Total Internal Reflection

Total internal reflection

Total internal reflection, in physics, complete reflection of a ray of light within a medium such as water or glass from the surrounding surfaces back into the medium. The phenomenon occurs if the angle of incidence is greater than a certain limiting angle, called the critical angle. In general, total internal reflection takes place at the boundary between two transparent media when a ray of light in a medium of higher index of refraction approaches the other medium at an angle of incidence greater than the critical angle. For a water-air surface the critical angle is 48.5°. Because indices of refraction depend on wavelength, the critical angle (and hence the angle of total internal reflection) will vary slightly with wavelength and, therefore, with colour. At all angles less than the critical angle, both refraction and reflection occur in varying proportions.

Uses of total internal reflection in daily life

  • Total internal reflection is the operating principle of optical fibers, which are used in endoscopes and telecommunications.
  • Total internal reflection is the operating principle of automotive rain sensors, which control automatic windscreen/windshield wipers.
  • Another application of total internal reflection is the spatial filtering of light.
  • Prisms in binoculars use total internal reflection, rather than reflective coatings, to fold optical paths and show erect images.
  • Some multi-touch screens use frustrated total internal reflection in combination with a camera and appropriate software to pick up multiple targets.
  • Gonioscopy employs total internal reflection to view the anatomical angle formed between the eye’s cornea and iris.
  • A gait analysis instrument, CatWalk XT, uses frustrated total internal reflection in combination with a high speed camera to capture and analyze footprints of laboratory rodents.
  • Optical fingerprinting devices use frustrated total internal reflection in order to record an image of a person’s fingerprint without the use of ink.
  • A total internal reflection fluorescence microscope uses the evanescent wave produced by TIR to excite fluorophores close to a surface. This is useful for the study of surface properties of biological samples.
  • Total internal reflection is the operating principle of LED Light Panels. This technology utilizes LGPs (Light Guide Plates) as the vehicle for transmitting light over large areas. By etching a grid pattern on the second surface of the LGP, frustrated total internal reflection occurs allowing the light to escape the LGP as visible illumination.


Lens: converging and diverging lens

Lens, in optics, piece of glass or other transparent substance that is used to form an image of an object by focusing rays of light from the object. A lens is a piece of transparent material, usually circular in shape, with two polished surfaces, either or both of which is curved and may be either convex (bulging) or concave (depressed). The curves are almost always spherical; i.e., the radius of curvature is constant. A lens has the valuable property of forming images of objects situated in front of it. Single lenses are used in eyeglasses, contact lenses, pocket magnifiers, projection condensers, signal lights, viewfinders, and on simple box cameras. More often a number of lenses made of different materials are combined together as a compound lens in a tube to permit the correction of aberrations. Compound lenses are used in such instruments as cameras, microscopes, and telescopes.

Converging Lens

A biconvex lens or a plano convex lens converges parallel light rays that enter on one side to a point on the axis on the other side of the lens. Hence, a biconvex or a plano convex lens is called a converging lens. The point where the rays actually converge is called the focus of the lens and the distance of the focus from the lens is called focal length of the lens.

Diverging lens

When a set of parallel rays enter a biconcave lens or a plano concave lens, it diverges the light rays on the other side of the lens. Hence, a biconcave or a plano concave lens is called a diverging lens. However these divergent rays give an impression that they emerge from a point on the axis at the same side of entry of the rays. It is a virtual point and not real. This point is called the focus and its distance from the lens is called focal length of the concave lens.


Focal length

Focal length, usually represented in millimeters (mm), is the basic description of a photographic lens. It is not a measurement of the actual length of a lens, but a calculation of an optical distance from the point where light rays converge to form a sharp image of an object to the digital sensor or 35mm film at the focal plane in the camera. The focal length of a lens is determined when the lens is focused at infinity.


Optical centre image formation by lens

When an object is placed between the principal focus and optical centre of a convex lens, then a parallel ray of light AO passes through the focus after refraction along the direction OX. while the other ray AC pass through the optical centre and goes straight without any deviation along the direction CY. But, in this case the two refracted light rays i.e. OX and CY are diverging away from one another, so these cannot intersect each other to form a real image on the right side of the convex lens. Thus, the refracted rays OX and CY are extended backward by dotted lines. On extending back, these rays appear to intersect at point A’. Hence, the image A’B’ formed in this case is a virtual image which is formed on the same side of the lens behind the object. Also the image formed is erect and highly enlarged.

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