# Q. The magnetic field associated with a light wave is given, at the origin, by $B = B_0 [\sin(3.14 \times 10^7)ct + \sin(6.28 \times 10^7)ct]$. If this light falls on a silver plate having a work function of 4.7 eV, what will be the maximum kinetic energy of the photo electrons ? $(c = 3 \times 10^{8} ms^{-1}, h = 6.6 \times 10^{-34} J-s)$

JEE Main JEE Main 2019

Solution:

## $B = B_0 \sin (\pi \times 10^7C)t + B_0 \sin (2 \pi \times 10^7C)t$ since there are two EM waves with different frequency, to get maximum kinetic energy we take the photon with higher frequency $B_{1} = B_{0} \sin \left(\pi \times10^{7} C\right)t \,v_{1} = \frac{10^{7}}{2} \times C$ $B_{2} = B_{0} \sin \left(2 \pi \times10^{7} C\right)t \,v_{2} = 10^{7} C$ where C is speed of light $C =3 \times10^{8}$ m/s $v_2 > v_1$ so KE of photoelectron will be maximum for photon of higher energy. $v_2 = 10^7 C$ Hz $v = \phi + KE_{max}$ energy of photon $E_{ph} = h_{V} = 6.6 \times10^{-34} \times10^{7} \times3 \times10^{9}$ $E_{ph} = 6.6 \times3 \times10^{-19 }J$ $= \frac{6.6 \times3 \times10^{-19}}{1.6 \times10^{-19}} eV = 12.75 eV$ $KE_{max } = E_{ph} - \phi$ = 12.375 - 4.7 = 7.675 eV $\approx$ 7.7 eV

You must select option to get answer and solution

## 1. The increase in pressure required to decrease the 200 L volume of a liquid by 0.004% (in kPa) is (Bulk modulus of the liquid ==2100 MPa)

Maharashtra CET 2006 Mechanical Properties Of Solids

## 2. Two discs of same moment of inertia rotating about their regular axis passing through centre and perpendicular to the plane of disc with angular velocities $\omega_1$ and $\omega_2$ . They are brought into contact face to face coinciding the axis of rotation. The expression for loss of energy during this process is:-

NEET 2017 System Of Particles And Rotational Motion

## 3. The period of revolution of planet A around the sun is 8 times that of B. The distance of A from the sun is how many times greater than that of B from the sun?

AIPMT 1997 Gravitation

## 4. To draw maximum current from a combination of cells, how should the cells be grouped?

AFMC 2006 Current Electricity

AFMC 2010 Atoms

## 6. The diameter of a flywheel is increased by 1%. Increase in its moment of inertia about the central axis is

JIPMER 2015 System Of Particles And Rotational Motion

## 7. The spectrum of an oil flame is an example for ...........

KCET 2010 Dual Nature Of Radiation And Matter

## 8. The temperature of 100 g of water is to be raised from 24°C to 90°C by adding steam to it. The mass of the steam required for this purpose is

JIPMER 2015 Thermal Properties Of Matter

## 9. A body of mass 4 kg is accelerated upon by a constant force, travels a distance of 5 m in the first second and a distance of 2 m in the third second. The force acting on the body is

KCET 2008 Laws Of Motion

## 10. Two equal forces are acting at a point with an angle of 60$^{\circ}$ between them. If the resultant force is equal to $40\sqrt{3}$, the magnitude of each force is

KEAM 2008 Laws Of Motion