A magnifying glass, a camera and the spectacles of a friend all use lenses. Here we learn how convex and concave lenses form images, how to draw and calculate them, and how the power of a lens is measured in dioptres.
4 lessons, about 1 h 50 min4 simulations14 problems8 practice questionsPython in every lesson
Created by StemPy LabA lens is a piece of transparent material with two curved surfaces that bends light by refraction. A convex lens is thick in the middle and brings a parallel beam together at its focus. A concave lens is thin in the middle and spreads a parallel beam apart.
A convex lens gives a real, inverted image when the object is beyond the focus. With the object between the focus and the lens, it gives a virtual, upright, larger image: a magnifying glass. A concave lens always gives a virtual, upright and smaller image between the lens and the focus.
With the New Cartesian sign convention, the lens formula is , and the magnification is . A convex lens has a positive focal length and a concave lens has a negative one. A positive magnification means an upright image, and a negative one means an inverted image.
The power of a lens is the reciprocal of its focal length in metres, , and it is measured in dioptres. A convex lens has a positive power and a concave lens has a negative power. When lenses are placed in contact, their powers add.
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