Q.No.135: - Describe the dispersion on atomic scale.

Q.No.135: - Describe the dispersion on atomic scale.

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

Q.No.134: - What is the microscopic reason of dispersion?

Q.No.134: - What is the microscopic reason of dispersion?

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

Q.No.133: - Describe the dispersion phenomena.

Q.No.133: - Describe the dispersion phenomena.

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

Q.No.132: - What will be the effect on the measure of dielectric constant if light with high frequency (electromagnetic wave with varying E and B field) passes through the material medium?

Q.No.132: - What will be the effect on the measure of dielectric constant if light with high frequency (electromagnetic wave with varying E and B field) passes through the material medium?

Ans: - When we apply a field varies at high frequency then the dipoles may not have time to respond and we can’t use the static dielectric constants in the case of a rapidly varying E and B field. Further more the value of dielectric constant varies with frequency, so that the speed of light in matter depends on the wavelength or frequency of light.

Q.No.131: - Define dielectric constant.

Q.No.131: - Define dielectric constant.

Ans: - When light (electromagnetic wave) pass through the material then the dipoles (permanent or induced) give response to the applied electric field basically the dielectric constant is in effect a measure of the response of the dipoles. 

Q.No.130: - On which thing (effect) the dielectric constant respond in a material?

Q.No.130: - On which thing (effect) the dielectric constant respond in a material?

Ans: - When light (electromagnetic wave) pass through the material then the dipoles (permanent or induced) give response to the applied electric field basically the dielectric constant is in effect a measure of the response of the dipoles. 

Q No.129: - (a):-Is it possible to have a purely electric wave propagate through empty space that is, a wave made up of an electric field but no magnetic field? (b) What about a purely magnetic wave, with a magnetic field but no electric field?

Q.No.129: - (a):-Is it possible to have a purely electric wave propagate through empty space that is, a wave made up of an electric field but no magnetic field? (b) What about a purely magnetic wave, with a magnetic field but no electric field? 
Answer: - (a) no (b) no, A purely electric wave would have a varying electric field. Such a field necessarily generates a magnetic field through Ampere's law, So a purely electric wave is impossible. In the same way, a purely magnetic wave is impossible: The varying magnetic field in such a wave would automatically give rise to an electric field through Faraday's Law.
Reference: -
Sears and Zemansky's University Physics with modern Physisc by Hugh D. Young Roger A. Freedman 12th Edition Chapter no 32 question is on page no 1096 and answer is on 1116

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

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.

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

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.

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?

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.

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?

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?

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?

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”?

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?

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?

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

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