Ch 9Ray Optics and Optical Instruments — Class 12 Physics, concept-first.

WBCHSE's Semester IV Unit 6, "Optics", is split into two sub-topics. This first sub-topic, Ray Optics and Optical Instruments, studies light as a collection of straight-line rays -- the geometrical-op…

The laws of reflection, first met for a plane mirror, apply unchanged at every point of a curved (spherical) mirror as well: the incident ray, the reflected ray and the normal to the surface at the po…

To turn the geometry of mirror (and, later, lens) image formation into a single formula that works for every combination of concave/convex mirror and real/virtual object and image, ray optics adopts o…

The relation between the object distance , the image distance and the focal length of a spherical mirror is derived from the geometry of a single ray leaving the tip of an object on the axis, striking…

When a ray of light passes obliquely from one transparent medium into another, it bends at the boundary; this bending is called refraction, and it happens because light travels at a different speed in…

When light travels from an optically denser medium towards a rarer medium (for example, from glass or water towards air), Snell's law shows the refracted ray bends away from the normal, so the angle o…

An optical fibre is a very thin, flexible strand of high-quality glass or plastic, engineered so that light entering one end undergoes total internal reflection over and over again along its length an…

The bending of light at a single curved (spherical) boundary between two transparent media of refractive indices (the medium containing the object) and (the medium the light enters) is worked out, exa…

A lens is a piece of transparent material bounded by two surfaces, at least one of which is curved (spherical); a convex (converging) lens is thicker at its middle than at its edges and, for rays trav…

The Lens-Maker's formula expresses a thin lens's focal length directly in terms of quantities the person actually grinding the lens controls -- the refractive index of the lens material relative to it…

The power of a lens is defined as the reciprocal of its focal length, expressed in metres: Power measures how strongly, and in which sense, a lens converges or diverges a beam of light -- a short-foca…

Many real optical instruments (a camera lens, a microscope objective, a telescope eyepiece) use not one single lens but two or more thin lenses placed close together, in contact, on a common axis, in…

A lens can also be combined with a mirror, most commonly by silvering (coating with a thin reflective layer) one of its two curved faces, so that light entering through the un-silvered face is refract…

The displacement method is a practical laboratory technique for measuring the focal length of a convex lens accurately, without needing to know either the object distance or the image distance separat…

A prism is a transparent optical block bounded by two plane refracting surfaces inclined to each other at an angle , called the refracting (or apex) angle of the prism.

When light passes through a medium containing particles much smaller than its own wavelength -- such as the molecules of the gases making up the earth's atmosphere -- some of the light is absorbed and…

The human eye is, structurally, a natural optical instrument built almost entirely from refracting elements, that forms a real, inverted image of the outside world on a light-sensitive layer at its ba…

Accommodation is the automatic process by which the ciliary muscles of the eye adjust the curvature, and hence the focal length, of the flexible crystalline eye lens, so that objects at very different…

A normal eye can accommodate comfortably over the full range from its near point (about ) out to infinity.

An optical instrument, in the sense used throughout the rest of this chapter, is any device built from one or more lenses (or, for a telescope, possibly a mirror in place of the main lens) whose purpo…

A simple microscope, more commonly called a magnifying glass, is nothing more than a single convex lens of short focal length, used to view a small object placed within its focal length, so that the l…

A compound microscope uses two convex lenses in place of the simple microscope's single lens, to reach far higher magnifying powers than a single short-focal-length lens safely can.

A refracting astronomical telescope, like the compound microscope, uses two convex lenses, but is built to view an object that is enormous in actual size yet so far away that it already subtends only…

A reflecting telescope replaces the refracting telescope's large objective lens with a large concave (parabolic) primary mirror, which collects incoming parallel light from a distant object and brings…

This chapter developed the ray (geometrical) picture of how light forms images, from the simplest single reflecting or refracting surface up to complete multi-element optical instruments.

Sample papers and previous-year board questions for this subject.