The power for a convex lens is positive and the power for a concave lens is negative. The third prototypical case is the peep-hole lens, a diverging lens used to make a virtual image that is smaller than the object. do is the distance of the object from the lens, and it is positive if the object is in front of the lens. Using the Gaussian form of the lens equation, a negative sign is used on the linear magnification equation as a reminder that all real images are inverted. Magnification is defined as the height of image / height of the object so its is negative for real images. di is the distance of the image, and it is positive if the image is behind the lens. Either form can be used with positive or negative lenses and predicts the formation of … Where f is the focal length of the lens used. Linear Magnification For Lens. Lens Equation Problems and Solutions. Thin Lens Equation. The power of the concave lens is negative, while the power of the convex lens can be positive. A common Gaussian form of the lens equation is shown below. A form using the Cartesian sign convention is often used in more advanced texts because of advantages with multiple-lens systems and more complex optical instruments. Stay tuned with BYJU’S to learn more about lens formula, magnification, and power of the lens. Here f is still positive (converging lens), but d i is negative. SI unit of power is Dioptre (D). This is the form used in most introductory textbooks. Hence the magnification M is positive, so the virtual image is not inverted. If the image is virtual, the image distance will be negative, and the magnification will therefore be positive for the erect image. The magnification … The lens formula may be applied to convex lenses as well as concave lenses provided the ‘real is positive’ sign convention is followed.