converge
Summary
In optics, light rays converge when they bend toward one another and meet at a point. Converging elements (convex lenses in air, concave mirrors) have positive focal length f. Testable point: they can form a real image or a virtual one, depending on whether the object is beyond or inside f.
Detail
Meaning: Rays converge when they bend toward each other and cross. A converging lens is thicker at the center and refracts rays parallel to the axis through its focal point on the far side. A concave mirror reflects parallel rays to a focal point in front of the mirror. Both have f > 0. If the object lies beyond f, the image is real and inverted. If the object lies inside f, the image is virtual, upright and enlarged. The lens label assumes the lens has a higher index of refraction than the surrounding medium; a convex air bubble in water diverges light.
Equation: 1/f = 1/o + 1/i, where f is focal length (m), o is object distance (m) and i is image distance (m). Power P = 1/f, in diopters (m⁻¹). Magnification m = −i/o (unitless). For a converging element, f is positive.
On the MCAT: Passages give a lens or mirror and an object distance, then ask for the image type, location or size. Use the ray rule that a ray parallel to the axis passes through the focal point. In multi-lens setups, the image from the first lens is the object for the second. Vision questions pair a converging corrective lens with farsightedness and a diverging lens with nearsightedness.
Common confusion: Converging does not mean the image is real. A converging lens makes a virtual image when o < f, as in a magnifying glass. Check the sign of f: positive means converging, negative means diverging.
Sources
- Physics (Giancoli)
- College Physics (Serway and Vuille)
- Fundamentals of Physics (Halliday, Resnick and Walker)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.