# Short Questions and Answers of Physics Wave & Optics Class 12

#### 1. What is Huygen’s Wave Theory of Light?

Huygen’s Wave Theory of Light states that light energy is transmitted in the form of waves. Initially, it was assumed that Wave travels through an invisible medium called Ether, but the idea was later discarded. Huygen just gave a theoretical treatment but the experimental verification was done by Thomas Young with his Young’s double-slit experiment.

#### 2. What is wavefront? [HSEB 2068]

According to Huygen’s Wave Theory, when the waves originate from a light source, all the particles of the medium located at the same distance from the source receive the disturbance simultaneously and vibrate in the same phase. The locus of all such particles vibrating in the same phase may be a straight line, circle, sphere, cylinder, or plane. Such surfaces are called Wavefronts of Light. If the source is spherical or just point size, the wavefront is spherical. If the source is linear or elongated, the wavefront is cylindrical.

#### OR State Huygen’s principle. Does it apply to sound waves in air? [HSEB 2067]

Huygen’s Theory of light suggests that if the present position of a wavefront is known, then its position can be known at some later time. Huygen’s principle can be stated as under:

(a) Every point on a given wavefront acts as a source of secondary wavelets that move in the forward direction at the same speed at which the wave moves.

(b) The new position of the wavefront at any instant is a line drawn tangent to the edges of the wavelets at that instant. This theory applies to any wave, either light or sound wave.

#### 4. What are spherical, cylindrical, and plane wavefronts?

Depending upon the source of light, wavefront can have several shapes:

(a) Spherical Wavefront: If a point source or spherical solid source is releasing energy, the waves will take the shape of concentric spheres centered at the source. Such a wavefront is called Spherical Wavefront.

(b) Cylindrical Wavefront: When the source of light is linear (straight), all the points of the wave are equidistant from the source. This forms the shape of a cylinder, such a wavefront being called Cylindrical Wavefront.

(c) Plane Wavefront: If the wave produced from a source has traveled large distances, the wavefronts of any shape will take up the shape of a plane surface (eg: water waves formed at the seashore), especially if only a small portion of the wave is considered. Such a wavefront is called Plane Wavefront.

#### 5. Diff ere ate between wavefront and wavelet? [HSEB Model Question 2057, 2065, 2070, 2072]

Wavefront is the locus or the shape formed by the particles in a wave, at the same phase or the same state of Motion. Eg: the points in the crest of a circular waveform a circle. The same applies to points in the trough. Wavelets are the small waves originating from each point of the wavefront, once the point gets disturbed and start acting like a new source. The wavelets from a wavefront combine to form a new wave.

#### 6. Differentiate between plane wavefront and spherical wavefront? [HSEB 2062]

A plane wavefront is a wavefront which is of the shape of plane surfaces. When wavefronts are plane, the energy is moving parallelly. Aspherical wavefront is the wavefront which is of the shape of circular arcs. When wavefronts are spherical, the energy is radiating uniformly in all directions.

[Note: In the long run, the spherical wavefront changes to a plane wavefront. Both the wavefronts can be changed from one type to each other.]

#### 7. What do you understand by the ‘dual’ nature of light?

Some observations in nature show that light behaves as a wave and others indicate that it has a particle-like nature. These two theories seem to contradict each other but both have their strong points to prove. So, physicists concluded that light must exist in dual form.

#### 8. Does the speed of light in a medium depend on color?

‘YES’ is the simplest answer. The color of any radiation (if it is within the visible region) depends on the wavelength. On the other hand, the velocity of any radiation is v = fλ, where ‘f’ is the frequency of the radiation, and ‘λ‘ is the wavelength. Since frequency is constant, the velocity depends directly on the wavelength and so on color. This means red light has the highest wavelength and so highest speed in a medium whereas violet light has the least wavelength and so least speed. [However, in a vacuum, all of them have the same velocity, given by c = 3.0x108 m/s]

#### 9. The refractive indices of glass, diamond, and water are 1.3, 1.5, and 2.0 respectively. In which of these media the speed of light will be maximum?

The refractive index of a medium is given by μ = c/v, or v = c/μ Since ‘c’ is constant, ‘μ‘ is inversely proportional to the velocity of light in the medium or vice versa. So, the medium which has the least value of ‘μ‘ (i.e. water) will have a high velocity of light and which has the highest value (diamond) will have the least velocity of light.

#### 10. What are the disadvantages of Fizeau’s method of measuring the speed of [HSEB 2064] light?

The disadvantages of Fizeau’s method are as follows:

(a) The exact position of the eclipse cannot be determined exactly.

(b) The distance between the toothed wheel and the plane reflecting mirror is very far, about 8km. So, the experiment is difficult to do inside a liquid medium.

#### OR Write down the advantages of Foucault’s method over Fizeau’s method of measuring the speed of light? [HSEB 2065]

The main advantage of Foucault’s method is that since the distance between various mirrors is less in such an arrangement, this experiment was possible to be performed even inside liquids in addition to air or vacuum. This helped in the determination of the speed of light even in liquids such as water, alcohol, spirit, etc. This led to a new finding at that time that the speed of light in any media is less than in air or vacuum.

#### 12. When two light waves interfere, then at some points there is darkness. Where does the energy of these points go?

During interference, some regions will be brighter and some darker than the original sources of light. When they arrive at a point out of phase, they interfere destructively and the ‘energy at that point is zero. At this point, the energy seems to disappear. However, instead of disappearing, this energy has been redistributed to other neighboring regions. This makes those regions brighter than the original.

#### 13. Why can’t the light waves emitted from various parts of a single light source produce sustained interference?

Light is emitted from a source when the excited electrons of the atoms drop to the second orbit (the series of radiation often called as Balmer Series). Since there are innumerable electrons, they definitely cannot occur simultaneously, which makes them have different phases. This produces waves whose phase difference varies with time. So, they are not coherent though their frequency is the same. This makes them unable to produce sustained interference.

#### 14. Explain why two flashlights held close together will not produce an interference pattern on a distance screen?[HSEB Model Question 2057]

The main requirement of interference is that the sources used should be coherent, which requires that they have the same phase or constant phase difference. When two flashlights are used, the electrons are under various states of excitation and de-excitation. So waves from different portions of even the single source can not be coherent, let alone the waves from two sources. So interference isn’t possible.

#### 15. What do you understand by coherent sources? [HSEB 2065]

Coherent sources are those sources whose waves have the same frequency and have either the same phase or same phase difference. Eg: two twin slits placed at some distance from a monochromatic source, a biprism, a Lloyd mirror, etc.

#### 16. Why can’t two candles (or two independent sources) produce interference fringes?

The sustainable interference fringes are obtained when light waves from two coherent sources overlap. Two candles do not emit coherent waves because the waves are not of the same frequencies or of compatible amplitudes. So the waves reaching a point from their interaction will not have the same phase or phase difference over time. This makes sustained interference impossible.

#### 17. Your automobile has two headlights. What sort of interference pattern do you expect to see from them? Why?

The sustainable interference fringes are obtained when light waves from two coherent sources overlap. Two headlights of automobiles do not emit coherent waves because the wave obtained from them is white in color. So, at a certain time, their frequencies do not match and neither do the amplitudes. So the waves reaching a point from their interaction will not have same phase or phase difference over time. This makes sustained interference impossible.

#### 18. What should be the path difference between two interfering waves, constructive interference and destructive interference? [HSEB 2070]

To have the constructive interference, the path difference between two waves should be an integral multiple of their wavelength (λ)

i.e. Path difference = nλ, where n = 0, 1, 2, . . .

But to have destructive interference, the path difference between two waves should be an odd integral multiple of half the wavelength of the waves,

i.e. Path difference = (2n + 1)λ/2 Where, n = 0, 1, 2

#### 19. Can we conclude from the interference phenomenon whether light is a transverse wave or longitudinal?

The occurrence of interference means that light has a wave nature. But it does not clarify whether light is transverse or longitudinal. This is because both types of waves show interference. Such nature is distinguished by the phenomenon of polarization. Only transverse waves undergo polarization but longitudinal does not. “But light undergoes polarization”, which means light travels in the form of transverse waves.

#### 20. The phase difference between two light waves emerging from the slits of Young’s experiment is n radians; will the fringe be bright or dark?

At the central region of the screen, the waves from the slits travel the same distance. So their path and phase difference is zero. This means the crest from one wave overlaps with the crest of the other and trough with the trough. This forms a bright image there. If the phase difference is n radians, the path difference between the two waves is  λ/2, which means crest from one wave will fall to the trough of the other and trough to crest. So, the fringe there will be dark.

#### 21. What will be the effect on the interference fringes obtained in Young’s double-slit experiment if the monochromatic source of light is replaced by a white light source?

The first thing to remember is that white light is composed of a lot of other colors; naming V, I, B, G, Y, 0, and R, which have different wavelengths.

Then according to the relation β= λD/d, all of them will have different fringe widths, though the difference will be very less.

For the central fringe, the difference does not mean much, so it will nevertheless appear white and bright. But as other fringes are observed at the two sides of the central fringe, each will show its existence and colored fringes are seen, which do not have any sharp boundaries. More to the sides, it might happen that the bright fringe of one color overlaps with the dark of the other and vice versa. This makes the color uniform (white) again as if there is no interference. The overall picture would be that fringes will appear for the first few and then no fringes are seen.

#### 22. In Young’s double-slit experiment, the waves emerging from the two slits are perfectly in phase. Why?

In this experiment, the two slits are at the same distance from the main source. So every feature of the light waves (either crest or trough or mid-points) which originate from the source reaches those two slits at the same time. This makes the path lengths and phase angles also the same. Therefore, the waves that emerge from the slits are in phase.

#### 23. Can two coherent sources be obtained from an incoherent source?

Yes. In Young’s double-slit experiment, the two slits, act as two secondary sources when illuminated by a monochromatic primary source. This primary source is itself an incoherent source. So, coherent sources can be obtained from incoherent sources.

#### 24. In Young’s double-slit experiment, the source gives out white light. One slit is covered with a red filter and the other with a blue filter. Discuss the nature of the interference pattern.

The principal requirement of interference is that the sources be coherent. When done as in the question as stated above, the frequencies of the two h sources will be different. Each of them will thus have their, own fringe widths, bright fringes, and dark fringes. Such a pattern will be difficult to observe and distinguish.

#### 25. Why does interference occur in thin film-like soap bubbles?

A film will have a certain small thickness of such liquid. When light tries to pass through them, some portion is reflected from the air-film boundary and some will be reflected from the film-air boundary. So, the two reflected lights will have some path differences. Somewhere, this path difference might be equal to integral multiples λ of or odd multiples of λ/2 depending on the thickness of the film or the angle of incidence. This gives rise to interference patterns. Somewhere, things are not so, which makes such things impossible.

#### 26. How do color filters base on interference work?

When two waves going towards the same direction have the same phase or phase difference, constructive interference occurs and they give a bright fringe or zone. If they are of the opposite phase, they result in destructive interference and cause darkness. The colored filters are designed of such thickness that when a light ray passes through them, the directly transmitted ray and those going after a certain degree of reflection, will be of the same phase for a particular frequency (only one). This requires that the thickness should be equal to the integral multiple of the wavelength of the wave which has been targeted to pass through.

While doing so, that thickness will not be equal to an integral multiple of other wavelengths. So, they cannot undergo constructive interference and so, such color cannot be seen in the resultant pattern.

#### 27. How is the phenomenon of interference different from the process of formation of standing waves?

Both of these phenomena are related to the superposition of waves. In the case of the formation of standing waves, the two waves coming from two sources of the same frequency in opposite directions interact with each other to give a resultant pattern in which there are regions of maximum and minimum amplitude at certain locations. The regions of high and low amplitude will occur along the line joining the two sources.

In the case of interference, the two waves coming from two coherent sources traveling in the same direction interact with each other to give a resultant pattern in which there are regions of maximum and minimum amplitude. The regions of high and low amplitude will occur perpendicular to the line joining the two sources.

#### 28. Why is it so much easier to perform interference experiments with a laser than with an ordinary light source?

Interference requires that the light sources be coherent. The foremost requirement of coherence is the same frequency and phase. LASER fulfills this criterion fully. So laser is preferred over other sources.