Physics

CBSE Class 12

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11.

Why does sun appear red at sunrise and sunset?

Sun appears red at sunrise and sunset because of least scattering of red light as it has the lowest wavelength.

As per Rayleigh scattering, the amount of light scattered .

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Name the essential components of a communication system.

Essential components of communication system:

1. Transmitter

2. Channel

3. Receiver

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13.

Define ionisation energy.

How would the ionisation energy change when electron in hydrogen atom is replaced by a particle of mass 200 times that of the electron but having the same charge ?

The minimum energy, required to free the electron from the ground state of the hydrogen atom, is known as ionization energy.

Ionisation energy is given by,

Since, ionisation energy is directly proportional to mass, the Ionization energy will become 200 times.

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14.

Sketch the graphs showing variation of stopping potential with frequency of incident radiations for two photosensitive materials A and B having threshold frequencies _{A} > _{B}.

(i) In which case is the stopping potential more and why?

(ii) Does the slope of the graph depend on the nature of the material used? Explain.

Graph showing the variation of stopping potential with frequency of incident radiations:

From the above graph, we can observe that,

i) The stopping potential is inversely proportional to the threshold frequency. Hence, the stopping potential is higher for metal B.

ii)The slope of the graph does not depend on the nature of the material used:

We know that,

Dividing the equation throughout by e, we get

On comparing the above equation with the straight line equation, we get

Slope of the graph =

That is, the slope of the graph does not depend on the nature of the material used.

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15.

(i) State law of Malus.

(ii) Draw a graph showing the variation of intensity (I) of polarised light transmitted by an analyser with angle () between polarizer and analyser.

(iii) What is the value of refractive index of a medium of polarising angle 60^{o}?

i) Law of Malus states that when a completely plane polarised light beam is incident on an analyser, the intensity of the emergent light varies as the square of the cosine of the angle between the plane of transmission of the analyser and the polariser.

I = I_{o} cos^{2}

ii) The variation of intensity (I) of polarised light transmitted by an analyser with angle .

iii) According to the Brewster's law, we have

The refractive index of the material is 1.73

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16.

Define modulation index. Why is it kept low ? What is the role of a bandpass filter?

Modulation index is the ratio of the amplitude of modulating signal to that of carrier wave.

Modulation index is kept low in order to avoid distortion. The low frequency modulating signal is mixed with high-frequency carrier wave, the distortion is restricted due the high-frequency carrier wave for modulation index lying between 0 and 1.

Bandpass filter rejects low and high frequencies and allows a band of frequencies to pass through.

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17.

A ray PQ incident normally on the refracting face BA is refracted in the prism BAC made of material of refractive index 1.5. Complete the path of ray through the prism. From which face will the ray emerge ? Justify your answer.

Here,

sin i_{c} =

The light will emerge out from face AC.

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18.

Calculate the de-Broglie wavelength of the electron orbitting in the n=2 state of hydrogen atom.

Given,

n =2

K.E for the second state, E_{k,}

De-Broglie wavelength,

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19.

a) Write the basic nuclear process involved in the emission of beta plus in a symbolic form, by a radioactive nucleus.

b) In the reactions given below:

Find the values of x, y and z and a, b and c.

a)

The basic nuclear process involved in the emission of beta plus in a symbolic form, by a radioactive nucleus,

In a beta-plus decay, a proton transforms into a neutron within the nucleus.

b)

i)

The corresponding y and z are 5 and 11 respectively.

Here, x is the positron.

ii)

The corresponding values of a, b and c are 10, 2 and 4 respectively.

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20.

Calculate the shortest wavelength of the spectral lines emitted in Balmer series. [ Given Rydberg constant, R = 10^{7} m^{-1}]

The formula is,

For shortest wavelength,

Therefore,

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