Optics Physics Class-12 Unit-VI All Questions and Answer

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  1. What is Optics?

It is branch of physics which deals with the study of nature, production and propagation of light. It is divided into two main branches: Ray optics and Wave optics wave

(A) Ray Optics: The straight line path of light when it travels from one point to another is termed as a ray of light. The branch of optics based upon the concept of these rays is called Ray Optics. In fact, ray optics is a limiting case of wave optics.

(B) Light: Light is a form of energy which produces sensation of sight.

  1. Some terms connected with Light:

(I) What is Source?

A body which emits light energy in all direction is said to be the source of light. It is two types
(a) Luminous source
(b) Non-luminous source.

(a) What is Luminous source? The source which possesses own heat and light is known as luminous source

For example : Sun, candle, electric bulb etc.

(b) What is Non-luminous source? Sources having no own heat and light are known as non-luminous source. For example: Table, paper, book and moon etc.

(ii) What is Medium? Substances through which light sie art o energy propagates or tends to propagate are called medium. and It is three types.
(a)Transparent: It is the medium through which light energy propagates easily.

For example : Glass, distilled water etc.

(b) Transluscent: It is the medium through which light energy propagates partially

For example : Oil paper, ground glass etc.

Opaque: It is the medium through easle geswhich light energy can not propagate at all e.g. wood, rubber, iron etc.

(iii) Ray: The straight line path of light energy in a homogeneous medium is called ray.

(iv) Beam: It is collection of rays. It is three types: (a) narrow beam (b) wide beam (c) parallel beam.

3 (A) Reflection: It is the phenomenon by which the light energy is sent back into the same medium after incidenting on the wreflecting surface, when light travels from one medium to another medium.

There are two types of reflection.

(a) Regular reflection: When reflection takes place from smooth surfaces. e.g. polished metal surface, polished glass, still surfaces of water etc.

(b) Irregular or Diffuse reflection: When reflection takes place from rough or irregular surface e.g. unpolished metal, grounded glass, surface of water with ripples, blotting paper etc.

(B) Laws of Reflection:

(I) 1st Law :

It states that, “the incident ray, reflected ray and normal to the reflecting surface, all lie in one plane”

(ii) 2nd Law:

It states that, “the angle of incidence is equal to the angle of reflection. Mathematically Zi =Z For regular reflection both the laws are applicable but for iregular or diffused reflection only 2nd law is applicable or obeyed.

(C) Characteristics of reflection by plane mirror :

(i) The image is virtual and erect.
(ii) The size of image is equal to the size of object.
(iii) The image is laterally inverted.
(iv) Image distance is equal to the object distance from the mirror.
(V) When mirror is rotated through an angle 6, reflected ray rotates through an angle 20.
(vi) Angle of deviation for the incident ray is d 180°-2i. Where Zi = angle of incidence. Hence, if i=0, then Zd
= 180°.
(vii) If object moves with velocity “V towards plane mirror, then relative velocity of image with respect to elobject is “2V”.
(Vii) The minimum size of a plane mirror required to form the full image of a person is half the height of the person.
(ix) If two plane mirrors are inclined with each other at an angle “0”, then the no. of image will be formed by
n= 360/∅ot n= 360/∅- 1, which one is odd. If n is fraction, then it will be integral part of n.
(x) If angle of incidence is 0° then angle of reflection is also 0°. The reflected ray will back along same normal.

(D) Spherical mirror :It is a reflecting surface which forms a part of a hollow sphere.

It is two types
(i) Concave or converging mirror.
(ii) Convex or diverging mirror.

(E) New cartesian sign convention for spherical mirror:

(i) Ray diagram should be drawn from ok left to right
(ii) All the measurements should be from pole.
(iii) All the measurements towards left of so pole is taken as negative and towards of right of pole is taken as positive.
(iv) All the measurements above principal axis is taken as positive and below principal axis is taken as negative.

(F) Magnification of spherical mirror?

Magnification is a measure of the size of the image to the size of the object.

It is two types

(i) Linear or Transverse magnification (M): It is ratio between size of image to the size of the object in a direction normal or transverse to principal axis. It is to denoted by M’.

(ii) Longitudinal magnification (M): It is ratio between size of image to the size of object in a direction along the principal axis. It is denoted by M’.

Hence, longitudinal magnification =- (Transverse magnification)


If M> 1, the image is magnified.
If M<1, the image is diminished.
If M=1, size of image is equal to the size of object.
If M is positive, the image is virtual and erect.
If M is negative, the image is real and inverted.

  1. What is Refraction? The phenomenon of the change in go direction of a light when it crosses from one optical medium to another is called refraction of light.
  2. What is Laws of Refraction?
    (i) The incident ray, the refracted ray and the normal to the separating plane at the point to afgus of incidence lie in the same plane.
    (ii) The ratio of the sine of angle of incidence to the sine of the angle of refraction is a constant for a given pair of media and is called the refractive index of the second medium with respect to the first (This law is called Snell’s law).

(A) What is Refractive index? Refractive index of medium 2 with respect to medium 1 is defined as the ratio between velocity of light in medium I to the velocity of light in
medium 2.

Velocity of light in medium 1/Velocity of light in medium 2 = V¹/V²
This is known as “Relative Refractive Index”.

If 1st medium is vacuum and 2nd medium is any other, then

This is known as “Absolute refractive Index”

Refractive index can also be defined in terms of wavelength of light and Snell’s law. It is also known as “optical density'” of the medium.

It has no units and dimensions.

(B) Factors on which the refractive index of a medium depends?

(i) Nature of the medium.
(ii)Wavelength of the light used.
(iii) Temperature
(iv) Nature of the surrounding medium. But it is independent of the angle of incidence.

  1. What is Principle of Reversibility of Light? It states that if light after suffering a number of reflections or refractions, has its final path reversed. It travels along its original path but in opposite direction.
  2. What are Total Internal Reflection? The phenomenon of complete reflection of light back into the same denser medium for angles of incidence greater than the critical angle, is called total internal reflection. Two essential conditions for total internal reflection are:

(i)the light must be travelling from an optically denser to an optically rarer medium.
(ii) the angle of incidence at the surface of separation of the two media must be greater than the critical angle ie. for the given pair of media. TIR finds its application in formation of mirrage, optical fibres, brillance of diamonds, outside view of a fish.

  1. What is Critical Angle? The critical angle for a pair of media in contact is defined as the angle of incidence in the denser medium corresponding to which angle of refraction in the rarer medium is 90°
  2. What is Lenses in optics? A lens is a part of a refracting medium bound by two non-parallel curved surfaces or one curve and another plane surface.

What are the Types of Lenses?

(1) What are Convex or Converging Lens? These lenses are thicker in the middle but thin at the edges. A convex lens always converges the incident beam and may form a real as well as virtual image.

(2) What are Concave or Diverging Lens? These lenses are thicker at the edges and thin in the middle. A concave lens diverges the incident beam and always forms a virtual image only.

  1. What are Terms associated with Lenses?

(1) What is Principal Axes? It is a line passing through the centre of curvature of two surfaces.
(2) What is Optical Centre? The optical centre of lens is a point situated on its principal axis such that a light ray whose refracted path sign as passes through this point has its emergent path parallel to the incident ray.
(3) Principal Focus: It is the point situated the principal axis of a lens, where an ancident beam of light travelling parallel to the principal axis, after refraction from lens, 400actually converges to (in convex lens) or appear to diverge from (in concave lens).

  1. What is Lens Formula? :Arelation correlating distance of object (u), distance of image (v) and the focal length (f) of a lens is called the lens formula.

Assumptions for Lens Formula:
(i) Lens is homogeneous and thin.
(ii) Aperture of the lens is small.
(iii) Object is a small linear object placed normally on the principal axis of the lens.

  1. What is Linear Magnification? The linear magnification produced by a lens is defined as the ratio of linear size (height) of the image to the linear size (height) of the object placed in front of lens. It is also known as transverse magnification.
  2. What is Power of a Lens? The power of a lens is defined as the reciprocal of its focal length expressed in metres, The unit of power is dioptre (D).
  3. What is Dispersion? The phenomenon of the splitting of a polychromatic (white) light beam into its constituent wavelengths (or colours) on passing through a dispersive medium (prism) is called the dispersion of light.
  4. What is Dispersive Power? The dispersive power of any material between any two wavelengths is defined as the ratio of the angular dispersion of these two wavelengths to the mean deviation produced by a prism of small angle made of that material. Dispersive power of a prism depends only on the material of the prism and is independent of the size and the angle of prism.
  5. What is Microscope? A microscope is an optical insturment which magnifies small objects situated in our vicinity. In microscope generally we make use of the following two points.
    (i) If an object is placed between optical centre and the principal focus of a convex lens, its virtual, erect and magnified image is formed on the same side of the lens.
    (ii) If an object is placed slightly beyond the principal focus of a convex lens, its real, inverted and magnified image is formed on the other side of the lens. Lesser the focal length of given convex lens, higher is the magnifying power of given microscope.
  6. What is Telescope? Telescope is an optical instrument which is commonly used to see the distant objects in more details.

(1) Astronomical (Refraction) Telescope This is used to see havenly bodies e.g. planets, stars etc. This type of telescope forms an inverted image of the distant object.

(2)What is Reflecting Telescope? It uses a concave or bass mirror as objective and makes final image well resolved.

  1. What is the Limitations of Snell’s Law? When angle of incidence is zero, angle of refraction will be also zero. At that time ratio between sine of angle of incidence to sine of angle of refraction is undefined, which is limitation of Snell’s law.
  2. DEFINE What are Application of Total Internal Reflection:

(i) Totally reflecting prism (ii) Mirage (iii) Optical fibres (iv) Dazzling of diamonds (v) Shining of air bubble (vi) Cold mirrage or looming (vii) A coin floating in air (viii) Right angled is O scales prism.

  1. What are the Application of Refraction?

(i)Twinkling of stars.
(ii) Sun appears oval during sunrise and sunset but spherical during mid day.
(iii) A stick appears bent in a water tank.
(iv) Apparent shift in the position of the sun at sunrise and sunset.
(v) Water tank appears shallower or an object placed at the bottom appears to be raised.

  1. Power factor: The factor (μ²-μ¹/2) is called power factor of the spherical refracting surface

Where R is radius of curvature of spherical surface. It gives a measure of the degree to which the refracting surface can converge or diverge the rays of light passing through it.

  1. Lens Maker’s formula: The formula relates the focal length of a lens to the refractive index of the lens material and the radii of curvature of it’s two surfaces

This formula is so named as it is used to design the lenses of required focal length from a glass of given refractive index.

  1. What are the Combination of thin Lenses? The purpose of using a lens combination is

(i) To magnity an image.
(ii)To increase the sharphess of final image.
(iii) To erect the final image.
(iv) To increase the field of view.

  1. What is Equivalent Lens? A single lens which forms the image of an object at the same position as is formed by a combination of lenses is called an equivalent lens.
  2. What is Wave Optics? Wave optics is based on the wave theory of light put forward by Huygens and modified by Fresnel. According to wave theory, light is a form of energy which travels through a medium in the form of transverse waves.
  3. What is Wavefront? A wavefront is the continuous locus of all those particles of the medium which are vibrating in the same phase condition at any one instant of time. (The lines normal to wavefront are called rays of light along which the energy travels.) There are three types of wavefront i.e. spherical, plane and cylindrical wavefront.
  4. What is Huygens’ Principle?
    (i) Each and every point on a wavefront acts as the centre of new disturbance called secondary wavelets, which travel in directions with the velocity of light in that medium.
    (ii) The common envelope of all the secondary wavelets taken in the forward direction gives the position of new wavefront at any subsequent time.
  5. What is Snell’s Law of Refraction? It states that the no ratio of the sine of angle of incidence to the sine of refraction is a constant called the refractive index of the medium.
  6. What is Interference? It is the phenomenon of redistribution of energy on account of superposition of two waves of same nature, same frequency and comparable amplitudes and travelling in the given medium in the same direction with same velocity.
    (i) A path difference of half a wavelength or an odd multiple of half wavelengths gives destructive interference and
    (ii) A path difference equal to zero or an even multiple of wavelength gives constructive interference.
  7. What is Fringe Width? The fringe width in an interference pattern is defined as the linear distance between two consecutive bright or dark fringes.

31 What is Coherent Sources?

Two light sources are said to be coherent if they emit light waves of same wavelength and have either zero or a constant phase difference.

32 What are the Conditions of Interference? (sustained)
(i) Two sources should be coherent one.
(ii) Two sources should be monochromatic one.
(iii) Two sources should be very close to each other.
(iv) The frequency, wavelength of two waves should be equal.
(v) The amplitude of two waves should be equal or nearly equal.
(vi) Two waves should travel in same direction with same velocity.
(vii) The two sources of light should be very as narrow.
(viii) The distance between source and screen should be very large.

  1. What are the Conditions of constructive and destructive Interference?

(i) For constructive interference, phase difference between two waves should be even multiple of π, or path difference should be even multiple of π/2.
(ii) For destructive interference phase difference between two waves should be odd multiple of T, or path difference should be odd multiple of π/2.

34 What is Angular fringe width? It is defined as the ratio of fringe width (B) to distance D of screen from
the source.

35 What is Refractive Index? Refractive index of second medium w.r.t.first medium is defined as the ratio of velocity of light in first medium to velocity of light in second medium

  1. What is Absolute Refractive Index? The refractive index of a medium w.r.t. vacuum is called absolute refractive index of the medium.
  2. What is Optical fibre? It consists of thousands of fine v strands of quality glass, coated with a material of lower refractive index. Light entering the fibres at one end undergoes several total internal reflections and finally emerges out without any appreciable change in intensity.

A bundle of optical fibre is called a light pipe, used in medical and optical examination and in receiving and transmitting signals in telecommunication.

  1. What is Angular Dispersion? The angular separation between the two extreme colours (violet and red) in the spectrum is is called angular dispersion.
  2. Why the colour of the sky is blue? : According to Rayleigh’s law of scattering, the intensity of light of wavelength present in the scattered light is inversely proportional to the fourth power of wavelength. So, blue colour of sun light is scattered more by atmospheric molecules, due to which the sky appears blue.
  3. What is Reflecting Telescope? It uses a concave paraboloical mirror of large aperture to view the distant objects. Both spherical and chromatic aberrations are minimum.

When the final image is formed at the least distant of district vision
f⁰. fé
m= m= (1+ —)
fē. D

When the final image is formed at infinity,
f⁰. R/2
m= –= —-
fé. fé

  1. Define the Nature of light? The phenomena like interference, diffraction and polarisation establish the wave nature of light. However, the phenomena like black body rediation and photoelectric effect establish the particle nature of light.
  2. What is Diffraction of light? The phenomenon of bending of light around the corners of samll obstacles or appertures and their consequent spreading into the regions of geometrical shadow is called diffraction of light.
  3. Define Diffraction at a single slit? A plane wave of wavelength on passing through a narrow slit of width ‘a’ suffers diffraction producing a central bright fringes (∅=0⁰), flanked on both sides by minima and maxima. The intensity of secondary maxima decreases with the increase
    in distance from the centre. For nth minimum

a sin∅ñ =n¥, n = 1, 2, 3..

For nth secondary maximum:
a sin 6, = (2n + 1),n= 1, 2, 3 .
Angular position of nth minimum:
∅ñ = n. D£/a
Distance of nth minimum from the centre of the
screen, Xn.=n. D¥/a
Angular position of nth secondary maximum,
∅n= (2n+1)¥/2a
width of a secondary maximum
B= ¥D/a
Width of central maximum,
Bo =28-2D¥/a

  1. What is the Limit of Resolution? The smallest linear or angular separation between two point objects at which they can be just resolved by an optical instrument is calléd the limit of resolution of the instrument.
  2. What is Resolving Power? it is the ability of an optical instrument to resolve or separate the images of two nearby point objects so that they can be distinctly screen. it is equal to the reciprocal of the limit of resolution of the optical instrument.
  3. What is Diffraction as a limit on resolving power? All optical instruments like lens, telescope, and microscope etc. act as appertures. Light on passing through them undergoes diffraction. This puts the limit on their resolving power.
  4. What is Resolving power of Microscope? It is defined as the reciprocal of the smallest distance between two points objects at which they can be just resolved when seen in the microscope.

R.P. of a microscope = 1/d 2μsin∅/Π

Where usin 0 is called numerical aperature (N.A.) and 0 is half the angle of cone of light from each point object and u is R.I. of medium between object and objective.

  1. What is Resolving power of Microscope? It is defined to as the reciprocal of the smallest angular separation de between two distant objects whose images can be just resolved by it.
    1 R.P. of telescope de1.22sfaotol
    Where D is the diameter of the telescope.
    Objective and is the wavelength of light used.
  2. What is Polarisation of Waves? The process of restricting the oscillations of a wave to just one direction in the transverse plane is called polarisation.
  3. What is Unpolarised light? A kind of light in which the electric field vector vibrates in all possible directions in the transverse plane, rapidly and randomly, during the time of measurement is called unpolarised light. For example, the light of the sun, candle light etc.
  4. describe the Plane polarised light? It electric field vector to the direction of wave vibrates perpendicular propagation, the light is said to, be linearly Dolarised. In a linearly polarised wave, the vibrations at all poin at all
    times, lie in the same plane, o it is also calle plane polarised wave. it is produced by NICOL PRISM.
  5. What is Polariser? A device that plane polarises the unpolarised light passed through it is called a polariser. For example, a tourmative crystal, nicol prism, polaroid etc.
  6. What is Law of Malus? It states that when a beam of completely plane polarised light is passed through an analyser, the intensity of the transmitted light varies directly as the square of the angle 0′ between the transmission direction of polariser and analyser. Mathematically, I = I, cos’.

where I, is the maximum intensity of transmitted light.

  1. Define Plane of polarisation? The plane passing through the direction of wave propagation and perpendicular to the plane of vibration is called plane of polarisation.
  2. Define Plane of Vibration? The plane containing the direction of vibration and the direction of wave propagation is called the plane of vibration.
  3. What is Brewster’s Law? This law states that the tangent of the polarising angle of incidence of a transparent medium is equal to its refractive Index.
    So, μ= tan ip
  4. What is Brewster angle? The angle of incidence at which a beam of unpolarised, light falling on a transparent surface is reflected as a beam of completely plane polarised light is called polarising or Brewster angle. it is denoted by i,
  5. What is Nicol prism? It is an optical device based on the phenomenon of double refraction which is based for producing and analysing plane polarised light. It consists of two pieces of a calcite cut with a 68° angle and stuck together with Canada balsam.
  6. Describe about Polaroids? These are thin commercial sheets which make use of the property of selective absorption to produce an intense beam of plane polarised light.

Polaroids are used in sunglasses, camera filters, wind screens and car head lights of motor cars to reduce glare of light reflected from shiny Surfaces etc. It is also used in 3D motion pictures, in optical stress analysis, etc. It is also known as sheet polarisers.


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