Science (English Medium) Class 12 - CBSE Important Questions for Physics

The work function for a metal surface is 4.14 eV. The threshold wavelength for this metal surface is ______.

Radiation of frequency 10¹⁵ Hz is incident on three photosensitive surfaces A, B and C. Following observations are recorded:

Surface A: no photoemission occurs

Surface B: photoemission occurs but the photoelectrons have zero kinetic energy.

Surface C: photo emission occurs and photoelectrons have some kinetic energy.
Using Einstein’s photo-electric equation, explain the three observations.

The graph shows the variation of photocurrent for a photosensitive metal

1. What does X and A on the horizontal axis represent?
2. Draw this graph for three different values of frequencies of incident radiation ʋ₁, ʋ₂ and ʋ₃ (ʋ₃ > ʋ₂ > ʋ₁) for the same intensity.
3. Draw this graph for three different values of intensities of incident radiation I₁, I₂ and I₃ (I₃ > I₂ > I₁) having the same frequency.

An alpha particle is accelerated through a potential difference of 100 V. Calculate:

1. The speed acquired by the alpha particle, and
2. The de-Broglie wavelength is associated with it.

(Take mass of alpha particle = 6.4 × 10⁻²⁷ kg)

Give an example each of a metal from which photoelectric emission takes place when irradiated by

1. UV light
2. visible light.

The work function of a metal is 2.31 eV. Photoelectric emission occurs when the light of frequency 6.4 × 10¹⁴ Hz is incident on the metal surface. Calculate

1. the energy of the incident radiation,
2. the maximum kinetic energy of the emitted electron and
3. the stopping potential of the surface.

A photon of wavelength 663 nm is incident on a metal surface. The work function of the metal is 1.50 eV. The maximum kinetic energy of the emitted photoelectrons is ______.

The energy of a photon of wavelength 663 nm is ______.

The figure shows a plot of stopping potential (V₀) versus `1/lambda`, where λ is the wavelength of the radiation causing photoelectric emission from a surface. The slope of the line is equal to ______.

An increase in the intensity of the radiation causing photo-electric emission from a surface does not affect the maximum K.E. of the photoelectrons. Explain.

The photon emitted during the de-excitation from the first excited level to the ground state of a hydrogen atom is used to irradiate a photocathode in which the stopping potential is 5 V. Calculate the work function of the cathode used.

E, c and `v` represent the energy, velocity and frequency of a photon. Which of the following represents its wavelength?

Plot a graph showing the variation of photoelectric current, as a function of anode potential for two light beams having the same frequency but different intensities I₁ and I₂ (I₁ > I₂). Mention its important features.

A metallic plate exposed to white light emits electrons. For which of the following colours of light, the stopping potential will be maximum?

• Assertion (A): For the radiation of a frequency greater than the threshold frequency, the photoelectric current is proportional to the intensity of the radiation.
• Reason (R): Greater the number of energy quanta available, the greater the number of electrons absorbing the energy quanta and the greater the number of electrons coming out of the metal.

Define ionization energy.

Calculate the shortest wavelength of the spectral lines emitted in Balmer series.

[Given Rydberg constant, R = 10⁷ m^–1]

Given the ground state energy E₀ = - 13.6 eV and Bohr radius a₀ = 0.53 Å. Find out how the de Broglie wavelength associated with the electron orbiting in the ground state would change when it jumps into the first excited state.

State Bohr’s postulate of hydrogen atom which successfully explains the emission lines in the spectrum of hydrogen atom. Use Rydberg formula to determine the wavelength of H_α line. [Given: Rydberg constant R = 1.03 × 10⁷ m⁻¹]

A 12.5 eV electron beam is used to bombard gaseous hydrogen at room temperature. Upto which energy level the hydrogen atoms would be excited? Calculate the wavelengths of the first member of Lyman and first member of Balmer series.