What is Ray Optics?

Optics is the study of light in the field of physics. It refers to the study and properties of light. Optical phenomena can be classified into three categories: ray optics, wave optics, and quantum optics. Geometrical optics, also known as ray optics, is an optics model that explains light propagation using rays. In an optical device, a ray is a direction along which light energy is transmitted from one point to another. Geometric optics assumes that waves (rays) move in straight lines before they reach a surface. When a ray collides with a surface, it can bounce back (reflect) or bend (refract), but it continues in a straight line. The laws of reflection and refraction are the fundamental laws of geometrical optics. Light is an electromagnetic wave with a wavelength that falls within the visible spectrum.

Light Ray

A light ray is a straight or curved line that is perpendicular to the wavefronts of light and has a tangent that is collinear with the wave vector. It travels in a straight path They bend at the interface of two dissimilar media and can also be bent in a medium with a changing refractive index. The propagation of rays through an optical device is defined by geometric optics. The imaged objects are considered as a series of independent point sources, each of which produces spherical wavefronts and outward rays. To find the corresponding point on the image, rays from each object point can be mathematically propagated.

The path taken by a ray of light between two points is the path that can be traversed in the least amount of time, according to Fermat's principle.


Reflection of light is the effect of light reflecting from a material's surface. The angle of reflection is the angle formed by the surface normal and the reflected light.

“The image that shows Incident and Reflected rays”

Reflection from Spherical Mirror

A spherical mirror is a reflective surface on one of the two surfaces of a hollow glass sphere. There are mainly two types of spherical mirrors

1.Concave Mirror-It's a polished inner surface on the inside of a hollow glass sphere. The concave mirror's reflective surface is on the hollow side.

2.Convex Mirror-It's a polished surface on the outside of a hollow glass sphere. The convex mirror's reflective surface is on the sphere's outer edge.

Terms related to spherical Mirror

  • Pole: The geometrical centre of the mirror's surface is known as its pole. In the diagram, it's labeled as point (p).
  • Centre of Curvature: The mirror's centre of curvature is the centre of the hollow sphere of which it is a part. In the diagram, it's labeled as point (C).
  • Radius of the Sphere: The radius of the sphere of which the mirror is a part is called the radius of curvature.
  • Principal Axis: The line connecting the pole and the mirror's centre of curvature is referred to as the principal axis.
  • Aperture of a mirror: The aperture of a spherical mirror is the diameter of the circular cross-section of the sphere used to make it.
  • Focal length of spherical mirrors

when a parallel light beam strikes (a) a concave mirror and (b) a convex mirror The rays are said to be paraxial, meaning that they are incident at points near to the mirror's pole P and create small angles with the principal axis. On the principal axis of a concave mirror, the reflected rays converge at point F.

Reflected rays from a convex mirror tend to diverge from a point F on its principal axis. The mirror's principal emphasis is designated by the letter F. The reflected rays would converge (or appear to diverge) from a point in a plane through F normal to the principal axis if the parallel paraxial beam of light was incident at an angle with the principal axis. This is referred to as the focal point.

“The image that shows focal length of spherical mirrors”
  • Focal Plane:

The focal plane of a given spherical mirror is the vertical plane that passes through the focus point and is perpendicular to the principal axis.

“The image that shows focal planes of spherical mirror”

Real Image and  Virtual Image

Real image: Whether the rays of the light cross or meet, after reflection, the image created is called a real image. It can be obtained on a computer screen, and a real image is always inverted with respect to the object when it is formed. For example, when an object is placed at infinity, the image created by a concave mirror is a real image.

“The image that shows formation of real image”

Virtual image: The image created is called a virtual image if the rays of light after reflection do not necessarily converge but tend to diverge from a point. A virtual image cannot be created on a screen; once created, a virtual image is always erect in relation to the object. A virtual image is, for example, the image created by a convex mirror when the object is very far away, i.e. at infinity.

“The image that shows formation of virtual image”


The deviation from the straight path of light is refraction. The angle of refraction is the angle formed by refracted ray and the normal, and it is determined by Snell's Law.

Snells Laws of Refraction

  • The incident ray, the refracted ray, and the normal to the interface at the point of incidence, all lie in the same plane.
  • The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant. Remember that the angles of incidence (i ) and refraction (r ) are the angles that the incident and its refracted ray make with the normal, respectively. We have sin i/sin r=n21 where n21  is a constant, called the refractive index of the second medium with respect to the first medium.

Total Internal Reflection

At the interface, light is partly reflected into the same medium and partly refracted to the second medium as it passes from an optically denser medium to a rarer medium. This is called total internal reflection. This phenomenon occurs only when incident angle should be higher than critical angle.

“The image that shows total internal reflection”


Aberration is the deviation of light rays across lenses in optical structures such as lenses and curved mirrors, creating distorted images of objects. Aberration can be caused by several factors, including lens size, material, thickness, and object position. Aberrations are of two types monochromatic and chromatic

Monochromatic aberrations are caused by the lens or mirror's geometry and may occur when light is reflected or refracted.

Chromatic Aberrations: Dispersion, or the change of a lens's refractive index with wavelength, causes chromatic aberrations.

Spherical Aberration

Light rays from a point on the optical axis of a spherical lens do not always converge at the same image point in spherical aberration. Rays passing through a circular zone near the rim are focused further away than rays passing through the core. When a plane held perpendicular to the optical axis intersects a cone, a circular cross-section is created.

The cross-sectional area changes as the gap along with the optical axis increases. The circle with the least amount of uncertainty is the smallest size. At this distance, the picture has the least spherical aberration.

“The image that shows Spherical aberration”

Collimated Beam

Since a collimated beam of light or other electromagnetic radiation has parallel rays, it spreads very little as it travels. With no separation, a perfectly collimated light beam does not disperse with time. Diffraction, on the other hand, prevents the formation of any such ray.

Common Mistakes

  • Geometrical optics' key flaw is that it lacks light's wave properties, which are defined in wave optics. This suggests that the effects of diffraction, interference and polarization aren't taken into account.
  • Geometrical optics can't even come close to describing light transmission in single-mode fibers.
  • Since interference effects are significant, light propagation in multilayer coatings cannot be realistically measured using ray optics.

Context & Applications

  • Geometrical optics is a branch of optics that studies how images shape in optical systems.
  • It is used in clinical applications for the optical diagnosis of human body problems.
  • It is used in the treatment and surgery of human tissues.
  • Paraxial approximation
  • Dispersion of light
  • Wave front

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