Electromagnetic Waves

In short wave communication, waves of which of the following frequencies will be reflected back by the ionospheric layer having electron density 10¹¹ per  m³ ?

• 2 MHz

• 10 MHz

• 12 MHz

• 18 MHz

X-ray beam are affected by

• electric field

• magnetic field

• both (a) and (b)

• none of these

X-rays are

• electromagnetic radiation

• stream of electron

• stream of proton

• stream of uncharged particle

An electron beam is accelerated through a potential difference of V volt. The minimum wavelength of X-rays produced is :

• $\frac{\mathrm{he}}{\mathrm{cV}}$

• $\frac{\mathrm{cV}}{\mathrm{he}}$

• $\frac{\mathrm{eV}}{\mathrm{hc}}$

• $\frac{\mathrm{hc}}{\mathrm{eV}}$

In case of linearly polarised light, the magnitude of the electric field vector

• does not change with time

• varies periodically with time

• increases and decreases linearly with time

• is parallel to the direction of propagation

The minimum wavelength of X-rays emitted by X-ray tube is 0.4125 Å. The accelerating voltage is :

• 30 kV

• 50 kV

• 80 kV

• 60 kV

Light waves of wavelength $\mathrm{\lambda }$ is incident on a metal of work function $\phi$ . Maximum velocity of the electron is :

• ${\left[\frac{2(\mathrm{h\lambda }-\mathrm{\phi })}{\mathrm{m}}\right]}^{1/2}$

• ${\left[\frac{2(\mathrm{hc}-\mathrm{\lambda \phi })}{\mathrm{m\lambda }}\right]}^{1/2}$

• $\frac{2(\mathrm{hc}-\mathrm{\lambda \phi })}{\mathrm{m}}$

• ${\left[\frac{\mathrm{hc}+\mathrm{\lambda \phi }}{\mathrm{m\lambda }}\right]}^{1/2}$

Assertion: There are very small sporadic changes in the period of rotation of the earth. 

Reason: Shi fling of large air masses in the earth's atmosphere produce a change in the moment of inertia of the earth causing its period of rotation to change.

• If both assertion and reason are true and reason is the correct explanation of assertion

• If both assertion and reason are true but reason is not the correct explanation of assertion

• If assertion is true but reason is false

• If both assertion and reason are false

The maximum distance upto which TV transmission from a TV of height h can be received is proportional to

• h^1/2

• h

• h³

• h²

Assertion:  The true geographic north direction is found by using a compass needle.

Reason: The impurities meridian of the earth is along the axis of rotation of the earth.

• If both assertion and reason are true and reason is the correct explanation of assertion

• If both assertion and reason are true but reason is not the correct explanation of assertion

• If assertion is true but reason is false

• If both assertion and reason are false