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Std 12 PHYSICS > Light > Flashcards

Light Flashcards

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0
Q

Drawbacks of Newtons corpuscular theory

A

Could not explain partial reflection and refraction at the surface of a transparent medium.
Unable to explain phenomenon such as interference, diffraction, polarization etc.
Predicted speed of light in a denser medium is more than speed of light in rarer medium which was proved wrong experimentally by Foucault.
If particles emitted from the source, then mass of source should decrease. Experimentally observed : No change

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1
Q

State Newton’s Corpuscular Theory

A

Every source of light emits large number of tiny particles called corpuscles in a medium surrounding the source.
Corpuscles are perfectly elastic, rigid, and weightless.
Travel in a straight line with very high speeds which are different in different media.
One gets sensation of light when corpuscles fall on retina.
Different colors of light are due to different sizes of corpuscles.

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2
Q

Give the postulates of the wave theory of light

A

Light travels I’m the form of a longitudinal wave with uniform velocity in homogenous medium.
Different colors of light due to different wavelengths.
When light enters our eye, we get sensation of light.
A material medium is necessary for propagation of longitudinal waves. - Huygens suggested the existence of a hypothetical medium called “luminiferous ether” - attributed properties like zero density and perfect transparency.

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3
Q

Drawbacks of wave theory of light

A

Could not explain rectilinear propagation of light.
Could not explain phenomenon like Polarization and Compton effect and photoelectric effect.
Morley and Michelson studied properties of hypothetical ether- no ether drag when Earth moved through it- this proved ether doesn’t exist.

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4
Q

Success of wave theory of light

A

Could explain laws of reflection, refraction, interference, diffraction etc.
Predicted speed of light in optically denser medium is less than speed of light in optically rarer medium.

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5
Q

Define Wavefront

A

A locus of all the points of the medium to which the wave reaches simultaneously so that all the points are in phase is called the wavefront.

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6
Q

How can a plane wavefront be obtained?

A

Point source of light placed at the focus of convex lens.

Small part of spherical wave front can be considered plane wavefront.

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7
Q

What kind of wave front does a linear source (slit) give?

A

Cylindrical

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8
Q

Define wave normal

A

A perpendicular drawn to the surface of a wavefront at any point of the wavefront in the direction of propagation of light is called wave normal.

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9
Q

State Huygen’s principle

A

When a luminous point source of light creates a disturbance, the energy of the disturbance is propagated through a transparent medium in the form of waves.

(1) every point on a wavefront behaves as a secondary source of light sending secondary waves in all possible directions.
(2) new secondary wavelets are more effective in the forward direction only. (Direction of propagation of wavefront)
(3) the resultant wavefront at any position is given by the tangent to all the secondary wavelets at that instant.

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10
Q

What type of waves do not get polarized?

A

Longitudinal

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11
Q

Define Polarization of light

A

The phenomenon of restriction of vibration of light waves in a particular plane perpendicular to direction of propagation of wave motion is known as Polarization of light waves.

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12
Q

Examples of polarizers

A

Tourmaline crystals, Nicol prism

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13
Q

Define unpolarized light

A

A light in which the vibrations of the electric vectors are in all possible directions, which are perpendicular to the direction of propagation, is known as unpolarized light.

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14
Q

Define plane polarized light

A

When vibrations of electric vectors are confined in one plane, the light is known as plane polarized light.

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15
Q

Define plane of vibration

A

The plane in which vibrations of polarized light take place is called as the plane of vibration.

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16
Q

Define Plane of Polarization

A

The plane perpendicular to the plane of vibration in which there are no vibrations of polarized light is known as plane of polarization.

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17
Q

State Brewster’s law

A

Brewster’s law states that: The tangent of the polarizing angle is equal to the refractive index of the refracting medium at which partial reflection takes place.

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18
Q

Is Brewster’s law applicable to every surface? Is the polarizing angle affected by dispersion?

A

Not applicable to polished metallic surfaces.

Yeah but in most media the dispersion is too small to affect polarizing angle.

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19
Q

What is a Polaroid?

A

A Polaroid is a large sheet of synthetic material packed with tiny crystals of crystals of a dichroic substance oriented parallel to one another, so that it transmits light only in one direction of the electric vector.

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20
Q

What is dichroism?

A

The property by which some doubly refracting crystals absorb the ordinary rays (O-rays) completely and extraordinary waves whose direction is parallel to the optic axis while passing through the crystal is called Dichroism.

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21
Q

What are dichroic crystals?

A

The crystals possessing dichroism property are known as dichroic crystals.

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22
Q

Example of dichroic crystal

A

Tourmaline crystal

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23
Q

What is Herapathite?

A

Iodosulphate of Quinine
Shows strong dichroism
Crystals not stable
They are affected by slight strain

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24
Q

How did Lamb develop his polarizer?

A

Arranged Herapathite crystals side by side in such a way that the optic axis of each of them were parallel to each other, so that they acted as a single crystal of large dimensions.

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25
Q

Uses of Polaroid

A

Motor car headlights to remove headlight glare.
3D movie cameras.
To produce and analyze Polarized light.
Filter in photographic cameras.
Window of airplanes to control amount of light.
In polarizing sunglasses to protect eyes from glare of sunlight.
Improve color contrast in old paintings.
Calculators, laptops, monitors of laptops which have LCD screens.

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26
Q

Difference between Doppler effect in light and sound

A

Light- symmetric (only depends in relative velocity and not whether source or observer moving)
Sound- asymmetric

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27
Q

Why is the Doppler effect in sound asymmetric while in light it is symmetric?

A

Sound requires material medium- speed of source and observer are measured ep relative to the medium.
Light doesn’t require medium

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28
Q

Formula for Doppler effect in light

A

υ’= υ((1+/- (Vr/c))/root(1-((Vr/c)^2)))
Vr &laquo_space;c
υ’ = υ(1+/- (Vr/c))

Δυ/υ = Δλ/λ = Vr/c

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29
Q

When will a wavelength in the middle of the visible spectrum shift towards red?

A

Source and observer move towards each other

Frequency decrease, wavelength increase

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30
Q

When will a wavelength in the middle of the visible spectrum shift towards blue?

A

Source and observer approach each other.

Frequency increase, wavelength decrease.

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31
Q

Applications of Doppler effect

A

Measurement of radial velocities of galaxies.
Speed of rotation of the sun (absorption lines due to oxygen matched, iron lines displaced relative to each other. ~2km/s)
Measurement of Plasma temperature.

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32
Q

State the principle of super position (P2)

A

When two or more waves overlap, the resultant displacement at any point and at any instant is equal to the vector sum of instantaneous displacements that would be produced at the point by individual waves if each wave was present alone.

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33
Q

Define interference of light

A

The modification of the intensity of light (redistribution) produced by superposition of two or more light waves is known as interference of light.

34
Q

Path difference for constructive interference

A

Even multiple of λ/2 = [2n(λ/2)]

35
Q

Path difference for destructive interference

A

Odd multiple of (λ/2) = (2m-1)*(λ/2)

36
Q

What are interference bands?

A

Central bright band will be flanked on both sides with alternate bands, of equal width, of maximum and minimum intensity. These are known as interference bands or fringes.

37
Q

Interference - expression for resultant displacement

A

R= root(a1^2 + a2^2 +2a1a2cosφ)

38
Q

Interference - expression for resultant phase angle

A

θ= tan inverse( a2sinφ/(a1+a2cosφ))

39
Q

How does intensity of wave depend on amplitude?

A

Intensity is directly proportional to the square of amplitude.

40
Q

When is maximum intensity obtained and what is its value?

A

Imax α (a1+a2)^2
Ir= 4I(cosφ)^2
Imax=4I

41
Q

When is minimum intensity obtained and what is its value?

A

Imin α (a1-a2)^2
Ir= 4I(cosφ)^2
Imin=0

42
Q

Interference - expression for resultant intensity

A

Ir α (a1^2 + a2^2 + 2a1a2cosφ)

Ir= (I1^2 + I2^2 + 2I1I2cosφ)

43
Q

Conditions for producing steady interference pattern

A

Two sources of light must be coherent. (Must be derived from same original source)
Sources of light must be monochromatic. (Only one wavelength)
Two interfering waves must be in same state of polarization.
Must emit light waves of equal amplitude or intensity.
The sources of light must be narrow.
The separation between light sources must be as small as possible.
Distance of screen from two sources should be large.
The two interfering waves must travel in same direction.

44
Q

What are coherent sources?

A

Two sources of light emitting light waves of equal frequency and which are always in the same phase or with constant phase difference are known as coherent sources.

45
Q

Give reasons: For a steady interference pattern source should be monochromatic

A

Diffused and in indistinct pattern. At one point - constructive due to one wavelength and destructive due to another may be possible.

46
Q

Give reasons: For a steady interference pattern sources must emit light waves of equal amplitude or frequency.

A

Sharp interference pattern
Perfectly dark band
Maximum contrast

47
Q

Give reasons: For a steady interference pattern sources of light must be narrow

A

Else waves starting from different points on the source travel different distances to reach the same point on the screen. Waves will interfere with one another. Interference pattern not clear and distinct.

48
Q

Give reasons: For a steady interference pattern source should have a small separation

A

Widely spaced interference fringes and can be seen clearly.

49
Q

Give reasons: For a steady interference pattern source should be at a large distance from screen

A

Widely spaced interference fringes and can be seen clearly.

50
Q

Importance of Young’s experiment

A

First successful experiment to show interference of light.

Possible to calculate the wavelength of monochromatic light by using the formula λ=Xd/D

51
Q

Expression for path difference between two waves

A

= xd/D

52
Q

Define band width or fringe width

A

The distance between centers of two adjacent bright or dark bands is called band width of fringe width.

53
Q

Expression for fringe width

A

X= λD/d

54
Q

Expression for upward shift of interference fringes with thin transparent plate of thickness t and RI μ

A

x= (D/d)(μ-1)t

55
Q

What is a biprism?

A

A biprism consists of two identical thin prism of very small refracting angle (30’ to 1deg) with their bases joined together, thus acting as a thin glass prism of obtuse angle of about 179deg.

56
Q

Define diffraction

A

The bending of light near the edges of an obstacle or slit and spreading into the region of geometrical shadow is known as diffraction of light.

57
Q

Give examples of diffraction of light

A

Sun seen through fine piece of cloth, colorful spectra is observed.
Luminous border surrounds the profile of a mountain just before sun rises behind it.

58
Q

Types of diffraction

A

Fraunhofer

Fresnel

59
Q

What is Fraunhofer diffraction?

A

Source of light and screen on which diffraction pattern is obtained are at infinite distance from the diffracting system.
Plane wavefront considered.
Diffraction pattern obtained using convex lens.

60
Q

What is Fresnel diffraction?

A

Source of light and screen are kept at a finite distance from the diffracting system.
Cylindrical or spherical wavefronts.

61
Q

Position of minimum intensity (Fraunhofer diffraction)

A

sinθn=nλ/a

62
Q

Position of secondary minimum

A

sinθn=(2n+1)λ/2α

63
Q

Width of central maxima

A

2λf/a

64
Q

How does the width of secondary maximum depend in light?

A

Proportional to the wavelength of light.

65
Q

What is Airy’s disc?

A

The diffraction pattern is a bright disc surrounded by alternate dark and bright rings, whose intensity goes on decreasing. It is known as Airy’s disc.

66
Q

Define limit of resolution of an optical instrument

A

The smallest angular or linear separation between two point objects at which they appear to be just resolved is known as the limit of resolution of an optical instrument.

67
Q

Define resolving power of an optical instrument

A

Reciprocal of limit of resolution

68
Q

State Rayleigh’s criterion

A

When central bright image of one point object falls on the first dark ring of the second point object kept close to it the two images are said to be just resolved.
The two objects are said to be well resolved if the distance between the central maximum of the two objects is greater than the distance between the central maximum and first minimum of any of the two objects.
The two objects are said to be unresolved if the distance between the central maximum of the two objects is less than the distance between the central maximum and first minimum of any of the two objects.

69
Q

Define limit of resolution of a microscope

A

The minimum distance by which two point objects are separated from each other so their images as produced by the microscope are just seen separate is called the limit of resolution of the microscope.

70
Q

Expression for limit of resolution of a microscope (A and B self-luminous)

A

d= 1.22λ/(2sinα)

71
Q

Expression for limit of resolution of a microscope (Αbbe)

A

d= λ/(2sinα)

72
Q

Minimum separation for resolved images

A

d= λ/(2μsinα)

73
Q

What is meant by an immersion objective and what are its advantages?

A

In a high resolving power microscope, an oil immersion objective is used. Space between the objective and the objective is filled with an oil (cedar wood oil).
Advantages:
Loss of light by reflection at the first lens surface decreased.
Resolving power of microscope increased.

74
Q

Expression for numerical aperture of a microscope

A

NA= μsinα

75
Q

R.P. of microscope -expression

A

2μsinα/λ

76
Q

How can RP of a microscope be increased?

A

Increasing its numerical aperture
Decreasing wavelength of light used to illuminate the objects (using uv light)
Use quartz lens

77
Q

Why can we not increase α to increase RP of a microscope?

A

Aperture of lens would increase in that case.

78
Q

What is a telescope?

A

A telescope is an instrument which is used to see distant objects clearly.

79
Q

Define Resolving power of a telescope

A

The reciprocal of the angle subtended at the objective by two distant point objects which can be distinguished just resolved in the focal plane of the telescope is the resolving power of the telescope.

80
Q

Reciprocal of resolving power for a rectangular aperture of a telescope.

A

dθ= λ/a

81
Q

Reciprocal of resolving power for a circular aperture of a telescope.

A

dθ= 1.22λ/a

82
Q

Resolving power of a telescope

A

RP= a/1.22λ

83
Q

Differences between interference and diffraction

A

(1) interference- result of interaction of two waves coming from two different wave fronts from two coherent sources
diffraction- interaction of light coming from different parts of the same wavefront.
(2) interference- fringes same width
diffraction- fringes not same width
(3) interference- all bright bands of same intensity
diffraction- all bright bands not of same intensity (max for central maximum)
(4) interference- dark fringes perfectly dark
diffraction- dark fringes not perfectly dark

Std 12 PHYSICS (12 decks)

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