given and apply the principles of optics.

### Step-by-Step Solution:

1. **Understanding the Setup**: The student is focusing the image of the sun on a screen placed 24 cm from the optical device. Since the sun is a very distant object, we can consider it as being at infinity for practical purposes.

2. **Image Formation by Optical Devices**: When an object is at infinity, the image formed by a lens or mirror is located at its focal point. Therefore, if the student is able to focus the image of the sun at 24 cm from the device, this distance represents the focal length of the optical device.

3. **Determining the Focal Length**: The focal length (F) of the optical device is thus determined to be 24 cm.

4. **Identifying the Type of Optical Device**:
- If the device were a **convex lens**, it would focus light on the opposite side of the lens from the object (the sun). This means the image would be formed on the side opposite to where the sun is located.
- If the device were a **concave mirror**, it would also focus light on the same side as the object (the sun).

5. **Conclusion**: Since the student is focusing the image on the same side as the sun, the optical device must be a **concave mirror**. A convex lens would not allow the image to be formed on the same side as the object.

### Final Statement:
The correct statement about the device is that it is a **concave mirror** with a focal length of 24 cm.

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