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1

JEE Main 2021 (Online) 27th August Evening Shift

MCQ (Single Correct Answer)
A monochromatic neon lamp with wavelength of 670.5 nm illuminates a photo-sensitive material which has a stopping voltage of 0.48 V. What will be the stopping voltage if the source light is changed with another source of wavelength of 474.6 nm?
A
0.96 V
B
1.25 V
C
0.24 V
D
1.5 V

Explanation

$$k{E_{\max }} = {{hc} \over {{\lambda _i}}} + \phi $$

or $$e{V_o} = {{hc} \over {{\lambda _i}}} + \phi $$

when $$\lambda$$i = 670.5 nm ; Vo = 0.48

when $$\lambda$$i = 474.6 nm ; Vo = ?

So,

$$e(0.48) = {{1240} \over {670.5}} + \phi $$ ..... (1)

$$e({V_o}) = {{1240} \over {474.6}} + \phi $$ .....(2)

(2) $$-$$ (1)

$$e({V_o} - 0.48) = 1240\left( {{1 \over {474.6}} - {1 \over {670.5}}} \right)eV$$

$${V_o} = 0.48 + 1240\left( {{{670.5 - 474.6} \over {474.6 \times 670.5}}} \right)$$ Volts

Vo = 0.48 + 0.76

Vo = 1.24 V $$ \simeq $$ 1.25 V
2

JEE Main 2021 (Online) 27th August Morning Shift

MCQ (Single Correct Answer)
In a photoelectric experiment, increasing the intensity of incident light :
A
increases the number of photons incident and also increases the K.E. of the ejected electrons
B
increases the frequency of photons incident and increases the K.E. of the ejected electrons
C
increases the frequency of photons incident and the K.E. of the ejected electrons remains unchanged
D
increases the number of photons incident and the K.E. of the ejected electrons remains unchanged

Explanation

$$\to$$ Increasing intensity means number of incident photons are increased.

$$\to$$ Kinetic energy of ejected electrons depend on the frequency of incident photons, not the intensity.
3

JEE Main 2021 (Online) 26th August Evening Shift

MCQ (Single Correct Answer)
The de-Broglie wavelength of a particle having kinetic energy E is $$\lambda$$. How much extra energy must be given to this particle so that the de-Broglie wavelength reduces to 75% of the initial value?
A
$${1 \over 9}$$E
B
$${7 \over 9}$$E
C
E
D
$${16 \over 9}$$E

Explanation

$$\lambda = {h \over {mv}} = {h \over {\sqrt {2mE} }}$$, $$mv = \sqrt {2mE} $$

$$\lambda \propto {1 \over {\sqrt E }}$$

$${{{\lambda _2}} \over {{\lambda _1}}} = \sqrt {{{{E_1}} \over {{E_2}}}} = {3 \over 4}$$, $${\lambda _2} = 0.75{\lambda _1}$$

$${{{E_1}} \over {{E_2}}} = {\left( {{3 \over 4}} \right)^2}$$

$${E_2} = {{16} \over 9}{E_1} = {{16} \over 9}E$$ (E1 = E)

Extra energy given = $${{16} \over 9}E - E = {7 \over 9}E$$
4

JEE Main 2021 (Online) 26th August Morning Shift

MCQ (Single Correct Answer)
In a photoelectric experiment ultraviolet light of wavelength 280 nm is used with lithium cathode having work function $$\phi$$ = 2.5 eV. If the wavelength of incident light is switched to 400 nm, find out the change in the stopping potential. (h = 6.63 $$\times$$ 10$$-$$34 Js, c = 3 $$\times$$ 108 ms$$-$$1)
A
1.3 V
B
1.1 V
C
1.9 V
D
0.6 V

Explanation

$$K{E_{\max }} = e{V_s} = {{hc} \over \lambda } - \phi $$

$$ \Rightarrow e{V_s} = {{1240} \over {280}} - 2.5 = $$ 1.93 eV

$$ \Rightarrow {V_{{s_1}}} = $$ 1.93 V .... (i)

$$ \Rightarrow e{V_{{s_2}}} = {{1240} \over {400}} - 2.5 = $$ 0.6 eV

$$ \Rightarrow {V_{{s_2}}} = $$ 0.6 V .... (ii)

$$\Delta$$V = $${V_{{s_1}}} - {V_{{s_2}}}$$ = 1.93 $$-$$ 0.6 = 1.33 V

Questions Asked from Dual Nature of Radiation

On those following papers in MCQ (Single Correct Answer)
Number in Brackets after Paper Indicates No. of Questions
JEE Main 2022 (Online) 24th June Morning Shift [ Memory Based ] (1)
JEE Main 2021 (Online) 1st September Evening Shift (2)
JEE Main 2021 (Online) 31st August Evening Shift (2)
JEE Main 2021 (Online) 27th August Evening Shift (1)
JEE Main 2021 (Online) 27th August Morning Shift (1)
JEE Main 2021 (Online) 26th August Evening Shift (1)
JEE Main 2021 (Online) 26th August Morning Shift (1)
JEE Main 2021 (Online) 27th July Morning Shift (1)
JEE Main 2021 (Online) 25th July Evening Shift (1)
JEE Main 2021 (Online) 25th July Morning Shift (2)
JEE Main 2021 (Online) 20th July Evening Shift (1)
JEE Main 2021 (Online) 20th July Morning Shift (1)
JEE Main 2021 (Online) 17th March Evening Shift (1)
JEE Main 2021 (Online) 16th March Morning Shift (1)
JEE Main 2021 (Online) 26th February Evening Shift (1)
JEE Main 2021 (Online) 26th February Morning Shift (1)
JEE Main 2021 (Online) 25th February Evening Shift (2)
JEE Main 2021 (Online) 25th February Morning Shift (1)
JEE Main 2021 (Online) 24th February Evening Shift (3)
JEE Main 2021 (Online) 24th February Morning Shift (1)
JEE Main 2020 (Online) 6th September Evening Slot (1)
JEE Main 2020 (Online) 6th September Morning Slot (2)
JEE Main 2020 (Online) 5th September Evening Slot (1)
JEE Main 2020 (Online) 4th September Evening Slot (2)
JEE Main 2020 (Online) 4th September Morning Slot (2)
JEE Main 2020 (Online) 3rd September Evening Slot (1)
JEE Main 2020 (Online) 3rd September Morning Slot (1)
JEE Main 2020 (Online) 2nd September Evening Slot (1)
JEE Main 2020 (Online) 9th January Evening Slot (2)
JEE Main 2020 (Online) 9th January Morning Slot (2)
JEE Main 2020 (Online) 7th January Evening Slot (2)
JEE Main 2019 (Online) 12th April Evening Slot (1)
JEE Main 2019 (Online) 12th April Morning Slot (1)
JEE Main 2019 (Online) 10th April Evening Slot (1)
JEE Main 2019 (Online) 10th April Morning Slot (1)
JEE Main 2019 (Online) 9th April Evening Slot (1)
JEE Main 2019 (Online) 9th April Morning Slot (1)
JEE Main 2019 (Online) 8th April Evening Slot (1)
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JEE Main 2019 (Online) 12th January Evening Slot (2)
JEE Main 2019 (Online) 12th January Morning Slot (1)
JEE Main 2019 (Online) 11th January Evening Slot (1)
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JEE Main 2019 (Online) 10th January Evening Slot (1)
JEE Main 2019 (Online) 9th January Evening Slot (1)
JEE Main 2019 (Online) 9th January Morning Slot (1)
JEE Main 2018 (Online) 16th April Morning Slot (1)
JEE Main 2018 (Online) 15th April Evening Slot (1)
JEE Main 2018 (Online) 15th April Morning Slot (1)
JEE Main 2017 (Online) 9th April Morning Slot (1)
JEE Main 2017 (Online) 8th April Morning Slot (1)
JEE Main 2017 (Offline) (2)
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