Lenses In Optics

What Are Lenses?

The most obvious distinction between mirrors and lenses are that mirrors reflect light rays (light bounces back) while light rays are refracted (pass through) through a lens. A mirror will have only one focal point, which is in front of the mirror. A lens will have two focal points, one on either side of the lens. A very interesting difference between a concave mirror and a convex lens is this: A concave mirror converges lights rays to a focal point and a convex lens does the same thing. Similarly, a convex mirror diverges light rays like the concave lens.

Types Of Lenses

Lenses are classified based on the way the light rays entering them are bent. The two main types of lenses are:

  • Convex Lens (Converging)
  • Concave Lens (Diverging)

Convex lenses are thick in the middle and thinner at the edges. A concave lens is thin in the middle and thicker at the edges. A convex lens is also known as a converging lens because it bends the parallel light rays entering them inward and converges them at a point known as focal length. On the other hand, the concave lens is also known as a diverging lens because it bends the parallel light rays outward and diverges them at the focal point.

Lenses are also classified as simple lenses and compound lenses. Simple lenses are differentiated based on their surface of curvature. The following figure shows different types of simple lenses.

Lenses

Compound lenses are those constructed out of a combination of different simple lenses. The lens types used to build a compound lens may have different refractive indices and other properties. They will be placed along a single axis and properties like focal length are recalculated for the new compound lens.

Other Lens Types are:

  • Cylindrical – curvature in one direction
  • Fresnel – narrow ring-like surface
  • Lenticular – a collection of microlenses used in lenticular printing
  • Gradient Index – flat surfaces but with varying refractive indices
  • Axicon – conical surface

Applications Of Lens

What do magnifying lens, your eye and binoculars have in common? They all use convex lenses. And have any of you wondered why people look so weird when you see them through a peephole? That’s because the glass used there isn’t a normal one. It is a concave lens. Convex lenses are used to correct farsighted vision problems. Similarly, concave lenses are used to correct nearsightedness. Compound microscopes and telescopes use both convex and concave lenses. Using combinations of convex lenses may create blurry images. A concave eyepiece is used to correct this problem. Even in cameras, a combination of both concave and convex lenses are sometimes used. The use of concave lenses is very important in lasers. The actual light ray is highly specific which may damage equipment where it is used (like CD’s and scanners). The diverging lens rectifies this issue. Concave lenses are also used in flashlights where the light beam is diverged for larger area coverage.

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Practise This Question

A ray of light falls on the surface of a spherical glass paper weight making an angle α with the normal and is refracted in the medium at an angle β. The angle of deviation of the emergent ray from the direction of the incident ray is