The emission of free electrons from a metal surface when the light is incident on it is called the photoemission or the photoelectric effect. This effect led to the conclusion that light is made up of packets or quantum of energy called Photons
- A photon is an elementary particle.
- The momentum and energy of the photons are related as given below
E = p.c where
p = magnitude of the momentum
c = speed
- Irrespective of the intensity of radiation, every photon of a frequency v has the same momentum.
- The increase in the intensity of light only increases the number of photons crossing an area per unit time. It does not affect the energy of the radiation.
- A photon remains unaffected by electric and magnetic fields. It is electrically neutral.
- A photon is massless.
- They are a stable particle.
- The total energy and momentum are conserved during a photon-electron collision.
- A photon cannot decay on its own.
- The energy of a photon can be transferred during an interaction with other particles.
- A photon has a spin-1, unlike electrons which are ½ spin. Its spin axis is parallel to the direction of travel. It is this property of photons which supports the polarization of light.
Q1: Photoelectric effect was explained by
Answer: (a) Einstein
Q2: In a photoelectric experiment for 4000 Å incident radiation, the potential difference to stop the ejection is 2 V. If the incident light is changed to 3000 Å, then the potential required to stop the ejection of electrons will be
- 2 V
- Less than 2 V
- Greater than 2 V
Answer: (d) Greater than 2 V
Q3: Light of wavelength 4000 Å is incident on a sodium surface for which the threshold wavelength of photo-electrons is 5420 Å. The work function of sodium is
- 5 eV
- 3 eV
- 2.29 eV
- 0.57 eV
Answer: (c) 2.29 eV
Q4: Photocell is a device to
- Store photons
- Measure light intensity
- Convert photon energy into mechanical energy
- Store electrical energy for replacing storage batteries
Answer: (b) Measure light intensity
Q5: Photoelectrons stopping potential depends on
- Frequency of incident light and nature of the cathode material
- The intensity of the incident light
- The frequency of the incident light
- Nature of cathode material
Answer: (a) Frequency of incident light and nature of the cathode material
Q6: What is the value of the maximum kinetic energy acquired by electron due to radiation of wavelength 100 nm?
- 12 eV
- 6.2 eV
- 100 eV
- 300 eV
Answer: (b) 6.2 eV
Q7: The stopping potential value is 0.6 V when the light source is kept at a distance of 20 cm. When the source is kept at 40 cm away, the stopping potential will be
- 0.6 V
- 0.3 V
- 1.2 V
- 2.4 V
Answer: (a) 0.6 V
Q8: If the work function for a certain metal is 3.2 x 10-19 joule and it is illuminated with light of frequency 8 x 1014 Hz. The maximum kinetic energy of the photo-electrons would be (h= 6.63 x 10-34 Js).
- 2.1 x 10-19 J
- 8.5 x 10-19 J
- 5.3 x 10-19 J
- 3.2 x 10-19 J
Answer: (a) 2.1 x 10-19 J
Q9: A photocell is receiving light from a source placed at a distance of 1m. If the same source is to be placed at a distance of 2 m, then the ejected electron
- Moves in one-fourth energy as that of the initial energy
- Moves with one-fourth momentum as that of the initial momentum
- Will be half in number
- Will be one-fourth in number
Answer: (d) Will be one-fourth in number
Q10: The minimum energy required to remove an electron is called
- Stopping potential
- Kinetic energy
- Work function
- None of these
Answer: (c) Work function