What are the signs of focal length in case of (a) convex mirror and (b) concave mirror? Explain Why?
Answers
Answer:
the focal length, is positive for a concave mirror, and negative for a convex mirror. When the image distance is positive, the image is on the same side of the mirror as the object, and it is real and inverted. When the image distance is negative, the image is behind the mirror, so the image is virtual and upright.
According to the Cartesian sign convention, the focal length should be positive since the convex mirror forms a virtual image to the right of mirror (positive direction) when light comes from left.
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your answer ==】
focal length is positive for a concave mirror and negative for a convex mirror.
Explanation=】
As per the new Cartesian sign convention, distances from the mirror to the side of the object are negative and distances to the opposite side are positive.
In a concave mirror, the light rays from the object
converge after reflection. Light rays from infinity,
parallel to the principal axis, thus meet at the a point on the principal axis on the side of the mirror which is towards the object. This distance is considered as negative since it is on the side of the object.
Hence the focal length of a concave mirror is negative.
In a convex mirror, the light rays from the object diverge after reflection. Light rays from infinity, parallel to the principal axis, thus move away from each other and do not meet at a point. However, if we extend the reflected rays backwards, it would appear that all the reflected rays are originating from a point on the principal axis on the side of the mirror opposite to that of the object. This distance is considered as positive since it is on the side opposite to that of the object.
Hence the focal length of a convex mirror is +ve.