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

Define the term: stopping potential in the photoelectric effect.

Explain how does (i) photoelectric current and (ii) kinetic energy of the photoelectrons emitted in a photocell vary if the frequency of incident radiation is doubled, but keeping the intensity same?

Show the graphical variation in the above two cases.

Light of wavelength 2000 Å falls on a metal surface of work function 4.2 eV.

What is the kinetic energy (in eV) of the fastest electrons emitted from the surface?

Light of wavelength 2000 Å falls on a metal surface of work function 4.2 eV.

What will be the change in the energy of the emitted electrons if the intensity of light with same wavelength is doubled?

Light of wavelength 2000 Å falls on a metal surface of work function 4.2 eV.

If the same light falls on another surface of work function 6.5 eV, what will be the energy of emitted electrons?

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.

(i) A ray of light incident on face AB of an equilateral glass prism, shows minimum deviation of 30°. Calculate the speed of light through the prism.

(ii) Find the angle of incidence at face AB so that the emergent ray grazes along the face AC.

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 4.50 eV. Find the frequency of light to be used to eject electrons from the metal surface with a maximum kinetic energy of 6.06 × 10⁻¹⁹ J.

1. Calculate the frequency of a photon of energy 6.5 × 10⁻¹⁹ J.
2. Can this photon cause the emission of an electron from the surface of Cs of work function 2.14 eV? If yes, what will be the maximum kinetic energy of the photoelectron?

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?