How to Measure the Focal Length of a Mirror at Home
When light rays fall parallel on a mirror, they converge to a point called the focal point. Focal length measures the distance between center of the mirror to the focal point. The type of mirror determines where the image will be formed. Knowing the focal length is useful in learning the characteristics of the mirror. Does this Spark an idea?
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Instructions
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Convex and Concave Mirror
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1
Place the mirror in a darkened room. This helps in seeing the reflected image on the paper.
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Place the light source in front of the mirror and slightly to the side. Switch it on.
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Adjust the position of the paper until you get a clear sharp focused reflected image. For the convex mirror, the image will be formed behind the mirror. For the concave mirror, the image will be formed in front of the mirror if the object is outside the focal length. The image will be formed behind the mirror if the object is inside the focal length. Move the light source and the paper until a sharp image is seen.
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Measure the distance of the light source to the mirror. Let this distance be o. Measure the distance of the paper to the mirror. Let this distance be i. If the image is formed behind the mirror, the distance has to be negative.
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Substitute the values of o and i in the formula: 1/F = 1/o + 1/i. Take care to enter the correct sign of the distance values. Solve for F. F is the focal length of the mirror.
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