# To Measure the Focal Length of a Concave Mirror

When an object is placed in front of a concave mirror (outside the focal point), a real image is formed. The image can be seen on a screen that is moved to the position of the image. In this experiment a slit at the front of a ray box is the object. The ray box and the screen can be moved using sliders. The object distance (u) and image distance (v) is measured each time an image is found on the screen. The focal length (f), which is multiples of 5, can then be calculated using the formula: 1/f = 1/u + 1/v
Note: When you move the ray box inside the focal point you do not get a real image. It is virtual and behind the mirror. Not shown here.

Procedure:
1. Using the left hand slider, drag the ray box towards the mirror.

2. Drag the screen to the point where the reflected rays meet to form the sharpest image.

3. Press "Measure u" and record its value.

4. Press "Measure v" and record its value.

5. Use the formula: 1/u + 1/v = 1/f to calculate f.

6. Repeat steps 1 to 5 until you have at least six sets of readings.

7. Press "New f value" to get another random value and repeat steps 1 to 6.

Precautions:
• Avoid parallax errors (eye directly in front of metre stick) when measuring u and v.
• Measure from the centre of the back of the mirror (the point where reflection occurs) in all cases.

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