- When light strikes the boundary between two transparent media, it undergoes partial reflection and partial refraction.
- A ray passing from a rarer to a denser medium bends towards the normal, while one passing from a denser to a rarer medium bends away from the normal.
- The angles of incidence and refraction are generally unequal, causing light to bend.
- A ray incident normally (i = 0°) passes undeviated, even though its speed changes.
- Refraction occurs due to a change in the speed of light when it passes from one medium to another.
Definitions [32]
Definition: Light
Light is an electromagnetic wave that travels in straight lines in a homogeneous medium.
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Speed of light in vacuum: c = 3 × 10⁸ m/s.
Definition: Ray Optics
The branch of optics that is based on rectilinear propagation of light and deals with mirrors, lenses, reflection, refraction, etc. is called ray optics.
Definition: Wave Optics
The branch of optics that considers light as a wave which can bend around objects, diffract and interfere, etc. is called wave optics.
Definition: Polarisation
The phenomenon that is based on the fact that light waves are transverse electromagnetic waves is called polarisation.
Definition: Electromagnetic Wave (Maxwell)
Coupled time-varying electric and magnetic fields that propagate in space are called electromagnetic waves.
Definition: Ray Optics
A branch of optics that describes light propagation in terms of rays is called ray optics.
Definition: Convex Mirror
A curved mirror whose reflecting surface is on the outer side of the curved surface is called a convex mirror.
Definition: Spherical Aberration
The aberration that arises due to the spherical shape of the reflecting surface is called spherical aberration.
Definition: Curved Mirror
A mirror with a curved reflecting surface is called a curved mirror.
Definition: Concave Mirror
A curved mirror whose reflecting surface is on the inner side of the curved surface is called a concave mirror.
Definition: Lateral Magnification
The ratio of linear size of an image to that of the object, measured perpendicular to the principal axis, is called lateral magnification.
Definition: Refraction
The change in the direction of the path of light when it passes from one transparent medium to another transparent medium is called refraction. The refraction of light is essentially a surface phenomenon.
or
When light passes from one transparent medium to another, its speed and direction change. This is called refraction.
Definition: Critical Angle
The angle of incidence in the denser medium corresponding to an angle of refraction of 90° in the rarer medium is called the critical angle.
Definition: Total Internal Reflection
The phenomenon where light rays are completely reflected back into a medium instead of being refracted into another medium is called total internal reflection.
or
Complete reflection of a ray of light at the interface of an optically denser medium and a rarer medium, back into the denser medium.
Define critical angle for a given medium.
When a ray of light propagates from a denser medium to a rarer medium, the angle of incidence for which the angle of refraction is 90° is called the critical angle.
Definition: Refracted Light
Refracted light is the part of light enters into the other medium and travels in a straight path but in a direction different from its initial direction and is called the refracted light.
Define the principal focus of a concave mirror.
Light rays that are parallel to the principal axis of a concave mirror converge at a specific point on its principal axis after reflecting from the mirror. This point is known as the principal focus of the concave mirror.
Definition: Refraction of Light
When travelling obliquely from one medium to another, the direction of propagation of light in the second medium changes. This phenomenon is known as refraction of light.
OR
Light changes its direction when going from one transparent medium to another transparent medium. This is called the refraction of light.
OR
The bending of the light ray from its path in passing from one medium to the other medium is called 'refraction' of light.
OR
When a ray of light impinges on a polished, smooth, shiny surface, the rebounding of light within the same medium is called reflection of light.
Definition: Chromatic Aberration
The aberration that occurs due to the lens refracting different wavelengths of light at different angles, resulting in an image consisting of different colours without a single focussed image, is called chromatic aberration.
Definition: Spherical Aberration (Lens)
The aberration caused by the spherical shape of the lens, where light rays at the edges focus at a different point than those near the centre, leading to a blurred image, is called spherical aberration.
Definition: Spectrum
On passing white light through a prism, the band of colours seen on a screen is called the spectrum.
or
The band of the coloured components of a light beam is called its spectrum.
Define the term dispersion of light.
The phenomenon of the splitting of white light by a prism into its constituent colours is known as dispersion of light.
When a beam of white light or composite light is refracted through any transparent media such as glass or water, it is split into its component colours. This phenomenon is called ‘dispersion of light’.
Definition: Dispersion
The phenomenon of splitting of white light by a prism into its constituent colours is known as dispersion.
OR
The splitting of light into its component colours is called dispersion.
OR
The process of separation of light into its component colours while passing through a medium is called the dispersion of light.
OR
The phenomenon in which white light splits into its constituent colours when it passes through a prism or another medium is called dispersion of light.
Definition: Angular Dispersion
The angular separation between the two extreme rays of a dispersed beam of light is called angular dispersion.
Definition: Primary Rainbow
An arc of seven colours formed in the sky with red on the outer edge and violet on the inner edge, caused by single total internal reflection inside water droplets, is called a primary rainbow.
Definition: Mirage
The optical illusion of water or distant objects caused by refraction of light due to temperature differences in air layers is called a mirage.
Definition: Secondary Rainbow
An arc of seven colours with red on the inner edge and violet on the outer edge, caused by double total internal reflection inside water droplets, is called a secondary rainbow.
Define and describe the magnifying power of an optical instrument.
Angular magnification or magnifying power of an optical instrument is defined as the ratio of the visual angle made by the image formed by that optical instrument (β) to the visual angle subtended by the object when kept at the least distance of distinct vision (α).
Definition: Simple Microscope
An optical instrument that uses a single convex lens to magnify small objects is called a simple microscope.
Definition: Compound Microscope
An optical instrument that uses objective and eye piece lenses to magnify tiny objects in detail is called a compound microscope.
Definition: Telescope
An optical instrument that uses objective and eye piece lenses to magnify distant terrestrial or celestial objects is called a telescope.
Define the term ‘resolving power of a telescope’.
The resolving power of an astronomical telescope is defined as the reciprocal of the smallest angular separation between two point objects whose images can just be resolved by the telescope.
R.P = `(1.22 lambda)/D`
Resolving power is the ability of the telescope to distinguish clearly between two points whose angular separation is less than the smallest angle that the observer’s eye can resolve.
Formulae [6]
Formula: Refractive Index
n = \[\frac {\text {sin i}}{\text {sin r}}\] = \[\frac {c}{v}\] = \[\frac {\text {Real depth}}{\text {Apparent depth}}\]
Formula: Apparent Depth (Glass Slab)
d = t - \[\frac {t}{μ}\] = t\[\left(1-\frac{1}{\mu}\right)\]
Formula: Refraction at Single Spherical Surface
\[\frac{n_2-n_1}{R}=\frac{n_2}{v}-\frac{n_1}{u}\]
Formula: Magnifying Power of Simple Microscope
- MMax = 1 + \[\frac {D}{f}\]
- MMin = \[\frac {D}{f}\]
Formula: Magnifying Power of Compound Microscopе
M = mo × Me
Formula: Magnifying Power of Telescope
- \[\mathrm{M_{D.D.V}=\frac{f_{o}}{f_{e}}\left(1+\frac{f_{e}}{D}\right)}\]
- M = \[\frac{\mathrm{f}_{0}}{\mathrm{f}_{0}}\]
Key Points
Key Points: Nature of Light
- Light consists of energy-carrying photons guided by the rules of electromagnetic (EM) waves.
- Commonly observed phenomena of light are broadly classified into three categories: Ray optics, Wave optics, and Particle nature of light.
- Light thus exhibits a dual nature — it behaves both as a wave (wave optics) and as a particle (photon/particle nature), depending on the phenomenon observed.
Key Points: Reflection
- The plane mirror image is always erect, virtual, the same size, the same distance behind the mirror, and laterally inverted.
- A convex mirror always gives a virtual, erect, diminished image; it is a diverging mirror with positive focal length.
- A concave mirror is a converging mirror with a negative focal length; the nature of the image depends on the object position.
- Cartesian Sign Convention: All distances from the pole; along incident light = positive; against = negative; above principal axis = positive; below = negative.
Key Points: Refraction of Light
Key Points: Thin Lenses and Their Combination
Lens Formula:
\[\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\]
Magnification:
\[m=\frac{h_i}{h_o}=\frac{v}{u}=\frac{f}{f+u}=\frac{f-v}{f}\]
Combination of Thin Lenses in Contact:
- Effective focal length: \[\frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}+\frac{1}{f_3}+...\]
- Total power: P = P1 + P2 + P3 + ...
- When one lens is concave and other convex: \[F=\frac{f_1f_2}{f_2-f_1}\]
For Separated Lenses (distance d apart):
\[\frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}-\frac{d}{f_1f_2}\]
Key Points: Dispersion of Light
- Dispersion is the splitting of white light into seven colours (VIBGYOR) when it passes through a prism or similar transparent medium.
- Human eyes can detect light with wavelengths ranging from 400 nm (violet) to 700 nm (red).
- Different colours travel at different speeds in a medium like glass, so each colour has a different refractive index.
- Violet light bends the most, and red light bends the least, as it passes through a prism, producing a spectrum.
- A rainbow is formed due to dispersion, refraction, and internal reflection of sunlight by raindrops acting as tiny prisms.
Key Points: Some Natural Phenomena Due to Sunlight
- A rainbow forms due to refraction, dispersion, and internal reflection inside a single raindrop.
- Primary rainbow → 2 refractions + 1 internal reflection; red outer, violet inner (θR = 43°, θV = 41°).
- Secondary rainbow → 2 refractions + 2 internal reflections; red inner, violet outer (θR = 51°, θV = 54°).
- Mirage is an optical illusion of water on a hot day caused by upward bending of light due to temperature differences in air layers.
Key Points: Optical Instruments
- An optical instrument uses the principles of optics to enhance, modify, or analyse light for specific purposes.
- They manipulate light through reflection, refraction, diffraction, or interference.
- Common instruments: simple microscope, compound microscope, telescope.
Key Points: Telescope
Astronomical (Refracting) Telescope:
- Used to view distant objects.
- Objective lens (large focal length) forms image A'B' at its focus → acts as object for eyepiece.
- Eyepiece forms the final virtual, magnified image A''B''.
Magnifying Power: \[MP=\frac{\text{Visual angle with instrument}(\beta)}{\text{Visual angle for unaided eye}(\alpha)}\]
| Condition | Formula | Tube Length |
|---|---|---|
| Relaxed eye (normal adjustment) | \[m=-\frac{f_a}{f_c}\] | \[L=f_o+f_e\] |
| Distinct vision (D) | \[m=-\frac{f_o}{f_e}\left(1+\frac{f_e}{D}\right)\] | \[L=f_o+f_e\] |
Resolving Power of Telescope:
- Ability to produce distinct images of two closely spaced objects.
- Angular separation between two resolvable objects: \[\sin\theta=1.22\frac{\lambda}{d}\] (Rayleigh's criterion), where d = aperture.
- Resolving power is inverse of angular separation.
- Larger aperture → better resolving power.
Types of Telescope:
- Refracting telescope: Uses two convex lenses; large objective + small eyepiece.
- Reflecting telescope: Uses concave mirror to reflect light internally; secondary mirror directs to eyepiece.
- Keplerian telescope: Converging lens as eyepiece → inverted image.
- Galilean telescope: Diverging lens as eyepiece → erect image.
- Magnification of Refracting Telescope: \[M=\frac{f_{o}}{f_{e}}\]
Concepts [22]
- Fundamental Concepts of Light
- Nature of Light
- Ray Optics Or Geometrical Optics
- Cartesian Sign Convention
- Reflection>Reflection from a Plane Surface
- Reflection>Reflection from Curved Mirrors
- Total Internal Reflection
- Refraction of Light
- Applications of Total Internal Reflection
- Refraction at a Spherical Surface and Lenses
- Thin Lenses and Their Combination
- Refraction at a Single Spherical Surface
- Lens Makers' Equation
- Dispersion of Light
- Analysis of Prism
- Thin Prisms
- Some Natural Phenomena Due to Sunlight
- Defects of Lenses
- Optical Instruments
- Simple Microscope or a Reading Glass
- Compound Microscope
- Telescope
