Due to Doppler's effect the shift in wavelength observed is $$0.1 \mathop A\limits^o$$ for a star producing wavelength $$6000 \mathop A\limits^o$$. Velocity of recession of the star will be

An electromagnetic wave is travelling in $$x$$-direction with electric field vector given by, $$\mathbf{E}_y=E_0 \sin (k x-\omega t) \hat{\mathbf{j}}$$. The correct expression for magnetic field vector is

The phenomenon involved in the reflection of radio-waves by ionosphere is similar to