Find And Download : February 2013

Monday 18 February 2013

Q.No.128(b): - What is the microscopic reason of dispersion?

Ans: - The electric field of the incident field of the incident light causes the electron in an atom to oscillate with the frequency of the incident light. When the incident and scattered waves interfere then the phase of their combination depends on their phase difference and hence on the frequency. As a result, the speed of light in a material depends on the frequency or wavelength. This phenomenon is called dispersion.

Sunday 17 February 2013

Q.No.129: - What is Doppler Effect?

Ans: - The change in frequency due to the relative motion is called Doppler Effect.

Q.No.128(c): - Describe the dispersion phenomenon. Or What is the microscopic reason of dispersion? Or Describe the dispersion on atomic scale. Ans: - The electric field of the incident field of the incident light causes the electron in an atom to oscillate with the frequency of the incident light. When the incident and scattered waves interfere then the phase of their combination depends on their phase difference and hence on the frequency. As a result, the speed of light in a material depends on the frequency or wavelength. This phenomenon is called dispersion.

Q.No.128(c): -Describe the dispersion on atomic scale.

Ans: - The electric field of the incident light causes the electron in an atom to oscillate with the frequency of the incident light. When the incident and scattered waves interfere then the phase of their combination depends on their phase difference and hence on the frequency. As a result, the speed of light in a material depends on the frequency or wavelength. This phenomenon is called dispersion.

Q.No.128(a): - Describe the dispersion phenomenon.

Ans: - The electric field of the incident light causes the electron in an atom to oscillate with the frequency of the incident light. When the incident and scattered waves interfere then the phase of their combination depends on their phase difference and hence on the frequency. As a result, the speed of light in a material depends on the frequency or wavelength. This phenomenon is called dispersion.

Q.No.125: - Define phosphorescent also describe the factors upon which it depends?

Ans: - Objects that continue to glow longer than 10^-8s after the source of the excitation is removed are called phosphorescent and the materials that cause this effect is called a phosphor. For example dial of clock. It depends on the duration of light emission after the source of excitation is removed.

Q.No.126: - What are the causes of luminescence? Describe them briefly.

Ans: - Luminescence can have a variety of causes some of them are given below.

i) Chemiluminescence:- When the energy that excites the atoms originates from a chemical reaction is called Chemiluminescence .

ii) Bioluminescence:- Often the effect occurs in living things, such as in fireflies and many marine organisms, in this case it is called bioluminescence.

iii) Triboluminescence:- Light can also be emitted when certain crystals for example, sugar are crushed the effect called Triboluminescence.

Q.No.124: - What is meant by fluorescent? On which factor it depends?

Ans: - Objects in which the emission of light ceases immediately (within 10^-8s) after the source of the excitation is removed are called fluorescent. It depends upon the duration of light emission after the source of excitation is removed. The example is fluorescent lamp.

Q.No.123(a): - How fluorescent lamp works?

Q.No.123(a): - How fluorescent lamp works?

Ans:- In the case of fluorescent lamp, an electric current passed through the gas in the tube causes the electrons to move to higher energy states; when the electrons return to their original energy states, they give up their excess energy in the form of ultraviolet radiation. This radiation is absorbed by atoms of the coating on the inside of the glass tube, which then emit visible light.

Q.No.123(b): -Who fluorescent lamps radiate light?

Ans:- In the case of fluorescent lamp, an electric current passed through the gas in the tube causes the electrons to move to higher energy states; when the electrons return to their original energy states, they give up their excess energy in the form of ultraviolet radiation. This radiation is absorbed by atoms of the coating on the inside of the glass tube, which then emit visible light.

Q.No.122: - Can light emit from cool objects? If yes then what is it called?

Ans: - Yes cool bodies also emit visible light and this phenomenon is called luminescence. Examples are common fluorescent lamps, glowing of watch and clock dials, and television receivers.

Q.No.119: - Which velocity of wave may greater than the speed of light “c”?

Ans: - The phase velocity of light may increase the speed of light but the group velocity is always less than the speed of light.

Q.No.221: - What is meant by incandescent object? How can we body made incandescent?

Ans: - An object whose thermal radiation is visible is called incandescent like Sun. Typically if the temperature of the body exceeds 1000^oC then the body will become incandescent.

Q.No.120: - What are thermal radiations?

Ans: - All objects emit electromagnetic radiation, called thermal radiation, because of their temperature.

Q.No.118: - By which velocity the modulated wave (information) travels?

Ans: - The modulated information (wave) travels with group velocity not with phase velocity.

Q.No.117: - Why it is necessary to modulate radio wave?

Ans: - Because an infinitely long wave train of constant amplitude cannot carry information. The carrier of radio wave, for instance, carries no information until it is modulated.

Q.No.116: - If light emitted from a single molecule is polarized then why the light emitted from incandescent and fluorescent light bulb is unpolarized?

Ans: - Incandescent and fluorescent light bulbs contain molecules. These molecules emit polarized light individually but this light depends on the orientation of molecules and the orientation of each molecule is different in the bulb. That’s why the result of all these polarized lights coming from each molecule into unpolarized light.

Q.No.115: - Which type of light is emitted from a single molecule?

Ans: - A single molecule emits polarized light. The direction of light depends upon the orientation of molecule.

Q.No.113: - What is meant by linear polarized wave? How can we make a wave linearly polarized?

Ans: - When a wave have only one component along anyone dimension i.e. along-x, y or z-axis that wave is called linearly polarized. We can make a wave linearly polarized by using polarizing filter.

Q.No.112: - What is meant by dispersion?

Ans: - The dependence of wave speed and index of refraction on wavelength is called dispersion.

Q.No.111(c): - Why ordinary light disperses from any other dense medium?

Ans: - As we know that white light is the superposition of waves with wavelengths of visible spectrum and the speed of light in air or vacuum is the same for all wavelengths, but the speed in a material substance is different for different wavelengths. That’s why when white light passes through material medium then it splits because it doesn’t consists of single wavelength but many wavelengths. This is called dispersion

Q.No.111(b): -Explain the phenomenon of dispersion.

Ans: - As we know that white light is the superposition of waves with wavelengths of visible spectrum and the speed of light in air or vacuum is the same for all wavelengths, but the speed in a material substance is different for different wavelengths. That’s why when white light passes through material medium then it splits because it doesn’t consists of single wavelength but many wavelengths. This is called dispersion

Q.No.111(a): - Why white light splits in material than in air?

Ans: - As we know that white light is the superposition of waves with wavelengths of visible spectrum and the speed of light in air or vacuum is the same for all wavelengths, but the speed in a material substance is different for different wavelengths. That’s why when white light passes through material medium then it splits because it doesn’t consists of single wavelength but many wavelengths. This is called dispersion

Saturday 16 February 2013

Q.No.110: - How ordinary white light forms?

Ans: - Ordinary white light is the superposition of waves with wavelengths extending throughout the visible spectrum.

Q.No.109(a): - Which condition of material, refractive index, wave speed and wavelength must satisfy for total internal reflection?

Ans: - When a ray passes from two media of different refractive index. Then the medium of incoming ray must have greater refractive index than the other i.e. the medium must be denser of incoming ray. Similarly the wave speed and wavelength of incoming ray must be smaller or shorter. Furthermore total internal reflection will occur if the angle of incident is larger than or equal to critical angle.

Q.No.109: - When total internal reflection occurs?

Q.No.108: - What is meant by total internal reflection?

Ans: - If the angle of incidence is larger than the critical angle, the sine of the angle of refraction, as computed by Snell’s law, would have to be greater than unity, which is impossible. Beyond the critical angle, the ray cannot pass into the upper material; it is trapped in the lower material and is completely reflected at the boundary surface. This situation is called “total internal reflection”.

Q.No.106: - what is the relation between wave speed, refractive index, wavelength and the properties (condition) of medium. Explain.

Ans: - If the medium is dense then wave speed then wave speed in this material will low as compared to wave speed in vacuum so the ratio of speed in vacuum over dense medium will be greater than unity. This ratio is refractive index.

n = c/v = vacuum/dense medium > 1

or we can say that if refractive index is greater than unity then wave is passing through the denser medium where wave speed is less than as in vacuum.

As, v = f.λ

Frequency remains same in every material. So velocity v is directly proportional to λ (lambda) wavelength. So if speed of wave changes then wavelength also changes. On the other hand if refractive index also depends upon wavelength. So we can say that in denser medium wave speed decrease so wavelength also decrease (shorter or squeezed).

Q.No.105: - How can we say that when wave enters from one medium to the other, then the boundary of the two media will never create or destroy the wave?

Ans: - Because if it create or destroy wave then its frequency must change while the frequency before and after entering media remains same. So, we can say that boundary of media will not destroy or create the wave.

Q.No.104: - When a wave enters from one medium to the other, then the boundary of the two media will create or destroy the wave or not?

Ans: - No, the boundary will not create or destroy the way but only it will reflect or refract it.

Q.No.103: - What will happen in case of normal incidence and why reflected ray disappears?

Ans: - In case of normal incidence the incident and refracted rays lie on normal and reflected rays lie on normal and reflected ray does not disappear but it reflected backward along the same normal.

Q.No.102: - Why straw or ruler appears bent in water?

Ans: - Light rays coming from below the surface change in direction at the air-water interface, so the rays appear to becoming from a position above their actual point of origin.

Q.No.101: - On which factors the refractive index depends?

Ans: - Refractive index depends upon wave speed, material of medium and wavelength of light.

Q.No.100: - What happens when light enters into much dense material from a low dense material and vice versa?

Ans: - When light enter from rare to denser medium then ray moves towards normal and if it enters from denser to rare then ray moves away from normal.

Q.No.99: - Under what condition instantaneous velocity and average velocity of a body becomes equal?

Ans: - If a body moves with uniform velocity then the instantaneous velocity and average velocity of a body becomes equal.

Q.No.98: - What is meant by variable acceleration?

Ans: - If the change in velocity is not equal in equal interval of time then the body will accelerate with variable acceleration.

Q.No.97: - State Newton’s third law.

Ans: - Action and reaction are equal in magnitude but opposite in direction.

Q.No.107: - Is there any medium in space?

Ans: - According to the new dark matter theory some scientist said that there may be a dark matter exists in the space and dark matter is itself a medium. But before this scientist said that there is no specific medium in space.

Friday 15 February 2013

Q.No.95: - Find unit of force from Newton’s second law.

Ans: - F=ma (Newton’s law)

F=kgms^-2

So, F= kgms^-2 = N

kgms^-2 is called Newton read as ‘N’ so we can say kgms^-2 and ‘N’ are the units of force.

Q.No.94: - State Newton second law.

Ans: - A force applied on a body produces acceleration in its own direction. The acceleration produced is directly proportional to the applied force and inversely proportional to the mass of the body.

Q.No.93: - If we increase the force then acceleration increase and if we increase the mass then acceleration decrease. Drive the mathematical relation between force, mass and acceleration according to this statement.

Q.No.93: - If we increase the force then acceleration increase and if we increase the mass then acceleration decrease. Drive the mathematical relation between force, mass and acceleration according to this statement.

Ans: - Mathematically,

Q.No.92: - What happens if we increase the mass of the body which is moving with some positive acceleration? What result will you get from it?

Ans: - If is experimental fact that if we increase the mass of the body then its acceleration will decrease. So, we can say that mass and acceleration are inversely proportional to each other.

Q.No.91: - What happens with the velocity of body if an unbalanced external force exert on the body?

Ans: - An unbalanced external force always changes the velocity and thus the body will accelerate.

Q.No.91: - What happens with the velocity of body if an unbalanced external force exert on the body?

Ans: - An unbalanced external force always changes the velocity and thus the body will accelerate.

Q.No.90: - How can we increase or decrease the inertia of a body?

Ans: - Simply by increasing or decreasing the mass of body we can change the body’s inertia.

Q.No.89: - Define inertia.

Ans: - The property of an object that tends to maintain the state of rest or state of uniform motion is known as object’s inertia.

Q.No.89: - Define inertia.

Ans: - The property of an object that tends to maintain the state of rest or state of uniform motion is known as object’s inertia.

Q.No.88: - State Newton’s first law.

Ans: - A body at rest will remain at rest and a body moving with uniform velocity will continue to do so unless acted on by some unbalanced external force.

Q.No.87: - Which laws are valid in inertial frame of reference?

Ans: - Newton’s laws are valid in the inertial frame of reference.

Q.No.86: - Differentiate inertial and non-inertial frame of reference.

Ans: - “A frame of reference which is not being accelerated and it may be at rest or moving with uniform velocity is called inertial frame of reference” while such a frame of reference which has non-uniform motion but certain acceleration is called non-inertial frame of reference.

Q.No.85: - What is frame of reference?

Ans: - A set of reference axis required to locate the position of an object in space is called a frame of reference.

Q.No.84: - Do objects of different weights fall with the same acceleration on the earth?

Ans: - Yes, objects fall with the same acceleration regardless of their weights at the surface of earth in situations air friction is negligible.

Tuesday 12 February 2013

Q.No.83: - Why the direction of acceleration due to gravity ‘g’ is always downward? Explain with example.

Ans: - The gravitational acceleration depends upon the change in direction and ∆v is always downward so ‘g’ is always downward.

Example: - Let us take an example, if we through a ball upward straight with velocity 10ms^-1 after some time its velocity will decrease and we take it

8ms^-1 (for simplicity) so it’s Vi = 10ms^-1 and Vf = 8ms^-1 and

∆v=Vf-Vi

∆v=8ms^-1 – 10ms^-1

∆v=–2ms^-1

∆v have –ve sign which is opposite to its motion which means downward now we take the second trip when body starts moving downward at highest point its velocity will be zero ”0” after some time its velocity will increase and we take 2ms^-1 (for simplicity) so its Vi=0 and Vf=2ms^-1 and,

∆v=Vf-Vi

∆v=2ms^-1 – 0ms^-1

∆v=–2ms^-1

The ∆v is positive which means that gravitational acceleration will be along the direction of motion means again downward direction. Hence, ∆v is always downward so ‘g’ is always downward.

Q.No.82: - Write down the basic three equation of linear motion.

Ans: -

V_f = V_i + at

S = V_it + at²

2aS = V_f² – V_i²

Q.No.81: - If the body is moving with uniform linear velocity in magnitude but its direction is changing then it will have acceleration. Can we check this type of acceleration from v-t graph? If not then why?

Q.No.81: - If the body is moving with uniform linear velocity in magnitude but its direction is changing then it will have acceleration. Can we check this type of acceleration from v-t graph? If not then why?

Ans: - The v-t graph is drawn between the magnitudes of velocity and time. So we can’t check this type of acceleration from graph, because in this case the acceleration is produced only due to change in direction.

Q.No.80: - If a body is moving and the graph of its acceleration is parallel to the time axis, then what will be the magnitude of the acceleration?

Ans: - In this case the acceleration is parallel to the time axis means its slope will be zero so acceleration will be zero.

Q.No79: - What will be the result of distance time (x-t) graph?

Ans: - The result of x-t graph will always be velocity because,

v = x/t

Q.No.78: - What will be the result of v-t graph?

Ans: - The result of v-t graph will always be acceleration because,

a = v/t

Q.No.77: - Why we take the time along x-axis when we plot velocity time graph?

Ans: - When we plot graph then we take quantities which must be dependent and independent. The independent quantity is often taken along x-axis and dependent quantity along y-axis. Time ‘t’ is independent so we take it along x-axis.

Q.No.76: - What is meant by retardation also give an example.

Ans: - If the velocity of a body is decreasing, then its acceleration will be negative. The negative acceleration is also called retardation or deceleration. For example, when we apply breaks to the vehicle then its velocity will decrease and the vehicle will decelerate.

Q.No.75: - Define positive acceleration.

Ans: - If the velocity of a body is increasing, then its acceleration will be positive.

Q.No.74: - Define uniform acceleration.

Ans: - If the velocity of a body changes by equal amount in equal interval of time the body is said to have uniform acceleration.

Q.No.73: - If a body is moving with constant velocity (magnitude) but its direction changes with time then the body will accelerate or not? Explain.

Ans: - The body will posses acceleration if its magnitude or direction or both changes. So, in the present case when direction is changing then the body will accelerate.

Q.No.72: - What are the units and dimension of velocity? Is the direction of acceleration depends upon the direction of velocity or not?

Ans: -

Unit: - SI unit of acceleration is ms^-2.

[a] = v/t = [1/t].[d/t]

[a] = d/t² = [L/T²][LT^-2]

The direction of acceleration does not depend on velocity but on the “change in velocity”.

Q.No.67: - If a body is moving with constant speed (magnitude) but its direction is changing. Then the velocity will be uniform or not?

Ans: - No, because velocity is a vector quantity so if the direction is changing then velocity will also change. It will not be a uniform velocity.

Q.No.71: - Define acceleration.

Ans: - Time rate of change of velocity of a body is called acceleration

Mathematically,

a = ∆v/∆t

Q.No.70: - Why the average velocity will be zero along a closed path?

Ans: - Because average velocity is the ratio of total displacement over the total time taken by the body. In case of a closed path the total displacement will be zero so the average velocity will be zero.

Q.No.69: - What will be the average velocity of a body around a closed path?

Ans: - It is understood that the average velocity of a body around a closed path will be zero.

Q.No.68: - What is the difference between uniform and constant velocity?

Ans: - There is no difference between uniform and constant velocity. The body will have uniform or constant velocity if its magnitude as well as direction will remain same.

Q.No.66: - If the instantaneous velocity of a moving body does not change then which type of velocity the body will possess?

Ans: - If instantaneous velocity does not change then the body will possess uniform velocity.

Q.No.65: - Define instantaneous velocity.

Q.No.65: - Define instantaneous velocity.

Ans: - The instantaneous velocity is defined as the limiting value of velocity ∆d/∆t as the time interval ∆t, following the time t approaches zero.

Q.No.64: - Define velocity.

Q.No.64: - Define velocity.

Ans: - Time rate of change of displacement of a body is called velocity.

Q.No.63: - Show that the straight line motion of a body is a special case of displacement?

Ans: - When a body moves along a straight line then we can’t distinguish between distance and displacement because we take the value of distance and displacement is the same way i.e. final minus the initial position. In case of straight line motion the magnitude of both distance and displacement is same.

Q.No6.2: - How can we find the direction of a body?

Ans: - We can find the direction of a body by taking difference between the final and initial positions of the body’s motion with some chosen axis.

Q.No6.1: - Define Displacement.

Ans: - The Shortest distance between two points is called displacement. It is a vector quantity.

Sunday 10 February 2013

Q.No.60: - If we said one plane contains incident, reflected, refracted and normal, the other plane is the boundary surface between two materials. Then at which direction these two planes oriented with respect to each other?

Q.No.60: - If we said one plane contains incident, reflected, refracted and normal, the other plane is the boundary surface between two materials. Then at which direction these two planes oriented with respect to each other?

Ans: - These two planes must held perpendicular to each other.

Q.No.59: - What is the law of reflection?

Ans: - The incident, reflected, refracted rays and normal to the surface all lie in the same plane.

Q.No.58: - What is the relation between wave speed and refractive index?

Ans: - The refractive index and wave speed are inversely proportional to each other as speed of light in vacuum is constant.

Q.No.57: - What is the SI unit of refractive index?

Ans: - It is a unit less quantity because it is a ratio of similar quantities.

Q.No.56: - When light travels from any material medium then why refractive index increases from unity and why refractive index is unity when light passes through vacuum?

Ans: - The light travels more slowly in materials so “v” decreases while “c” remains constant. So the ratio n=c/v increases as denominator decreases. On the other hand when light travels in vacuum then v=c and refractive index becomes n=c/v=v/v=1

The refractive index “n” is unity when light travels through vacuum.

Q.No.55: - How can we find the refractive index of an optical material?

Ans: - The refractive index of any optical material can be found by computing the ratio of c/v the speed of light in vacuum over material.

n = c/v (n is refractive index)

Q.No.54: - How things appear visible?

Ans: - Things are visible because they reflect light in diffuse manner from their surface.

Q.No.53: - Which materials support diffuse and specular reflection and which condition must satisfy for it?

Ans: - Both kinds of reflection can occur with either transparent material or opaque material but they do not transmit light.

Q.No.52: - What is the difference between the specular reflection and diffuse reflection?

Ans: - Reflection at a definite angle from a very smooth surface is called specular reflection while scattered reflection from a rough surface is called diffuse reflection.