Physics Viva Q&A PDF

# ARVIND KUMAR NIT SURAT ## Experiment - 1 To determine the internal diameter and depth of a cylindrical container (like a tin can, calorimeter) using a vernier calipers and find its capacity. Verify the result using a graduated cylinder. ### Questions: 1. What is vernier scale and why is it so...

# ARVIND KUMAR NIT SURAT ## Experiment - 1 To determine the internal diameter and depth of a cylindrical container (like a tin can, calorimeter) using a vernier calipers and find its capacity. Verify the result using a graduated cylinder. ### Questions: 1. What is vernier scale and why is it so called? 2. What is meant by vernier constant? 3. If the zero of V.S. is on the left of the zero of M.S., the zero error is positive or negative? 4. How is zero error determined? 5. What is the advantage of the vernier? 6. If zero error is -0.03 cm, what is the value of zero correction? 7. How can you find the thickness of the bottom of a hollow cylinder by using vernier calipers? ### Answers 1. A vernier scale is a scale with divisions slightly smaller than those on the main scale and is movable along the main scale. It is named after the name of its inventor Pierre Vernier. 2. Vernier constant is the difference between the length of one main scale division and one vernier division. It is the least count of the instrument, because we can measure a length with this much precision. 3. Negative 4. By adjusting the lower jaws for zero thickness (or the depth gauge for zero depth), observe the vernier reading and multiply it to the vernier constant. 5. Vernier scale enables us to observe the position of its zero mark on the main scale with a precision of 1/10, 1/20 or 1/50 of the main scale division. 6. +0.03 cm. 7. First measure the inside depth of the hollow cylinder by using its depth gauge. Next measure its outside depth using the lower jaws. Subtract the former from the latter to get the thickness of the bottom. ## Experiment - 2 To determine the diameter of a given wire using a screw gauge. ### Questions: 1. Why is this instrument called a screw gauge? 2. What do you understand by pitch of a screw gauge? 3. What do you understand by least count of a screw gauge? 4. What is back-lash error and how it can be avoided? 5. What is the use of a ratchet arrangement in a screw gauge? 6. If the zero of the circular scale is ahead of the zero of the main scale by 7 divisions of the circular scale and the least count is 0.005 mm, what is the zero error and the zero correction? ### Answers 1. Because it measures the fraction of the smallest division on the main scale accurately with the help of a screw. 2. Pitch of a screw gauge is the distance through which the screw moves along its axis in one complete rotation. 3. Least count is the distance through which the screw moves along its axis in a rotation of one circular scale division. 4. Back-lash error is the error in the circular scale reading caused by no movement of the screw along its axis while we rotate it. It is due to play in the screw. It can be avoided by taking care to only advance the screw every time the final adjustment is made for finding zero error or the diameter of the wire. 5. The ratchet arrangement prevents you from accidentally pressing hard on the fixed stud by the screw while measuring zero error or the wire while measuring the diameter of the wire. 6. Zero error = -0.035 mm Zero correction = +0.035 mm. ## Experiment - 3 To determine the radius of curvature of a concave mirror using a spherometer ### Questions: 1. Why is a spherometer so called? 2. What is pitch, and how is it related to the least count? 3. Why does a spherometer have three legs? 4. What is back-lash error, and how is it avoided? ### Answers 1. Because it is used for the measurement of radii of curvature of spherical surfaces. 2. Pitch of a screw is the distance between two consecutive threads of the screw and is equal to the linear distance moved by the screw when it is given a full rotation. Pitch divided by the number of divisions on the circular scale gives the least count. 3. Three legs provide the most stable structure to stand on any surface. 4. A screw has back-lash error when it can rotate a little without moving forward. It is due to its being a loose fit in the threads of the spherometer in which it moves. It is also a necessity that it may be a loose fit; otherwise, it may not move. It is avoided by letting the spherometer hang on the screw for every reading. ## Experiment - 4 To find the time period of a simple pendulum for small amplitudes and draw the graph of the length of the pendulum against the square of the time period. Use the graph to find the length of the second's pendulum. ### Questions: 1. Time period is defined as the time interval in which the pendulum makes one oscillation. To measure it, you are advised the indirect approach of first measuring the time of 20 oscillations and then calculating the time of one oscillation, instead of simply measuring the time of 1 oscillation by the stopwatch? 2. How does it help in making accurate measurements of the time period if you measure the time of 50 oscillations instead of 20 oscillations? 3. If the length of a pendulum is (a) decreased to 1/9th (b) increased to 9 times its previous length. Then its time period becomes (Choose the correct answers in the two cases). 4. Without changing the length of your pendulum, you carry it to another place where the acceleration due to gravity is larger. * Does its time period change? If so, how? * Does the length of the seconds' pendulum change? If so, how? ### Answers: 1. Chance error of measuring a time interval by stop watch, which depends on your personal skill, remains the same whatever is the length of the time interval. By taking 20 oscillation, the fractional error (i.e. percentage error) in the measurement is smaller by a factor 1/20, as the time interval is 20 times longer. 2. When you measure time of 50 oscillations, instead of 20 you measure a time interval 2.5 times longer. Thus percentage error in measuring this time interval (and also the calculated time of one oscillation) is smaller by a factor 1/2.5. 3. ( a) 1/3rd (b) 3 times. 4. * Time period changes. Because bob accelerates faster, T decreases. * Length of second's pendulum also changes. It increases - a longer pendulum will be required for the same time period of 2 s. ## Experiment - 5 To find the weight of a given body using the law of parallelogram of vectors. ### Questions: 1. When do we say a body is at rest? 2. Why does the thread junction not come to rest at the same position always? 3. Why are the suspended weights kept away from the board or table? 4. A student has values of P = 200 g, Q = 250 g, and the angle between them is (a) 90°, (b) 60°, (c) 30°. Find the resultant by drawing a suitable parallelogram. (Take 50 g = 1 cm). 5. For pulling down a tall tree, why are ropes pulled in two different directions making an acute angle between them? ### Answers 1. A body is said to be at rest if it does not change its position relative to its surroundings with the passage of time. 2. The junction may not come to rest at the same position due to friction. 3. The weights are kept away from the table or board so as to avoid the effect of friction. 4. (a) 320 g wt. (b) 390 g wt. (c) 443 g wt. 5. The resultant force is almost equal to the sum of the individual forces, and when it falls down, it does not fall on any of the workers. ## Experiment - 6 To study the Newton's law of cooling by plotting a graph between cooling time and temperature, difference between calorimeter and surroundings. ### Questions: 1. The two graphs of water/oil are quite similar. Why? 2. Why do animals curl up and sleep in winter? 3. For the same temperature difference with the surrounding, you find that the rate of cooling of oil is faster than that of water. Why? 4. Can the doctor's thermometer be used to perform the experiment? Give a reason for your answer. 5. Why should the liquid be stirred continuously? 6. Would the nature of the graph change if a large calorimeter was taken? 7. Why should the same volume of liquid be taken in the case of oil and water? How does it help you in the comparison of the graph? ### Answers: 1. Cooling curves are similar because the rate of cooling depends on the temperature difference between the calorimeter and surroundings. 2. Animals curl up to sleep during winter. By doing so, they reduce the surface area of the exposed body and avoid the loss of heat. 3. Mass and specific heat of oil are less than that of the same loss of heat in one second, its fall of temperature is more. 4. No. The doctor's thermometer cannot be used because of the low range of temperature (say up to 44°C approximately only). Also, it has to be given a jerk to lower its reading. 5. The liquid is stirred continuously so that the exchange of heat is done soon, and equilibrium temperature is obtained. 6. No. Because its range is only from 35°C to 43°C, and its reading will not decrease with the cooling of the calorimeter. 7. So that the comparison is possible and the effect of density and specific heat on cooling can be observed. ## Experiment - 7 To determine the specific heat of a solid using the method of mixtures. ### Questions: 1. Can you find the specific heat of the brass bob by putting the cold brass bob in hot water in the calorimeter? Explain! Can you still find the final steady temperature? Why? 2. Can you use this method to determine the specific heat of a wooden bob? Explain. 3. Why does the tap water not boil at 100°C? 4. How do you measure the final temperature of the mixture? 5. Why should the mixture be stirred continuously? 6. A brass piece of 200 g at 100°C is dropped into 500 ml of water at 20°C. The final temperature is 23°C. Calculate the specific heat of brass. 7. What is meant by the statement specific heat of marble is 0.215 cal g-1 °C-1 or that of Aluminium is 900 J kg-1 °C-1? 8. Can you use this 'method of mixtures' for finding the specific heat of a liquid? Explain? 9. Is it necessary that the solid bob should be spherical in shape? ### Answers: 1. Yes. This method can be used. In this case, hotter water and the calorimeter will give heat to the colder solid brass bob. However, it will be difficult to find the steady final temperature of the mixture because the temperature of water with the bob dipped in it will keep on falling continuously. 2. No, wood is a bad conductor of heat. It cannot acquire a uniform temperature throughout. 3. The pure water boils at 100°C only when the atmospheric pressure is 76 cm of mercury. 4. The temperature of the water during stirring initially rises; becomes maximum and steady for some time and then starts falling again due to heat losses by radiation. This steady maximum temperature of the water is the final temperature of the mixture. 5. The mixture is stirred continuously to keep the temperature uniform throughout. 6. Specific heat of water = 1 cal g-1 °C-1 Let the specific heat of brass = S Heat lost by brass piece = 200 x S x (100-23) Heat gained by water = 500 × 1 × (23-20). Assuming no loss of heat to the surrounding, 500×3 15 S= = 0.098 cal g-1 °C-1 200×77 157 7. For marble of 1 gm; to raise its temperature by unity, 0.215 cals of heat are required. Similarly, 1 kg of Aluminium requires 900 J of heat to raise its temperature by one degree Celcius. 8. Yes. Instead of water, the given liquid is used. In this case, however, the specific heat of the material of solid bob is taken as known. 9. No. It can be of any shape. ## Experiment - 8 To measure extensions in the length of a helical spring with increasing load. Find the spring constant of the spring extension graph. ### Questions: 1. Why should the oscillations be small? 2. Why should oscillations be only vertical? 3. How will the time period of large vertical oscillations, but within the elastic limit compare with that for small vertical oscillations? 4. A spring, with a certain load suspended on it, is carried to the Moon. Thus, the load decreases due to less gravitation of the Moon. What change occurs in its extension? Give reasons for your answer. ### Answers: 1. If the oscillations are too large, the maximum extension of the spring during a downward swing can be beyond the elastic limit. 2. We are concerned with oscillations that occur in the suspended mass M due to the elastic force of the spring only. If there is a horizontal component of motion, somewhat like a pendulum, then the gravitational force makes the motion complicated. 3. These will be equal. The oscillations are S.H.M. If these are within the elastic limit, i.e., the maximum extension during a downward swing is within the elastic limit of the spring. For a simple harmonic motion, the time period is independent of amplitude. 4. Extension decreases due to the smaller gravitational force pulling down the spring. ## Experiment - 9 To find the time required to empty a burette, filled with water, to ½ of its volume, to ¼ of its volume, to 1/8 of its volume and so on. Then plot a graph between volume of water in the burette and time and thus study at each stage that the fractional rate of flow is same (analogy to radio-active decay). ### Questions: 1. Why does fractional rate of flow at low values of V tend to differ from its constant value during the experiment? 2. Why could fractional rate of flow at high values of V sometimes differ from its constant value during the experiment? 3. Why do we need to attach a thistle funnel in the lower end of the burette in level with its bottom mark? 4. Which time interval should be larger: (a) for the flow of water out of the burette from V = 50 mL to 40 mL. (b) for the flow of water out of the burette from V = 40 mL to 30 mL. 5. Intensity of radioactive radiation from a given sample of carbon-14 decays in a manner similar to water flow in this experiment. Identify the physical quantities in this experiment corresponding to, (a) intensity of radioactive radiation in the sample, (b) the number of carbon-14 atoms yet undecayed in the sample at a point of time, (c) half-life of decay. 6. (a) Roughly in, how many half-lives do you expect nearly complete decay of radioactivity of carbon-14 (i.e. to less than 1% of initial intensity) in a given sample? (b) Extrapolate your V versus t graph and state in how many half-lives does V reduce to about 1% of the initial value. ### Answers: 1. At low values of V, the force of surface tension becomes comparable to the pressure of the water column, which causes the flow of water. 2. At high values of V, if there is turbulent flow of water in the narrow stopper of the burette, fractional rate of flow could be too. 3. This ensures that only the pressure of the water column in the burette above the bottom mark causes the flow of water through the narrow stopper of the burette. 4. (b) is larger. The rate of flow of water at V = 40 ml is 4/5th of that at V = 50 ml because the fractional rate of flow is the same. 5. (a) Rate of flow of water. (b) Volume of water, V, in the burette at any point in time. (c) Half-life of water flow: T (1/2) or T (1/4)/2, etc. 6. (a) About 7 half-lives. ## Experiment - 10 To determine the wavelength of sound produced (i) in an air column, (ii) the velocity of sound in air at room temperature using a resonance column and a tuning fork. ### Questions: 1. A 128 Hz sound source is held over a resonance tube. What are the first and second lengths of air column at which resonance will occur at a temperature of 20°C? (The velocity of sound in air is temperature-dependent and is given by the relationship V₁ = 331.4 + 0.6 t ms¹ where t is the air temperature in degree Celsius? 2. Why do you use the difference in lengths, of resonating air column for the first and second position, for calculating the wavelength and the velocity of sound? Explain. 3. Suppose that the laboratory temperature were 5°C higher than the temperature at which you prepared this experiment, what effect would this have had on the length of the resonating air column for each reading? Explain. ### Answers: 1. 0.67 m and 2.01 m. 2. Equation (1) says that from even one length, we can determine the wavelength and hence the velocity of sound. But the antinode does not occur exactly at the open end of the tube. It is at a slight distance above it. This is approximately equal to 0.3D where D is the internal diameter of the tube. Therefore, the real length of the resonating air column is not equal to length of air column I, but is I + e. Taking the difference in the lengths of resonating air columns for two positions climate this end correction. 3. For a given source of sound, frequency is constant and hence wavelength is directly proportional to the velocity of sound. Since the velocity increases with temperature, wavelength will also increase accordingly. Now length of resonating air column L = ηλ/4. Hence, if the temperature is 5°C more, length of air column for each resonance will increase. ## Experiment - 11 To compare the frequencies of two tuning forks by finding first and second resonance positions in a resonance tube. ### Questions: 1. Should a tuning fork be set into oscillation by striking it with a rubber mallet/block or any other object? Explain. 2. For a resonance tube apparatus with a total tube length of 1m, how many resonance positions would be observed when the water level is lowered through the total length of the tube for a tuning fork with a frequency of (a) 500 Hz, (b) 1000 Hz? (Velocity of sound in air = 347 ms¹). 3. A sound source is held over a resonance tube, and resonance occurs when the surface of the water in the tube is 10 cm below the source. Resonance occurs again when the water is 26 cm below the source. If the temperature of the air is 20°C, calculate the source frequency. Velocity of sound in air at temperature t in degree celsius, is V₁ = 331.4 + 0.6 t ms-¹. ### Answers: 1. A tuning fork should be set into oscillation by striking it with a rubber mallet/block whichever is available. Striking the tuning fork with any hard object may damage the fork and cause a change in its characteristic frequency. 2. (a) 3; (b) 6 3. 1073 Hz. ## Experiment - 12 To establish graphically the relation between the tension and length of a string of a sonometer vibrating in its fundamental mode resonating with a given tuning fork. Use the graph to determine the mass per unit length of the string. ### Questions: 1. Show that both sides of equation (12.1) have the same dimension. 2. What is the purpose of the sound board in a sonometer? 3. A wire of mass 0.0003 kg/m and 0.5 m long is vibrating 200 times per second. What must be the tension in newtons? What mass hung on the wire would produce this tension? 4. If a wire 50.8 cm long produces a note of 128 Hz, a similar wire 25.4 cm long string at the same tension, should produce a note of ___________ Hz. 5. Two wire strings have the same vibrating length and are under the same tension, but one string has a linear mass density twice that of the other. Will the fundamental frequency of the string with the greater mass density be half that of the other? Explain ### Answers: 1. Tension F has the dimensions of MLT-2 and u has the dimension of u/L or ML-¹. Therefore, RHS of equation (12.1) has the dimension of _1 [MLT-2]_ ² / L _ML_ ¹ Left hand side of the equation is frequency which has the dimension of T-¹. Thus both sides of the given equation have the same dimension. 2. Soundboard communicates the vibrations of tuning fork the string. When natural frequency of the string is same as that of tuning fork, resonance takes place and paper rider flutters vigorously and falls. 3. 12N, 1.225 kg. 4. 256 Hz. 5. For constant F and L, the fundamental frequency of a string = _1_ /_√μ_ (See Eqn. 13.1 in the text). Therefore, the fundamental frequency of the string with greater mass density could be not half but √2 times the fundamental frequency of the other. ## Experiment - 13 To find the value of v for different values of u in case of a concave mirror and find its focal length (f) by plotting graph between 1/u and 1/v. ### Questions: 1. What do you mean by parallax? How is it removed between the tips of a pin and the real image of another pin? 2. How does the size of the image Change as the object is moved away from a concave mirror? 3. When will you get a virtual image of an object in a concave mirror? 4. What is the importance of determining rough focal length before starting the actual experiment? 5. You are given a round piece of mirror. How will you identify whether the mirror is plane, concave or convex. 6. Why do we use small spherical mirror? 7. Can you determine the focal length of a convex mirror using this method? Explain. 8. Suggest any other alternative graphical methods for the determination of 'f' in this experiment. 9. In this experiment if you are given a candle and a screen in place of the two pins. Can you still perform this experiment? Explain. 10. If you are given only one pin in this experiment. Can you find the focal length of the mirror? Explain. ### Answers: 1. Relative shift in the position of a body with respect to another body, on viewing it from two different stand-points, is called parallax. Parallax between the tip of the real image of a pin and the tip of another pin is removed by moving the image-pin on the optical bench till we find that their tips remain coincident as we see them from different positions by moving our head side-ways. 2. As we move an object away from a concave mirror between its pole and focus the size of its virtual image increases. On placing it at a point beyond focus the image formed is real and the size of the real image decreases as we move the object from focus to infinity. 3. We will get a virtual image from a concave mirror when the object is positioned between the focus and the pole of the mirror. 4. Rough focal-length is determined so that the object pin may be placed between f and 2 f. Thus we will manage to keep our image-pin beyond 2 f and the real image of object pin may be formed on it. 5. Place an object very close to the mirror. If its image in the mirror is enlarged, the mirror is concave if the image is diminished in size, the mirror is convex. 6. We use spherical mirrors of aperture (diameter) small in comparison to focal length, because the mirror formula is applicable only for paraaxial rays. 7. No. Because the image formed by a convex mirror is always virtual. 8. We could also determine f by plotting graphs between (i) on y-axis (uv) and (ii) on x-axis (u + v). Slope of this straight line graph passing through origin is the focal length. 9. Yes. Because the real image of candle may be obtained on screen and thus the value of u and v may be accurately determined. 10. Yes. We can obtain the real image of a pin on itself when it is placed at the centre of curvature. Thus we can determine R. Then f = R/2 ## Experiment - 14 To find the focal length (f) of a convex lens by plotting graph between 1/u and 1/v. ### Questions: 1. Give some practical uses of lenses. 2. You have a plano-convex lens having μ = 1.5 and R the radius of curvature of its spherical surface. What is the value of its focal length in terms of R. 3. Power of a lens is - 2.5 Dioptre (a) what is the focal length of the lens? (b) Is it a converging or a diverging lens. 4. Can you perform the experiment using a candle and a screen. How? 5. If a lens of μ = 1.5, be immensed in water (μ = 4/3), how will its focal length change? 6. What is the position of the object for which the image formed by a convex lens is of the same size as the object? 7. Is the image formed by a convex lens always real? 8. How will you determine the focal length of a convex lens using one pin and a plane mirror? ### Answers: 1. Lenses are used in (i) spectacles, (ii) microscopes, (iii) telescopes, (iv) Photo-cameras etc. 2. _1_ / f = (μ-1) (_1_ / R) ⇒ f = 2R 3. (a) P = -2.5 m^-1, f= _1_ / P = _1_ / 2.5 m = -40 m (b) Negative sign of focal length indicates that the lens is a diverging (concave) lens. 4. Yes. Because the image formed by a convex lens in this experiment is real, we can use a candle in place of object pin and a translucent screen in place of image-pin. 5. When in air _1_ /f = (1.5 - 1) (_1_ /R) When in water _1_ /f1 = (4/3 -1) (_1_ /R_1_) + (1 - 1/4)(_1_ /R_1_) = _1_ /8 (_1_ /R_1_) _1_ /f1 = _1_ / 5 = _1_ / 4f ⇒ f1= 4f i.e. in water the focal length will be four-times the value in air. 6. The image is the same size as object when the object is placed at 2f. 7. No. The image will be virtual when the object is placed between focus and optical centre of the lens. 8. If the object pin is placed at the focus of the lens, rays from any point of it will emerge out as a parallel beam (Fig. 14.4). Hence if the lens is backed by a plane mirror the rays will retrace their path and hence the real and inverted image of the object pin will be formed at the same position. Thus f can be measured. ## Experiment - 15 Find the focal length of a convex mirror using a convex lens. ### Questions: 1. How will you find out the focal length of a convex mirror using a spherometer? 2. Why do we always get a virtual diminished image from a convex mirror? 3. A convex mirror is used as a rear-view mirror in auto-mobiles. Why? 4. Instructions in this experiment advise that focal length of convex lens should be more than that of convex mirror. If you have a lens against this advice i.e. of focal length f₁ less than that of convex mirror (f), then can this experiment be done? If so, how? 5. A convex mirror always forms a virtual image of a real object but it can form a real image of a virtual object. Explain with the help of an example. ### Answers 1. Radius of curvature of a spherical mirror may be determined using the formula R = (h² / 6h) + l/2 where I = average distance between the legs of the spherometer and h = height of the spherical surface above any planner section (measured by spherometer), then f = R/2 2. The magnification for convex mirror is given by M = u / (u+f) The formula shows that the image formed will be virtual and diminished. 3. A convex mirror is used as a fear-view mirror in automobiles, because, the erect, diminished images formed in the mirror help in seeing the wider portion of the rear-traffic. 4. Yes. Referring to Fig. 16.1, if OL is slightly more than f, image distance LI can be as large as we like and more than R. Hence, experiment can be done even if f₁ >R/2. However, if f₁, is too small, precision of the experiment is less as the image of O at I becomes highly magnified. Procedure for doing the experiment remains the same even if f₁ < R/2. 5. Ordinarily when we place a real object in front of a convex mirror its virtual image is formed behind the mirror. But in case of the present experiment, we are forming the real image of a virtual object by the convex mirror. The virtual object is image at I formed by the lens, but rays forming that image are reflected by the mirror before reaching I. ## Experiment - 16 Determine the focal length of a concave lens by combining it with a suitable convex lens. ### Questions: 1. What are the factors on which the focal length of a lens depends? 2. Out of red and violet colour lights, which travels faster in (i) air, (ii) water? 3. Is the focal length of a lens more for red light or for violet light? 4. Can you determine rough focal length of a concave lens? 5. What is the minimum distance between an object and its real image formed by a lens? 6. In the experiment you performed, the focal length of the convex lens should be smaller than the focal length of the concave lens. How will you check this? Why is it necessary? 7. It is difficult to mount two thick lenses in contact in the same upright. Can you perform the experiment holding the lenses in separate uprights. Explain. ### Answers: 1. Focal length of a lens depends on * refractive index of the material of the lens. * refractive index of the surrounding medium. * radii of curvature of the surfaces of the lens. * wavelength of light used 2. (a) In air red and violet colour lights travel with the same speed. (b) In water red light travels faster than violet light. 3. Focal length is more for red light, because _1_ / F = (μ-1) (_1_ / R₁ - _1_ / R₂) and μ = A + B/λ² (Cauchy's formula). Red light has larger wavelength λ and hence smaller μ. Therefore, for red light the lens shows larger focal length. 4. No. Because, the image formed by a concave lens is virtual. 5. Minimum distance between an object and its real image formed by a lens = 4f 6. * The combination of the two lenses in contact should form an enlarged image of a near-by object. This ensure that the focal length of the convex lens is smaller than the focal length of the concave lens. * This is necessary because we want to form a real image with the combination. 7. Yes. When we mount the lenses in two separate uprights the real image formed by the convex lens serves as the virtual object for the concave lens which finally forms its real image. By measuring u and v. focal length can be calculated. ## Experiment - 17 To draw a graph between the angle of incidence (i) and angle of deviation (δ) for a glass prism and to determine the refractive index of the glass of the prism using this graph. ### Questions: 1. A prism made of glass {μ = 1.5) and refracting angle 60° is kept in minimum deviation position. What is the value of the angle of incidence? 2. What is the condition for the angle of minimum deviation? In particular, what is the relation of the transmitted ray to the base of the prism? 3. Find the index of refraction of a 60° prism that produces minimum deviation of 50°. 4. Is the refractive index of the glass prism different for different wavelengths? Explain. 5. A prism, n = 1.65, has a refracting angle of 60°. Calculate the angle of minimum deviation. ### Answers: 1. In minimum deviation A=2r 60 r=- = 30° 2 Sin i / Sin r = μ = Eki / Tkr ⇒ Sin i=1.5x=0.75 i = Sin¯¹ (0.75) 2. The angle of minimum deviation occurs for a particular wavelength when a ray of that wavelength passes through the prism symmetrically, i.e. parallel to the base of the prism. 3. 1.64 4. The index of refraction is slightly different for different wavelengths. When the incident beam is not monochromatic, each wavelength (colour) is refracted differently because the wave velocity is slightly different for different wavelengths in a material medium. Here different wavelengths for different colours refers to their wavelengths in air (or in vacuum). But the frequencies of the waves are unchanged, when they enter from one medium to another. Thus we can also take of different n for different frequencies (for different colours). 5. 51.2° ## Experiment - 18 To compare the refractive indices of two transparent liquids using a concave mirror and a single pin. ### Questions: 1. Where is the image formed when the object pin is placed at the centre of curvature of the concave mirror? 2. Where is the image formed when the object pin is placed beyond the centre of curvature on the principal axis of the concave mirror? 3. Where is the image formed of the object pin is placed at a distance less than half the radius of curvature of the concave mirror? Explain. 4. You have adjusted the position of the object pin after removing parallax in finding the distance h1 with empty concave mirror. Now filling the concave mirror with any transparent liquid, to which side is the image displaced - towards the concave mirror or away from the concave mirror? 5. When you move the object pin towards the mirror, does the position of the image remain fixed or the image also moves? 6. Suppose, the object pin is at a distance of 30 cm from the concave mirror and its image is at a distance of 20 cm from the concave mirror. How much distance the object pin is be moved towards the concave mirror to coincide with the image. * 10 cm, b) less than 10 cm, c) more than 10 cm 7. When you fill a concave mirror with refractive index 1.3, the value of h2 measured is 25 cm. What will be the value of h2 when the same concave mirror is filled with a liquid of refractive index 1.25. 8. Can you use this method to find the refractive index of mercury? Explain. ### Answers: 1. At the centre of curvature i.e. at the object pin itself. 2. Below the object pin at a smaller distance than the radius of curvature.