Ray Optics or Geometrical Optics and Optical Instruments use a graphic of a ray of light to explain numerous phenomena such as reflection, refraction, and dispersion. Ray Optics or Geometrical Optics argues that light travels in a straight path, and each object has an image. 

Characteristics of Light

  • Light always travels in a Rectilinear Propagation: Light, on the whole, prefers to travel in a straight line. However, light rays cannot continue their propagation if the path has curves or bends. This suggests that light prefers to propagate through the shortest distance.
  • No need for the medium: Light may travel in any medium, including vacuum, where the speed of light is maximum.
  • Speed: There is no other energy source that can travel as quickly as light.

  • Reflection of Light: When the light ray approaches an extremely smooth surface and the ray of light bounces back, it is referred to as the reflection of light. 

Spherical Mirrors

A sphere includes a spherical mirror. When one of the surfaces is silvered, the other becomes the reflective surface. The mirror is called a concave mirror when the convex face is silvered and the reflecting surface is concave. It is called a convex mirror when its concave face is silvered, and the convex face is the reflecting face. A few terms related to spherical mirrors are:

  • Centre of curvature
  • Normal
  • The radius of curvature
  • Pole of the mirror
  • Principal axis

Refraction of Light

Refraction occurs when light is transferred obliquely from one medium to another, causing a perceptible shift in the image’s light path.

Laws of Refraction:

  • The incident ray, the normal, and the refracted ray are all on the same plane.
  • For a given medium, the ratio of sine angle of incidence to sine angle of refraction remains constant.

The ratio of the speed of light in a vacuum to the speed of light in the medium is known as the refractive index.

Snell’s law of refraction states that for the refracting medium to incident medium, a refractive index is the ratio of sine angle of incidence to sine angle of refraction i.e. sin i/sin r= n2/n1. On the other hand, the absolute refractive index refers to the ratio of the speed of light in a medium to the speed of light in a vacuum. n = c/v

An absolute refractive index can be used to identify whether a medium is optically denser or rarer. If medium 1 has a greater absolute refractive index than media 2, medium 1 is said to be optically denser. Medium 1 is optically rarer if its refractive index is lower than medium 2.

Examples: Diamonds, Prisms, Optical Fibre, Mirage, etc.

Refraction can also occur through spherical surfaces like lenses. For a ray of light refracting through the lens, the lens maker’s formula is followed.

Lens maker’s formula for the thin lens will be 

1/f = ((n2-n1)/n1) (1/R1 – 1R2).

For convex lenses, R1 is positive, R2 is negative, whereas for concave lenses, R1 is negative, and R2 is positive. 

Benefits of using a lens combination

  • Increasing the image’s magnification.
  • Increasing the sharpness of an image by reducing the flaws generated by a single image.
  • Giving the constructed image about the thing.
  • Expanding the field of view.

Optical Instruments

In our daily lives, we employ various optical devices created by combining the properties of reflection, refraction, and dispersion.

  • Eye: The intensity of light and colour is sensed by an array of interconnecting nerve fibres and cells in our eyes. The retina, for example, comprises rods and cones that take light, convert it to electrical signals, and transmit them to the brain via the optic nerves. In addition, the ciliary muscles aid in modifying the shape and focal length of the lens in the eye.
  • Lens: Lenses can be described as magnifying glasses which have curved sides. It is a piece of transparent glass that disperses rays of light. Everything passes through them through refraction. 
  • Simple Microscope: A converging lens with a short focal length is used in a basic microscope, commonly known as a magnifying glass. As a result, an erect and virtual image is generated when it is amplified and held close to the eye. On the other hand, a compound microscope has two converging lenses, a moderate focal length, a big aperture eyepiece, and a tiny focal length and fast aperture objective lens.
  • Telescope: This equipment is used to observe objects located at a great distance. On the other hand, a telescope has an objective lens with a large aperture and a long focal length and an eye lens with a tiny aperture and focal length.

Conclusion

Light propagation is described in geometrical optics in terms of rays, which is useful for approximating the routes along which light propagates in specific situations. Certain optical effects, such as diffraction and interference, are not taken into consideration by geometrical optics. The bouncing of incident light waves at the medium’s border is thought of as reflection. According to the law of reflection, the angle of incidence equals the angle of reflection when measured from the conventional.