Understanding a Concave Lens
A concave lens is a type of lens that is thinner in the middle and thicker at its edges. Unlike a convex lens, it causes parallel rays of light to diverge, or spread out, after passing through it, making it also known as a diverging lens.
How Light Interacts with a Concave Lens
When light rays parallel to the principal axis strike a concave lens, they bend outwards. If these diverged rays are traced backward, they appear to originate from a single point on the principal axis on the same side as the object. This point is known as the principal focus or focal point.
Practical Example: Correcting Nearsightedness
A common application of concave lenses is in eyeglasses to correct nearsightedness (myopia). For someone with myopia, light focuses in front of the retina. A concave lens diverges the light rays before they enter the eye, pushing the focal point back onto the retina, thus allowing for clear distant vision.
Applications Beyond Eyeglasses
Besides vision correction, concave lenses are used in various optical instruments. They can be found in telescopes (as part of the eyepiece or as a primary element in Galilean telescopes), binoculars, flashlights (to spread the beam), and some laser systems to expand the beam.