Focal Length of a Concave Mirror

To determine the focal length of a given concave mirror: 

Focal Length of Concave Mirror 

1. A concave mirror obeys the laws of reflection of light. 

2. The rays of light coming from a distant object such as the sun or a distant tree can be considered parallel to each other. 

3. The parallel rays meet at a point in front of the mirror and the image formed of the object is real, so, the image can be obtained on a screen. The image formed is also inverted and very small in size.

4. The distance between the principal axis P of the concave mirror and the focus F is the focal length of the concave mirror. It is denoted by the letter ‘ f ’.

1. Select a distant object from the laboratory window (distance should be more than 50 ft). 

2. Fix the concave mirror on the mirror stand placed on the table, facing the distant object. 

3. Place the screen in front of the reflecting surface of the mirror. Move the screen back and forth until a clear, sharp image of the distant object is obtained on the screen. 

4. Measure the distance between the concave mirror and the screen with a metre scale. This distance is the focal length of the given concave mirror. Record the focal length. 

5. Repeat the above procedure twice and record the readings. Take three readings and calculate the average focal length. 

The mean value of the focal length of the concave mirror = (f₁ + f₂ + f₃) cm / 3 = -19.25cm. 

1. The distant object must be well-illuminated to produce a well-illuminated and distinct image.

2. Always place the concave mirror near an open window.

3. The polished surface of the concave mirror must face the distant object.

4. There should be no obstacle or hurdle in the path of rays of light from the distant object, incident on the concave mirror.

5. The base of the stands' of the concave mirrors and screens should be parallel to the measuring scale.

6. The mirror holder along with the mirror should be kept perpendicular to the measuring scale for precise measurements.