NCERT Solutions for Class 12 Help.html Physics Part I Chapter 8 Electromagnetic Waves

The threshold frequency for a metallic surface corresponds to an energy of 6.2 eV, and the stopping potential for a radiation incident on this surface 5 V. The incident radiation lies in

• X-ray region

• ultra-violet region

• infra-red region

• visible region

Who discovered X-rays?

• Roentgen

• Madam Curie

• Rutherford

•  All of these

If  ${\mathrm{\lambda }}_{\mathrm{\nu }}, {\mathrm{\lambda }}_{\mathrm{x}} \mathrm{and} {\mathrm{\lambda }}_{\mathrm{m}}$represent the wavelengths of visible light, X-rays and microwaves respectively, then

• ${\mathrm{\lambda }}_{ \mathrm{m}} > {\mathrm{\lambda }}_{ \mathrm{x}} > {\mathrm{\lambda }}_{ \mathrm{\nu }}$

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

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

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

In gamma ray emission from a nucleus

• both the neutron number and the proton number change

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

• only the neutron number changes

• only the proton number changes

Which of the following electromagnetic waves have the  smallest wavelength?

• γ - rays

• X- rays

• UV waves

• infrared rays

X-rays are used in determining the molecular structure of crystalline because

• its energy is high

• it can penetrate the material

• its wavelength is comparable to interatomic distance

• its frequency is low

Assertion:  The electromagnetic waves are transverse in nature.

Reason:  Waves of wavelength 10 µm are radiowave and microwave.

• 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.

Assertion:  UV radiation causes photodissociation of ozone into O₂ and O, thus causing damage to the stratospheric ozone layer. 

Reason:  Ozone hole is resulting in global warming and climate 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

Assertion:  Electromagnetic waves with frequencies smaller than the critical frequencies of ionosphere cannot be used for communication using sky wave propagation. 

Reason: The refractive index of the ionosphere becomes very high for frequencies higher than the critical frequency.

• 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.