Why are the focal lengths of convex and concave lens , considered positive and negative respectively ?
Why are the focal lengths of convex and concave lens , considered positive and negative respectively ?
Let us understand,
Positive or negative is determined by the characteristics of the image produced by lenses.
Now, if the image is formed at a point due to the actual convergence of rays at that point, the corresponding point is positive. The rays may also diverge from that point. If the rays actually diverge from that point it is positive.
If the image APPEARS to be formed at a point due to the ray appear to diverge from a point or a ray APPEARS to converge to a point, the corresponding point is negative. Thus negative sign indicates that there are no actual rays converging or diverging, but it only appears to do so. Focus of a convex lens is the point where the rays actually meet. Hence it is positive. Focus of a concave lens is the point from where the rays appear to diverge, where no rays actually meet. Hence it is negative. Focus of a concave mirror is the point where there rays actually meet. Hence it is positive. Focus of a convex mirror is the point from where the rays appear to diverge, where no rays actually meet. Hence it is negative.
Let us understand,
Positive or negative is determined by the characteristics of the image produced by lenses.
Now, if the image is formed at a point due to the actual convergence of rays at that point, the corresponding point is positive. The rays may also diverge from that point. If the rays actually diverge from that point it is positive.
If the image APPEARS to be formed at a point due to the ray appear to diverge from a point or a ray APPEARS to converge to a point, the corresponding point is negative. Thus negative sign indicates that there are no actual rays converging or diverging, but it only appears to do so. Focus of a convex lens is the point where the rays actually meet. Hence it is positive. Focus of a concave lens is the point from where the rays appear to diverge, where no rays actually meet. Hence it is negative. Focus of a concave mirror is the point where there rays actually meet. Hence it is positive. Focus of a convex mirror is the point from where the rays appear to diverge, where no rays actually meet. Hence it is negative.
