A lens is a transparent object with curved surfaces that refracts light. A convex lens (also called a converging lens) is thicker in the middle and converges (brings together) parallel rays of light at a focal point. A concave lens (also called a diverging lens) is thinner in the middle and diverges (spreads out) parallel rays of light.
Convex lens: thicker in middle, converges light, positive focal length and power.
Concave lens: thinner in middle, diverges light, negative focal length and power.
Convex lens = converging lens; Concave lens = diverging lens.
Power = 1/focal length (in metres). Unit: Dioptre (D).
Concave lens always forms virtual, erect, diminished images.
Convex lens corrects hypermetropia (long-sight); Concave corrects myopia (short-sight).
Magnifying glass = convex lens (virtual, erect, magnified image when object is between F and lens).
Spectacles with negative power = concave lens (for myopia).
Feature | Convex Lens | Concave Lens Shape | Thicker in middle, thinner at edges | Thinner in middle, thicker at edges Also called | Converging lens | Diverging lens Effect on light | Converges (brings together) | Diverges (spreads apart) Focal length (f) | Positive (+) | Negative (-) Power (P = 1/f) | Positive | Negative Real/virtual image | Can form both | Forms only virtual images Image type | Can be real or virtual | Always virtual, erect, diminished Focal point | Actual meeting point of rays | Virtual (rays appear to diverge from it) Uses | Magnifying glass, camera, projector, human eye | Spectacles for myopia (short sight) Dioptre sign | + (plus) | − (minus) Example | Reading glass, camera lens | Spectacles for -3 power prescription
Convex lens (converging lens): • Thicker at centre, thinner at edges • Surfaces: both curved outward (biconvex) OR one curved, one flat (plano-convex) • Converges parallel rays to a real focal point (F₂)
Ray diagrams — image formation:
Power of convex lens: • P = 1/f (f in metres) • Convex lens: f is positive → P is positive (e.g., +2D means focal length = 0.5 m)
Uses of convex lens: • Magnifying glass • Camera lens • Microscope (eyepiece and objective lenses) • Telescope • Projector • Correction of hypermetropia (long-sightedness — +ve lens)
Concave lens (diverging lens): • Thinner at centre, thicker at edges • Surfaces: both curved inward (biconcave) OR one curved inward, one flat (plano-concave) • Diverges parallel rays — appears to originate from a virtual focal point (F₁)
Image formation: • A concave lens ALWAYS forms a virtual, erect, diminished image • Image is always on the same side as the object • As object moves farther, image remains virtual but approaches the virtual focus
Power of concave lens: • P = 1/f (f in metres) • Concave lens: f is negative → P is negative (e.g., −3D means focal length = −0.33 m)
Uses of concave lens: • Spectacles for myopia (short-sightedness — −ve lens) • Flashlight reflectors • Used in Galilean telescopes • Peephole (door viewer)
Spectacles example: • Person has myopia (near-sightedness): image forms in front of retina • Correction: concave lens diverges light BEFORE entering eye, so image now falls on retina • Prescription: −1.5D means a concave lens of focal length −0.67 m
Convex lens: thicker at centre, converges parallel light rays to a real focus, positive focal length, positive power. Used in magnifying glasses and camera lenses. Concave lens: thinner at centre, diverges light rays to a virtual focus, negative focal length, negative power. Used in spectacles for short-sightedness (myopia).
A concave lens always forms a virtual, erect, and diminished image on the same side as the object. It never forms a real image. The image appears smaller than the object and cannot be projected on a screen.
A concave lens is used to correct myopia (short-sightedness or near-sightedness). In myopia, the image forms in front of the retina. A concave lens diverges light before it enters the eye, moving the image back to fall exactly on the retina. Concave lenses have negative power (−D).
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