Q.

An em wave is propagating in a medium with a velocity v→=vi^. The instantaneous oscillating electric field of this em wave is along +y axis. Then the direction of oscillating magnetic field of the em wave will be along             [2018]
 

1 -z direction   
2 +z direction   
3 -y direction   
4 -x direction   

Ans.

(2)

Velocity of em wave in a medium is given by v→=E→×B→

∴    vi^=(Ej^)×(B→)    [∵E→=Ej^ (Given)]

As i^=j^×k^,  so  B→=Bk^

Direction of oscillating magnetic field of the em wave will be along +z direction.