81.

If v_g, v_x and v_m are the speeds of gamma rays, X rays and microwaves respectively in vacuum then :

• v_g > v_m > v_x

• v_g > v_x > v_m

• v_g = v_m = v_x

• none of these

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

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

• $\frac{\mathrm{he}}{\mathrm{cV}}$

• $\frac{\mathrm{cV}}{\mathrm{he}}$

• $\frac{\mathrm{eV}}{\mathrm{hc}}$

• $\frac{\mathrm{hc}}{\mathrm{eV}}$

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

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

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

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

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

Spectrum of a star is shifting towards voilet colour, then the star

• is going away from earth

• is coming towards earth

• intensity of light of star is increasing

• intensity of light of star is decreasing

X-rays are not used for radar purposes because they are not

• reflected by target

• partly absorbed by target

• electromagnetic waves

• completely absorbed by target

The minimum wavelength of X-ray emitted by X-ray tube is 0.4125 $\stackrel{\circ }{\mathrm{A}}$. The accelerating voltage is

• 30 kV

• 50 kV

• 80 kV

• 60 kV

The nature of light waves is similar to

• cosmic rays

• cathode rays

• gamma rays

• alpha rays

The energy of an X-ray photon is 2 keV, then its frequency will be

• 3.2 x 10^-6 per second

• 5 x 10¹⁷ per second

• 2 x 10¹⁷ per second

• 2 × 10¹⁸ per second

The speed of electromagnetic waves in vaccum is

• µ₀ε₀

• $\frac{1}{\sqrt{{\mathrm{\mu }}_0{\mathrm{\epsilon }}_0}}$

• $\sqrt{{\mathrm{\mu }}_0{\mathrm{\epsilon }}_0}$

• $\frac{1}{{\mathrm{\mu }}_0{\mathrm{\epsilon }}_0}$