Physics Part I Chapter 8 Electromagnetic Waves

B.

ultra-violet region

A.

Roentgen

Wilhelm Conrad Roentgen. Wilhelm Roentgen, a German professor of physics was the first person to discover electromagnetic radiation in a wavelength range commonly known as X-rays today. Although, many people had observed the effects of X-ray beams before, but Roentgen was the first one to study them systematically.

C.

${\mathrm{\lambda }}_{\mathrm{m}}> {\mathrm{\lambda }}_{\mathrm{v}} > {\mathrm{\lambda }}_{\mathrm{x}}$

Wavelength order of given rays are listed below:

Obviously, ${\mathrm{\lambda }}_{\mathrm{x}}< {\mathrm{\lambda }}_{\mathrm{\nu }}< {\mathrm{\lambda }}_{\mathrm{m}}$

$\mathrm{or} \mathrm{\lambda }{ }_{\mathrm{m}}> {\mathrm{\lambda }}_{ \mathrm{v}} > {\mathrm{\lambda }}_{ \mathrm{x}}$

Visible light, X-rays and microwaves are all electromagnetic waves.

B.

there is no change in the proton number and the neutron number

In gamma-ray emission the energy is released from the nucleus, so that nucleus get stablished. The emission of gamma rays does not alter the number of protons or neutrons in the nucleus but instead of moving the nuleus from higher to lower energy state (unstable to stable).

A.

γ - rays

γ - rays wavelength range ⇒  < 10^-3 nm

UV wavelength range ⇒ 400 nm to 1 nm

X -rays wavelength range ⇒ 1 nm to 10^-3 nm

Infra-red rays wavelength range ⇒ 1 mm to 700 nm

So γ-rays have the smallest wavelength.

C.

its wavelength is comparable to interatomic distance

Crystal structure is explored through the diffraction of waves having a wavelength comparable with the interatomic spacing ( 10^-10 m) in crystals. Radiation of longer wavelength cannot resolve the details of  structure, while radiation of much shorter wavelength is diffracted through in conveniently small angles. Usually diffraction of X-rays is employed in the study of crystal structure as X-rays have wavelength comparable to interatomic spacing.

C.

If assertion is true but reason is false.

10μm wavelength lies in infrared region.

While range of Radio wave is > 0.1 m and range of microwave is lies between 0.1m to 1 mm.

Hence assertion is true but reason is false.

C.

If assertion is true but reason is false

UV radiation does not cause photodissociation of ozone into O₂ and O instead it takes pan in the formation of ozone into two steps. In the first step, UV radiation breaks apart the two atoms of oxygen and in the second step one of the liberated atoms ( O ) joins another oxygen molecule ( O₂ ) to form ozone ( O₃ )

          ${\mathrm{O}}_2 \stackrel{\mathrm{UV} \mathrm{radiation} }{\to } \mathrm{O} + \mathrm{O}$

           $$\begin{gathered} {\mathrm{O}}_2 + \mathrm{O} \stackrel{\mathrm{UV} \mathrm{radiation}}{\to } {\mathrm{O}}_3 \\ \left(\mathrm{ozone}\right) \end{gathered}$$

human activities, the ozone layer in the stratosphere becomes start thinning which is also called ozone hole.  Ozone hole is resulting in rain failure, increase in radiation, cancers (skin) and reduction in crop production. Global warming and climate changes are mainly due to green-house gases ( CO₂, NO_x ). 

D.

If both assertion and reason are false.

The highest frequency above which the ionosphere no longer returns the sky wave back to earth when transmitted in vertical direction is called critical frequency, so for sky wave communication, frequency of electromagnetic wave should be less than the
critical frequency. 

Refractive index of a medium is given by

          μ = A + $\frac{\mathrm{B}}{{\mathrm{\lambda }}^2}+ \frac{\mathrm{C}}{{\mathrm{\lambda }}^4} + ....$