1

### JEE Main 2017 (Online) 9th April Morning Slot

The electron in the hydrogen atom undergoes transition from higher orbitals to orbital of radius 211.6 pm. This transition is associated with :
A
Lyman series
B
Balmer series
C
Paschen series
D
Brackett series

## Explanation

Here electron is coming to the orbital of radius 211.6 pm. Now we have to find which series have radius of orbital 211.6 pm.

We know,

Radius, r = $0.529 \times {{{n^2}} \over Z}\,\mathop A\limits^ \circ$

Given, r = 211.6 pm = 211.6 $\times$ 10-12 m

and Z = 1 for hydrogen atom

$\therefore$ 211.6 $\times$ 10-12 = $0.529 \times {{{n^2}} \over 1}$ $\times$ 10-10

$\Rightarrow$ n = 2

As n = 2 so the series is Balmer Series.
2

### JEE Main 2018 (Online) 15th April Morning Slot

Ejection of the photoelectron from metal in the photoelectric experiment can be stopped by applying 0.5 V when the radiation of 250 nm is used. The work function of the metal is :
A
4 eV
B
4.5 eV
C
5 eV
D
5.5 eV

## Explanation

$\lambda$ = 250 nm = 2500 $\mathop A\limits^ \circ$

E = ${{hc} \over \lambda }$ = ${{12400} \over {2500}}$ = 4.96 eV

K. E = 0.5 eV

As, $\,\,\,\,\,\,$ K. E = E $-$ $\omega$0

$\Rightarrow$ $\,\,\,$ 0.5 = 4.96 $-$ ${\omega _0}$

$\Rightarrow$ $\,\,\,$ ${\omega _0}$ = 4.46 eV $\simeq$ 4.5 eV
3

### JEE Main 2018 (Online) 15th April Evening Slot

The de-Broglie's wavelength of electron present in first Bohr orbit of 'H' atom is :
A
0.529 $\mathop {\rm A}\limits^ \circ$
B
2$\pi$ $\times$ 0.529 $\mathop {\rm A}\limits^ \circ$
C
${{0.529} \over {2\pi }}$ $\mathop {\rm A}\limits^ \circ$
D
4 $\times$ 0.529 $\mathop {\rm A}\limits^ \circ$

## Explanation

Radius    r = 0.529 $\times$ ${{{n^2}} \over z}$

In first Bohr orbit of hydrogen atom radius,

r = 0.529 $\times$ ${{{1^2}} \over 1}$ = 0.529 $\mathop A\limits^ \circ$

Angular momentum,

mvr = ${{nh} \over {2\pi }}$

$\therefore\,\,\,$ for    n = 1,

mvr = ${h \over {2\pi }}$

$\Rightarrow$$\,\,\,$ 2$\pi$r = ${h \over {mv}}$ = $\lambda$   [ as   $\lambda$ = ${h \over {mv}}$]

$\therefore\,\,\,$ $\lambda$ = 2$\pi$r = 2 $\pi$ $\times$ 0.5 29
4

### JEE Main 2018 (Online) 16th April Morning Slot

Which of the following statements is false ?
A
Photon has momentum as well as wavelength.
B
Splitting of spectral lines in electrical field is called Stark effect.
C
Rydberg constant has unit of energy.
D
Frequency of emitted radiation from a black body goes from a lower wavelength to higher wavelength as the temperature increases.

## Explanation

When a black body is heated, black body emit high energy radiation, from higher wavelength to lower wavelength.