NEET Modern Physics MCQs & PYQs

Practice NEET Modern Physics Questions

Question 51:

easy

An electron is accelerated from rest through a potential difference of V volt. If the de Broglie wavelength of the electron is $1.227 \times 10^{-2} nm$, the potential difference is:

(2020)

The de Broglie wavelength of an electron accelerated through a potential difference V is given by $\lambda = \frac{1.227}{\sqrt{V}} nm$. Given $\lambda = 1.227 \times 10^{-2} nm$. Equating the two gives $\frac{1.227}{\sqrt{V}} = 1.227 \times 10^{-2}$, which simplifies to $\sqrt{V} = 10^2$. Squaring both sides gives $V = 10^4 V$.

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Question 52:

easy

The kinetic energy of an electron, which is accelerated in the potential difference of 100 volts, is

(1997)

Kinetic energy acquired by an electron accelerated through a potential difference V is $K.E. = eV$. Here $V = 100 V$, so $K.E. = (1.602 \times 10^{-19} C) \times 100 V = 1.602 \times 10^{-17} J$.

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Question 53:

easy

An electron of mass m and charge e is accelerated from rest through a potential difference V in vacuum. Its final velocity will be

(1996)

The kinetic energy gained by the electron is equal to the work done by the electric field: $\frac{1}{2}mv^2 = eV$. Rearranging for velocity gives $v = \sqrt{\frac{2eV}{m}}$.

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Question 54:

easy

The velocity of photons is proportional to (where $\upsilon$ = frequency)

(1996)

The velocity of a photon in vacuum is the speed of light ($c$), which is a constant and independent of its frequency. Thus, it is proportional to $\upsilon^0$.

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Question 55:

easy

When light of wavelength $300 nm$ (nanometer) falls on a photoelectric emitter, photoelectrons are liberated. For another emitter, however, light of $600 nm$ wavelength is sufficient for creating photoemission. What is the ratio of the work functions of the two emitters?

(1993)

The work function is inversely proportional to the threshold wavelength: $\phi = \frac{hc}{\lambda_0}$. The ratio of the work functions is $\frac{\phi_1}{\phi_2} = \frac{\lambda_{02}}{\lambda_{01}} = \frac{600 nm}{300 nm} = \frac{2}{1}$. Thus, the ratio is 2 : 1.

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Question 56:

easy

Number of ejected photoelectrons increases with increase

(1993)

The number of photoelectrons ejected per second from a photosensitive surface is directly proportional to the intensity of the incident light, assuming the frequency is above the threshold.

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Question 57:

easy

51. The cathode of a photoelectric cell is changed such that the work function changes from $W_1$ to $W_2$ ($W_2 > W_1$). If the current before and after changes are $I_1$ and $I_2$, all other conditions remaining unchanged, then (assuming $h\nu > W_2$) (1992)

Saturation photoelectric current depends only on the intensity of the incident light (number of photons per second) and is independent of the work function of the cathode material, provided the incident frequency is above the threshold. Therefore, $I_1 = I_2$.

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Question 58:

easy

If the kinetic energy of the particle is increased to 16 times its previous value, the percentage change in the de-Broglie wavelength of the particle is

(2014)

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Question 59:

easy

The wavelength $\lambda_e$ of an electron and $\lambda_p$ of a photon of same energy E are related by:

(2013)

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Question 60:

easy

An $\alpha$-particle moves in a circular path of radius 0.83 cm in the presence of a magnetic field of $0.25 Wb/m^2$. The de Broglie wavelength associated with the particle will be:

(2012 Pre)

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