11th Class Physics CHAPTER 9 PHYSICAL OPTICS Short Question Answers
11th Class Physics CHAPTER 9 PHYSICAL OPTICS Short Question Answers Below
We are providing all Students from 5th class to master level all exams preparation in free of cost. Now we have another milestone achieved, providing all School level students to the point and exam oriented preparation question answers for all science and arts students.
After Online tests for all subjects now it’s time to prepare the next level for Punjab board students to prepare their short question section here. We have a complete collection of all classes subject wise and chapter wise thousands questions with the right answer for easy understanding.
Here we are providing complete chapter wise Physics questions and Answers for the 11th class students. All the visitors can prepare for their 11th class examination by attempting below given question answers.
In this List we have included all Punjab boards and both Arts and Science students. These Boards students can prepare their exam easily with these short question answer section
Lahore Board 11th classes short questions Answer
Rawalpindi Board 11th classes short questions Answer
Gujranwala Board 11th classes short questions Answer
Multan Board 11th classes short questions Answer
Sargodha Board 11th classes short questions Answer
Faisalabad Board 11th classes short questions Answer
Sahiwal Board 11th classes short questions Answer
DG Khan Board 11th classes short questions Answer
Bahwalpur Board 11th classes short questions Answer
All above mention Punjab Boards students prepare their annual and classes test from these online test and Short question answer series. In coming days we have many other plans to provide all kinds of other preparation on our Gotest website.
How to Prepare Punjab Board Classes Short Question Answer at Gotest
- Just Open the desired Class and subject which you want to prepare.
- You have Green bars which are Questions of that subject Chapter. Just click on Bar, it slides down and you can get the right answer to those questions.
- You can also Rate those question Answers with Helpful or not to make it more accurate. We will review all answers very carefully and also update time to time.
Now you can start your preparation here below
1. Under what conditions two or more sources of light behave as coherent sources?
Two or more sources will behave as coherent sources if they emit continuously light waves of the same amplitude, same time period and same frequency (or wavelength) having the same phase (or constant phase difference).A common method for producing two coherent light sources is to use a single source to illuminate a screen containg two narrow slits. Thus, two or more sources derived from a signal source of light will behave as coherent sources, because they are in the same phase.
2. How the distance between interference fringes affected by the separation between the slits of Young‘s experiment? Can fringes disappear?
The formula for fringe spacing is given by Δy =λl/d This relation shows that the fringe spacing varies inversely with the separation‘d’ of the slits. It means that greater the separation between the slits, the smaller will be the fringe spacing. When the separation between the slits is made large enough, the fringes will so close that they cannot be distinguished and will disappear on further increase of separation ‘d’ between slits.
3. Can visible (white) light produce interference fringes? Explain.
Yes, the visible light or white light can produce interference fringes. As visible light is a mixture of seven component colors so each component colors will produce interference fringes corresponding to its own wavelength. Hence, the fringe pattern will be colored. But the fringes will overlap due to different wave lengths of the colors, so it would be difficult to observe the interference fringes of visible or white light.
4. In the Young’s experiment, one of the slits is covered with blue filter and other with red filter. What would be the pattern of light intensity on the screen?
The blue and red lights emerging out of the two slits will not be in phase coherence due to different wavelengths. Therefore, there will be no maxima and minima on the screen. We shall observe two colored images on the screen with constant intensity.
5. Explain whether Young’s experiment is an experiment for studying interference or diffraction effects of light?
Young’s double slits experiments is basically, used to study interference effect of light. However, spreading of light around the edges of the slits also produce some diffraction effect but interference phenomenon. Since diffraction occurs along with the interference in the experiment, therefore the same experiments is also used to study diffraction effect. But diffraction is a special type of interference, so interference phenomenon has an upper hand upon the diffraction phenomenon.
6. An oil film spreading over a wet foot path shows colors. Explain how it happens?
The colors are seen on the oil film spreading over a vet foot path due to interference of light waves. If white light is incident on an oil film of irregular thickness at all possible angles, we can see the interference patter due to each color separately. It is quite possible that at a certain place on a oil film, its thickness and angle of incidence are such that the condition of destructive interference for one of its seven colors is satisfied. This color disappears while the remaining six colors show their appearance. As a result, the different colors which are seen with ordinary light are produced due to interference of waves reflected from the opposite surfaces of the oil film.
7. Could you obtain Newton’s rings with transmitted light? If yes would the pattern be different from that obtained with reflected light?
Yes, Newton’s rings can be observed with transmitted light. In case of transmitted light, the fringe pattern is just opposite to the reflected pattern because of no phase change of 1800. It means that the central spot of Newton’s rings in this case will be bright instead of dark due to the transmitted light.
8. In the white light spectrum obtained with diffraction grating, the third order image of a wavelength coincides with the forth order image of a second wavelength. Calculate the ratio of the two wavelengths?
The grating equation is given by
d sin θ = nλ………………………………….(i)
For first wavelength,
Third order image, n = 3
λ =λ1
Equation (1) can be written as
d sinθ = 3λ1
For second wavelength,
fourth order image, n -4
λ = λ2
Again, equation (1) can be written as
d sin θ = 4 λ2
or λ1/λ2 = 1.33
Hence, the ratio is 4:3 or 1.33:1
d sin θ = nλ………………………………….(i)
For first wavelength,
Third order image, n = 3
λ =λ1
Equation (1) can be written as
d sinθ = 3λ1
For second wavelength,
fourth order image, n -4
λ = λ2
Again, equation (1) can be written as
d sin θ = 4 λ2
or λ1/λ2 = 1.33
Hence, the ratio is 4:3 or 1.33:1
9. How would you manage to get more orders of spectra using a diffraction grating?
. As we know from the formula d sinθ = nλ on increasing the value of ‘θ’ when θ becomes 900 then the maximum value of sinθ = 1, so it is ignored. Thus for a given value of λ, the order of spectrum ‘n’ is directly proportional to grating elements‘d’ i.e. n α d But d = l/N Thus, we conclude that in order to get more orders of spectra, we should increase the grating elements‘d’ i.e. spacing between slite on the grating or decrease the number of lines per centimeter ruled on the grating. Hence, the best way is to reduce the no of lines on the grating.
10. Why the Polaroid sunglasses are better than ordinary sunglasses?
The sunlight reflected from water, glass and rough road surface, for large angles of incidence, is partially polarized and so it produces glare. Glare can be reduced by using Polaroid sunglasses because they decrease the intensity of light passing through them. In this way, our eye is saved from unnecessary strain of glare. Hence, the Polaroid sunglasses are better than ordinary sunglasses because the ordinary sunglasses cannot provide much less intensity of light.
11. How would you distinguish between un- polarized light and plane polarized light?
When un- polarized light is passed through a polarized, the emerging light beam has all the electric vectors confined is one plane at right angles to its direction of propagation. Such light is called plane polarized light (Polaroid is named as polarizer). When another Polaroid is placed in the path of polarized light with its axis parallel to the first Polaroid, the total light will pass through the second Polaroid. If the second Polaroid is rotated through 900, no light passes through it. But ordinary or unpolarized light will totally pass through it in all directions. This second Polaroid distinguishes the plane polarized light from ordinary or unpolarised light. This Polaroid used to test polarized light is called analyzer.
12 An oil film floating on water surface has colored pattern. Why?
The colors are seen on the film floating on water surface due to interference of light waves. If white light is incident on an oil film of irregular thickness at all possible angles, we can see the interference pattern due to each color separately. It is quite possible that at a certain place on an oil film, its thickness and angle of incidence are such that the condition of destructive interference for one of its seven colors is satisfied. This color disappears while the remaining six colors which are seen with ordinary light are produced due to interference of waves reflected from the opposite surface of the oil film.
13. Define diffraction of light?
The property of bending of light around the edges of an opening or obstacle placed in its path is called diffraction of light.
14. What is diffraction grating?
A diffraction grating consists of a glass plate on which very fine equidistant parallel lines are drawn by means of a fine diamond point. These lines act as an opaque through which the light cannot pass, while the spacing’s between the lines on the glass plate act as slits through which light can pass. A typical diffraction grating has about 400 to 5000 lines per centimeter to produce diffraction of light.
15. What is distance between two adjacent dark fringes?
The distance between two adjacent dark fringes is given by Distance = Δy = λL/d where ‘L’ is the distance of the screen from the slit, ‘d’ is the distance between the centers if two slits and λ is the wavelength of light.
16. Describes the factors upon which the fringe spacing (or width) in Young double slit experiments can be increased?
The fringe spacing in Young’s double slit experiments is given by Δy = λL/d This relation shows that the fringe spacing (or width) can be incread by (i). Increasing ‘L’ the distance between the slits and the screen (ii). Using the light of large wavelength ‘λ’ (iii). Decreasing‘d’ the distance between the two slits.
17. In Michelson interference, why a fringe is shifted when a mirror is moved through λ/2?
When a mirror is moved through a distance of λ/2, a path difference of double of this displacement is produced i.e. λ/2+ λ/2 = λ (This occurs due to the onward journey and backward journey of the first beam from the mirror) Thus a fringe is seen shifted forward across the line of reference of cross wire in the eyepiece of the telescope used to see the fringe.
18. If white light is used to illuminate the diffraction grating what kind of diffraction pattern is produced?
We know that white length consists of seven colors of different wavelength. If it is used for grating a spectrum will be formed. The image of each wavelength for certain value of n is diffracted in defferent directions. Thus, separate images are obtained corresponding to each wavelength or color on either side of central maximum for each ‘n’. Here, n represents the order of the maxima and ‘n’ has the value 0,1,2,3,………………………………….. When n = 0 gives the central bright line or central maximum.
19. What is the necessary conditions for the path difference between two waves that interfere. (a) Constructively (b) Destructively
(a) Condition of constructive interference
The constructive will take place if the path difference between two waves is zero or integral multiple of wave length, that is
Path different = d = 0, λ, 2λ, 3λ + ……………………..mλ
Or in general:- d = mλ
Where m = 0, 1, 2, 3, 4, + ……………………….
b) Condition of Destructive interference:-
The destrictive interference will take place if the path difference between two waves is odd integral multiple of half wave-length.That is
path difference=d= λ/2,3λ/2,5λ,……………………….(m+1/2) λ
or d= (m+1/2) λ
Where m=0.1.2.3.4.+………………………
The constructive will take place if the path difference between two waves is zero or integral multiple of wave length, that is
Path different = d = 0, λ, 2λ, 3λ + ……………………..mλ
Or in general:- d = mλ
Where m = 0, 1, 2, 3, 4, + ……………………….
b) Condition of Destructive interference:-
The destrictive interference will take place if the path difference between two waves is odd integral multiple of half wave-length.That is
path difference=d= λ/2,3λ/2,5λ,……………………….(m+1/2) λ
or d= (m+1/2) λ
Where m=0.1.2.3.4.+………………………
20. Explain why two distant flash lights will not produce an interference pattern?
The two distant flash lights will not produce interference pattern because the light waves from two sources have no phase coherence. Since the various atoms of a single source emit light just at random; therefore no question can arise to have phase coherence between two separate light sources. Secondly, no interference will take place because these sources are not monochromatic
21. What is the difference between interference and diffraction?
INTERFRENCE 1. Interference is the result of superposition of only a few secondary wavelets from two wave fronts originating from the same source. 2. Interference fringes are equally spaced. They can be of the same width. 3. All bright fringes are of the uniform intensity. 4. The points of minimum intensity are perfectly dark, DIFFRACTION:- 1. Diffraction is the result of superposition of very large number of secondary wavelets coming from different parts of the same wave front. 2. Diffraction fringes are not of the same width and not equally spaced. They and wide near the obstacle and become narrower moving away towards the shadow region. 3. All bright fringes are not of the same intensity. 4. The points of minimum intensity are not perfectly dark.
22. What aspect of the nature of light is proved by the phenomenon of polarization?
The phenomenon of polarization has proved that light waves are transverse waves.
23. Describe the uses of a Polaroid as an analyzer and as a polarizer?
Polarizer:- When we use Polaroid disc to polarize light, it is called polarizer. Analyzer:- When unpolarized lihjt is passed through a Polaroid, the emerging light beam has all the electric vectors confined in one plane at right angles to its direction of propagation. Such light is called plane Polaroid light (and Polaroid is named as Polarizer). When polarized is placed in the path of polarized light with its axis parallel to the first Polaroid, the total Polarized light will pass through the second Polaroid. If the second Polaroid is rotated through 900 (first and second Polaroid’s are crossed), no light passes through it. But ordinary light will totally pass through it in all direction. This second Polaroid distinguishes the plane polarized light from ordinary light, it is named is analyzer.
24. Explain whether a path difference of λ/4 is associated with a constructive interference or destructive or neither of them?
A path difference of λ/4 is neither associated with constructive interference nor with destructive interference. For constructive interference, the path deference between two waves should be 0, λ, 2λ, 3λ, mλ. Or d = mλ Where m = 0, 1, 2, 3, ………………… And for destructive interference, the path difference between two waves should be λ/2, 3λ/2, 5λ/2……………………(m +1/2)λ or d = (m + ½)λ Thus, λ/4 is associated with neither of them Secondly, in the case of Michelson’s interference, when sliding mirror is moved through λ/2, even in that case the path difference is not λ/4 but λ/2.
25. Under what conditions can interference of light take place?
Conditions for interference of light:- (i) The source must be phase coherent, that is, they maintain a constant phase with respect to each other. (ii). The source should be monochromatic i.e. of simple wave length. (iii). The principle of linear superposition should be applicable.
26. What are the main facts which support the following statements: (a). The light is propagated in the form of waves and not a corpuscular. (b). Light travels in the form of transverse waves. (c). Light waves are much shorter than sound waves.
(a). The phenomenon of interference and diffraction support that light constant of waves and not a flow of particles (corpuscles) (b). Phenomena of polarization supports that light waves are transverse waves. (c). Phenomenon of diffraction support that wavelength of light is shorter than wave length of sound waves.
27. In a diffraction experiment related to a grating, a student calculates the maximum since it has to be a whole as 6.76. this answer is correct answer 6 or 7 and why?
If the students calculates the maximum order in the diffraction grating experiment as 6.76, the correct answer will be 6 (sixth order) because the order of spectrum must be a whole number.
28. State Huygens’s principle?
Statement:- This principle constant of two parts. (i). The first part states that every point of a wave front may be considered as a source of secondary wavelets which spread out in forward direction with a speed equal to the speed of propagation of the wave. (ii). The second part states that new position of the wave front after a certain interval of time can be found by constructing a surface that touches all the secondary wavelets.
29. Define the term, wave front?
Wave front;- Such a surface on which all the points have the same phase of vibrations is known as wave front. There are two types of wave fronts 1. Spherical wave front 2. Plane wave front. (1). Spherical Wave front:- The wave front in which the light waves are propagated in spherical from with the source Is called spherical wave front. (2). Plane wave front. At every large distance i.e. infinity from the source, a small portion of a spherical wave front will become very nearly plane. Such a wave front is known as plane wave front. The direction of propagation of wave is along the line normal to the wave front is called a ray of light.
30. Write three factors on which interference in thin film depends?
The value of path difference between the rays reflected from upper and lower surface of a thin film depends upon the following factors. (i). Thickness of the film. (ii). Angle of incidence of rays on the thin film. (iii). Nature of the film.
31. Define phenomenon of interference of light waves?
When two lights waves of the two same amplitude and the same frequency (or wavelength) traveling in the same direction are superimposed upon each in such a way that they reinforce (support) each other at some points and cancel each other at the other points. Such phenomenon is called interference. There are two types of interference (i) constructive interference (ii) Destructive interference
32. What is constructive interference?
If the two light waves meet at a point in such a way that they reinforce each other and bright constructive interference.
33. Explain with diagram “the second part of Huygens’s principle?
Let us considered wave front AB produced by source of light ‘S’ at an instant ‘t’ as shown in the diagrams. Consider the dots on the wave front as sources of secondary wavelets, suppose we wont to find new position of the wave front after time Δt. This is done by drawing a hemi sphere of radius cΔt, where c is the speed of the propagation of the wave and Δt is the time of propagation from one wave to the other. If we join the points on spherical surface, the plane tangential to the spherical secondary wavelets gives a new wave front A/B/ at time t + Δt as show in fig
34.Which of the following can occur in a) Longitudinal b) and transverse waves.Refraction,interference, diffraction dispersion and polarization?
a) The phenoma of refraction interference diffraction and dispersion take place in both longitudinal and transverse waves. b) But the phenomenon of polarization can only take place in the transverse waves.
35. Define polarization of light?
The phenomenon in which the vibrations of wave can be oriented to ant one direction that may be vertical, horizontal or any another direction is called polarization of light waves. Considering the example of transverse wave, the vibrations of particles of the medium are perpendicular to the direction of horizontal or any other direction. In each case, the transverse wave is said to be polarized.
36. What are Newton’s formed?
Circular dark and bright fringes formed due to interference in a thin air film enclosed between convex lens and a flat glass plate are called Newton’s rings.
37. How is Newton’s rings formed?
When a Plano –convex lens is placed in contact on a plane glass plate a thin air film of air is formed in between lens and plate. When parallel rays of monochromatic light are made incident normally on the flate surface of plano – convex lens, they are partly reflected and partly reflected and partly refracted. The two reflected rays (i.e. one reflected from the upper surface and the other from the lower surface of the film) are coherent and thus they interfere constructively or destructively producing the circular fringes called Newton’s rings.
38. Why is center of Newton’s rings dark?
The path difference between the rays reflected at the top and the bottom of the air gap at the point of constant is zero. Actually, the ray reflected from the top of the air film does not undergo any change in phase but the ray reflected from the bottom of air film (i.e. denser surface a phase change of 1800or an additional path difference of λ/2 thus a path difference of λ/2 is produced between two rays over this point of contact. Hence, instead of a bright spot, a dark spot is formed at the center.
39. In young’s double slit experiment, the wavelength of the light is doubled, what will happen to the spacing (or width) of fringes?
The spacing (or width) of fringes in Young’s double slit experiment is given by the formula Δy = λL/d Where L is the distance of the slits from the screen ad ‘d’ is the distance between the two slits. Thus, when ‘L’ and‘d’ are fixed, the fringe spacing is directly proportional to the wavelength ‘λ’ of the light used to illuminate the slits. Thus if the wavelength of the light is doubled the fringe spacing (or width) will also be doubled.
40. What are different methods for obtaining the polarized light?
Polarized light can obtain by the following methods. (i). Selective absorption (ii). Reflection from different surface (iii). Refraction through crystals (iv). Scattering by small particles.
41. What is Michelson interference?
The instrument which is used to measures the distance with extremely high accuracy and wavelength if light, is called Michelson interferometer. Let m be the number of fringes which are shifted through a reference point, if the mirror ‘M’ is displaced through a distance ‘L’ then we can write the question L = mλ/2…………………………….(1) secondly, for wavelength it can be written as λ = 2L/m……………………………………..(2)
42. What are different substances for obtaining polarized light by selective absorption?
The substances which are used to get polarized light by selective absorption are called dichroic substance. These materials transmit only those waves whose vibrations are parallel to a particular direction and will absorb those waves whose vibrations are in other directions. One such commercial polarizing material is called Polaroid.
43. What do you mean by grating elements of a diffraction grating?
44. Why it is impossible to have diffraction of x – rays by laboratory diffraction grating?
The grating elements and width of slits on levorotary grating are not so small as the wavelength of x – rays. So diffraction of x – rays cannot be observed. Moreover, the wavelength of x – rays is the order of the 10-10 m. This wavelength is much smaller as compared to the visible light, therefore ordinary diffracting object like diffraction grating cannot be used for diffraction grating of x = rays. However, crystal can be used for the diffraction of x = rays.
45. What is plane polarized light. Give some applications of polarized light?
Such a plane which contains the direction of vibration of the particles of the medium and the direction of propagation of the wave is called the plane of polarization. Applications of polarized Light (i). Plane polarized light is used to fined the concentration optically active substance such as sugar in blood and urine. (ii). It is used to control the head light glare in night driving of the motor cars. (iii). Plane polarizing light is used for checking the quality of crystals. (iv). It is used in making non- glaring spectacles.
46. Describe two uses of diffraction grating?
1). Diffraction grating is used to find out the wavelength of light using the formula d sin θ = nλ (2). Just as a prism disperses white light into component colors (wave lengths) The diffraction grating can also be used to disperse spectrum into component wavelengths
47. Describe four uses of Michelson’s interference?
Following are the uses Michelson’s interference. (i). It is used for the determination of wavelength of light. (ii). Michelson’s measured the length of the standard meter in terms of wavelength of red cadmium light and proved that I standard meter = 1553163.3 wavelength of this light. (iii). It is used to observe the interference of light. (iv). It can be used for resolution of spectral lines.
48. What is the condition for diffraction?
Diffraction can take place only if the size of the obstacle is so small that it may be comparable to the wavelength of incident light. For example a slit, wire, a small bore and a straight edge. The diffraction of light takes place due to interference between rays coming from different parts of the same wave front on the basis of Huygens’s principle; therefore diffraction is a special type of interference.
49. what do you understand by thin film?
A layer of extremely small thickness of certain transparent medium is called thin film. When light falls on a thin film, interference fringes are formed due to interference of light reflected from the two surfaces of the film. Examples:- (i). Surface of soap bubble. (ii). A thin layer of oil film on the surface of water. (iii). Crakes in glass plate.
50. What will happen when light is made incident on the thin film of irregular thickness at different angles?
If white light is incident on a thin film of irregular thickness at all possible angles of incidence, then the interference is observed separately for different colors of light. It may be possible that a certain place on the film, its thickness and the angle of incidences are such that the condition of destructive interference is being satisfied for one particular while on the other side of thin film some different colors may get suitable conditions of constructive interference. Thus bright fringes of different colors are visible.
In case of any Query or Question regarding 11th Class Physics CHAPTER 9 PHYSICAL OPTICS Short Question Answers then please leave your opinion below in comments section respectively.
Your site is really helpful.