Summary of Sign Conventions Mirrors Lenses The focal length f is positive for converging lenses and negative for diverging lenses. When the object, image, or focal point is on the reflecting side of the mirror, the distance is positive. The object distance s is positive if the object is on the side of the lens from which the light is coming; otherwise s is negative (and the object is virtual). When the object, image, or focal point is “behind” the mirror, the distance is negative. The image distance s’ and radius of curvature R are positive if the image is on the side of the lens into which the light is going; otherwise negative. The image height is positive if the image is upright, and negative if the image is inverted relative to the object. The image height is positive if the image is upright, and negative if the image is inverted relative to the object.

Summary of Sign Conventions Here’s a more compact way of expressing the sign conventions all at once. Object Distance. When the object is on the same side as the incoming light, the object distance is positive (otherwise is negative). Image Distance. When the image is on the same side as the outgoing light, the image distance is positive (otherwise is negative). Radius of Curvature. When the center of curvature C is on the same side as the outgoing light, R is positive (otherwise is negative).

“Image Distance. When the image is on the same side as the outgoing light, the image distance is positive (otherwise is negative).” If the image distance is negative, can the image be real?