A student obtained a sharp image of a candle flame placed at the distant end of the laboratory table on a screen using a concave mirror to de determine its focal length. The teacher suggested him to focus a distant building about 1 km far from the laboratory, for getting more correct value of the focal length. In order to focus the distant building on the same screen the student should slightly move the:

Options

A. mirror away from the screen
B. screen away from the mirror
C. screen towards the mirror
D. screen towards the building

Explanation

To solve the problem of focusing a distant building using a concave mirror, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Setup**: The student initially focused a candle flame, which is a relatively close object, on a screen using a concave mirror. The image formed by the concave mirror depends on the distance of the object from the mirror.

2. **Identifying the Object Distance**: When the student focuses on the candle flame, the object distance (distance from the mirror to the candle) is relatively small compared to the distance of the distant building (1 km away).

3. **Using the Mirror Formula**: The mirror formula is given by:
\[ \frac{1}{f} = \frac{1}{u} + \frac{1}{v} \]
where \( f \) is the focal length, \( u \) is the object distance, and \( v \) is the image distance. For distant objects, \( u \) becomes very large, and thus \( \frac{1}{u} \) approaches zero.

4. **Focusing on a Distant Object**: When focusing on a distant building, the object distance \( u \) is significantly larger than when focusing on the candle. As a result, the image distance \( v \) will also change.

5. **Adjusting the Screen Position**: To focus the distant building on the same screen, the student needs to adjust the position of the screen. Since the object distance \( u \) is increasing (moving from a close object to a distant one), the image distance \( v \) will also increase.

6. **Moving the Screen**: Therefore, to keep the image sharp on the screen, the student should slightly move the screen away from the mirror. This adjustment allows the image of the distant building to be formed clearly on the screen.

### Final Answer:
The student should slightly move the screen away from the concave mirror to focus the distant building.

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