Physics · Class 11 Science

Ch 9Optics — Class 11 Physics, concept-first.

Optics is the branch of physics dealing with light, in exact parallel to acoustics, the branch dealing with sound (already met in the earlier chapter on oscillations and waves).

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Nature of Light and Its Dual Character

Light was historically defined only by its effect -- 'that form of radiant energy which makes objects visible by stimulating the retina' -- rather than by what it actually IS.

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Chapter contents

The NCERT structure, section by section. Open a section to see its questions, then read the concept-first solution.

9.1

Introduction

Optics is the branch of physics dealing with light, in exact parallel to acoustics, the branch dealing with sound (already met in the earlier chapter on oscillations and waves).

9.2

For a long time, light was understood simply as 'that form of radiant energy which makes objects visible by stimulating the retina of the eye' -- a definition based purely on its EFFECT, not its actua…

9.3

Ray optics or geometrical optics

Geometrical (ray) optics is mainly concerned with image formation by mirrors, lenses and prisms, and the whole subject rests on four fundamental postulates, already met in earlier classes.

9.3.1

Cartesian sign convention

A sign convention is essential throughout geometrical optics, because the numerical relation between object distance , image distance and focal length for a mirror or a lens would otherwise look compl…

9.4

Reflection

Reflection is the bouncing back of light at a surface it cannot cross, obeying the two laws of reflection already stated in Section 9.3.

9.4.1

Reflection from a plane surface

For a single PLANE reflecting surface, an object placed in front of it always forms a virtual, laterally-inverted image of exactly the same size as the object, at exactly the same perpendicular distan…

9.4.2

Reflection from curved mirrors

Curved (spherical) mirrors -- concave (polished from the inside) or convex (polished from the outside) -- are needed whenever a parallel or divergent beam of light must be brought to a focus, since a…

9.5

Refraction

Refraction is the change a light ray's properties (speed, wavelength, direction of travel) undergo on crossing from one medium into another of different refractive index .

9.6

Total internal reflection

When light travels from a denser medium into a rarer one, the angle of refraction is always larger than the angle of incidence.

9.6.1

Applications of total internal reflection

Total internal reflection (TIR) has several important everyday and technological applications, every one of them relying on light striking an interface at an angle beyond the critical angle.

9.7

Refraction at a spherical surface and lenses

Section 9.5 derived the real-depth/apparent-depth relation only for a FLAT (plane) refracting interface.

9.8

Dispersion of light and prisms

The colour we perceive in light is set by its frequency, and a material's refractive index also depends on that frequency (increasing with frequency -- so violet light refracts more than red).

9.9

Some natural phenomena due to Sunlight

Two everyday optical illusions caused by sunlight interacting with air and water are explained here using tools already developed in this chapter.

9.10

Defects of lenses (aberrations of optical images)

Just as spherical mirrors of finite aperture suffer aberrations, so do lenses of finite aperture and finite thickness -- but lenses additionally suffer a defect mirrors never do, precisely because len…

9.11

Optical instruments

Whether an object 'looks big' has nothing to do with its actual size alone -- a huge, distant mountain can appear smaller than a nearby small tree, because APPARENT size depends on the VISUAL ANGLE th…