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# Modern Physics I (Part - 2) - Physics, Solution by DC Pandey NEET Notes | EduRev

## DC Pandey (Questions & Solutions) of Physics: NEET

Created by: Ciel Knowledge

## NEET : Modern Physics I (Part - 2) - Physics, Solution by DC Pandey NEET Notes | EduRev

``` Page 1

Introductory Exercise 30.2
Q 1.  A silver ball is suspended by a string in a vacuum chamber and ultraviolet light of wavelength
2000 Å is directed at it. What electrical potential will the ball acquire as a result? Work function of
silver is 4.3 eV.
Q 2.  1.5 mW of 400 nm light is directed at a photoelectric cell. If 0.1 % of the incident photons produce
photo electrons, find the current in the cell.
Q 3.  Is it correct to say that K
max
is proportional to f ? If not, what would a correct statement of the
relationship between K
max
and f ?
Q 4.  Light of wavelength 2000 Å is incident on a metal surface of work function 3.0 eV. Find the
minimum and maximum kinetic energy of the photoelectrons.
Q 5.  When a metal is illuminated with light of frequency f the maximum kinetic energy of the
photoelectrons is 1.2 eV. When the frequency is increased by 50% the maximum kinetic energy
increases to 4.2 eV. What is the threshold frequency for this metal.

Solutions
1.   K
max
(in eV) = E - W

= 1.9 eV
Therefore stopping potential is 1.9 V.
2.  Number of photons incident per second.

Number of electrons emitted per second = 0.1% of

?  Current = Charge (on photoelectrons per second)

= 0.48 × 10
-6
A = 0.48 ?A

3.  K
max
= E - W = hf - hf
0
= h(f - f
0
)
K
max
? (f - f
0
)
4.  K
min
=0 and K
max
= E - W
Page 2

Introductory Exercise 30.2
Q 1.  A silver ball is suspended by a string in a vacuum chamber and ultraviolet light of wavelength
2000 Å is directed at it. What electrical potential will the ball acquire as a result? Work function of
silver is 4.3 eV.
Q 2.  1.5 mW of 400 nm light is directed at a photoelectric cell. If 0.1 % of the incident photons produce
photo electrons, find the current in the cell.
Q 3.  Is it correct to say that K
max
is proportional to f ? If not, what would a correct statement of the
relationship between K
max
and f ?
Q 4.  Light of wavelength 2000 Å is incident on a metal surface of work function 3.0 eV. Find the
minimum and maximum kinetic energy of the photoelectrons.
Q 5.  When a metal is illuminated with light of frequency f the maximum kinetic energy of the
photoelectrons is 1.2 eV. When the frequency is increased by 50% the maximum kinetic energy
increases to 4.2 eV. What is the threshold frequency for this metal.

Solutions
1.   K
max
(in eV) = E - W

= 1.9 eV
Therefore stopping potential is 1.9 V.
2.  Number of photons incident per second.

Number of electrons emitted per second = 0.1% of

?  Current = Charge (on photoelectrons per second)

= 0.48 × 10
-6
A = 0.48 ?A

3.  K
max
= E - W = hf - hf
0
= h(f - f
0
)
K
max
? (f - f
0
)
4.  K
min
=0 and K
max
= E - W
-3.0 ? 3.19 eV
5.  K
max
=E - W
1.2 = E - W   ...(i)
4.2 = 1.5 E - W  ...(ii)
Solving this equation, we get W = 4.8 eV = hf
0

= 1.16 × 10
15
Hz
```
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