For a lens, magnification m = −di/do. If do = 0.30 m and di = 0.15 m, what is m?

For a lens, magnification m = −di/do. If do = 0.30 m and di = 0.15 m, what is m?

• A. -0.25
• B. -0.75
• C. -1.00
• D. -0.50

Answer: (D)

Magnification tells you how big the image is compared to the object, and for a lens it’s given by m = -di/do. With do = 0.30 m and di = 0.15 m, the magnification is m = -0.15/0.30 = -0.5. The negative sign means the image is inverted relative to the object, and the magnitude 0.5 means the image is half as tall as the object. Since di is positive, the image is real and formed on the far side of the lens. So the image is real, inverted, and half the size of the object (m = -0.5).

Magnification tells you how big the image is compared to the object, and for a lens it’s given by m = -di/do. With do = 0.30 m and di = 0.15 m, the magnification is m = -0.15/0.30 = -0.5. The negative sign means the image is inverted relative to the object, and the magnitude 0.5 means the image is half as tall as the object. Since di is positive, the image is real and formed on the far side of the lens. So the image is real, inverted, and half the size of the object (m = -0.5).