Photo electric effect & Laser–9th Standard Science Question Bank

Photo electric effect & Laser :

choose the correct answer.

1. The electromagnetic radiation that can produce photoelectric effect in almost all metals is -----

a) Radio wave b)Visible light c)Infrared rays d)Gamma rays

2.A person wearing a black shirt and a white cap is standing infront of a T.V camera. The

part of the photocell which captures the image of his cap emits more number of electrons. This is related to ------

a)Frequency b)intensity c)velocity d)wavelength

3.Laser is used in finding the distance between the moon and the earth . This property of

laser used in this process is --------

a)Monochromaticity b)Non diverging nature

c)High energy density d)Higher velocity

4. Radio waves cannot produce photoelectric effect on the known metal because radio waves -----

a)have higher frequency b)have lower frequency

c)are absorbed by air d)are reflected by the metal

5.The following cannot be explained by wave nature of light ------

a)Interference b)Photo electric effect c)Diffraction d)Refraction’'


Fill in the blanks with suitable answer.

1.The energy of a photon is given by the formula -----( E = hν )

2.Light energy can be converted into electricity by --------( Photoelectric cells)

3. The minimum frequency of incident radiation to produce photoelectric effect is called ---( threshold frequency)

4. The kinetic energy of photoelectrons is directly proportional to ---------( frequency )

5. Laser light when passed through a glass prism cannot produce band of colours because it is -------( monochromatic )


Answer the following questions :

1.What is thermionic emission ?

When a metal is heated to high temperature electrons are emitted from it . This phenomenon is called thermionic emission.

2. What is photoelectric effect? Explain

This phenomenon of emission of electrons by materials under the action of light is called photoelectric effect.The electrons so emitted are called photoelectrons. Ultraviolet rays, X- rays and γ -rays also produce this effect on certain materials.

3. What are photoelectrons?

The emission of electrons by materials under the action of light are called photoelectrons .

4. Who discovered photoelectric effect ?

Photoelectric effect was discovered by Hallawaches & Hertz in 1887

5. State the experimental facts regarding photoelectric effect.

The experimental facts regarding photoelectric effect are

(i)The phenomenon is instantaneous.

(ii)There is a certain frequency called threshold frequency for radiation, below which no photoelectric effect takes place. Threshold frequency is different for different meterials.

(iii)For radiation of a given frequency, the number of photoelectrons released is proportional to the intensity of the radiation.

(iv) When radiations of different frequencies are used, the velocity (energy) of photoelectron increases with increasing frequency.

6. What are photons?

Light consists of tiny energy packets called photons.

Energy E of aphoton is given by E = hν

7. What is a photoelectric cell?

The device which convert light energy into electrical energy is called photoelectric cell.

8. Mention four applications of photoelectric cell.

i.They are used in reproduction of sound in cinematography.

ii. They are used in exposure meters.

iii. Photoelectric cells are used for automatic switching on and off of street lights.

iv. They are used in automatic control of traffic signals.

v. They are used in counting machines.

vi. Photoelectric cells are used in the operation of burglar alarm.

vii. They are used in television transmission.

9. What is a laser?

The acronym LASER stands for Light Amplification by Stimulated Emission

of Radiation or Laser is a device for producing a highly intense narrow beam

of nearly monochromatic light.

10. Explain the term spontaneous emission.

When an electron from an orbit of higher energy jumps to an orbit of lower energy , a photon of energy hν is emitted. As this takes place spontaneously, it is called spontaneous emission

11. Explain stimulated emission.

An electron in a higher energy level may remain in that level for sometime. If another

photon of energy hν = (E2 – E1) is incident on it, then the electron in the higher energy level is made to jump to the lower energy level emitting a photon of exactly the same frequency as the incident photon. This type of emission is called stimulated emission.

12. Explain the term population inversion.

The process of raising atoms from lower energy levels to higher energy levels is called population inversion.

13. what is threshold frequency?

It is the minimum frequency of the incident light below which no photo electric effect takes place.

14. State the differences between laser light and ordinaty light.

Laser light

Ordinary light

1. It is monochromatic

2. It is directional

3. It is coherent

4. It has high intensity

1. It is polychromatic

2. It is not directional

3. It is inchorent

4. It has comparatively lower intensity

15. What is dual nature of light ?

Light behaves both as wave and as well as particle. This is called dual nature of light.

16. Draw a neat labelled diagram of Helium-Neon laser tube.

In figure :

1 – Perfectly Reflecting mirror.

2 – Partially Reflecting mirror.

3 – Discharge tube.

4 – Power source.

5 – Laser beam.

6 – Helium-Neon mixure.

17. Mention four applications of lasers.

(i)The distance between two objects can be found accurately using laser reflectors. This technique is known as ‘laser ranging’. (The distance between the earth and the moon has been calculated using laser ranging technique. In 1969 astronauts of Apollo-II had left behind laser reflector on moon. A laser light is sent from the earth to the moon. Light reflected by the reflector on the moon is received. Distance to moon can we calculated knowing the time taken for to and fro journey. The distance can be measured to an accuracy of 5 cm in 4 lakh km.)

(ii) Laser is used in laser Raman spectroscopy to understand the molecular structure of a material.

(iii) It is used in laser optical surgery in welding back detached retina into proper position and save eye sight.

(iv) It is used in the treatment of dental decay and skin diseases.

(v) Laser cutting, drilling and welding have wide ranging industrial applications.

(vi) One of the most useful applications of laser is optical communication using optical fibres.

(vii) Laser is used in the measurement of pollutants in the atmosphere.

(viii) Lasers are extensively used in holography and its applications.

18. What is laser ranging ?

The distance between two objects can be found accurately using laser reflectors. This technique is known as ‘laser ranging’.


The Quanta

In 1900, Max Planck, in his Quantum hypothesis explained the spectrum of black body radiation. He postulated that “Radiation consists of packets of definite energy called ‘Quanta’ or ‘photons’. Each packet travels with the velocity of light and E the energy associated with a packet (quantum) is given by:

E=nhv where,

n = is a positive integer called quantum number

H= is a constant called the Planck's constant (= 6.626 x IO’34Js)

v= is the frequency of radiation.

Planck proposed that radiation is emitted and absorbed only in the integral multiples of'hv1.

When a molecule is in a particular quantum state, there is no change in energy.

When a molecule moves from a lower energy state to a higher energy state, energy is absorbed.

When a molecule moves from a higher energy state to a lower energy state, energy is released (emitted).

According to Planck, radiation is emitted or absorbed in integral multiples of'hv1 and not in fractions of it.

In other words a molecule can absorb or emit 1 quantum or 2 quantums of energy, but not in fractional amounts.

The energy states are represented by equally spaced horizontal lines called energy levels.

When a molecule is in quantum state with n = 1, energy associated = hv.

When a molecule is in quantum state with n=2, energy associated = 2hv.

Since molecules have only discrete values of energy, energy is said to be quantised.



The phenomenon of emission of electrons from the surface of certain substances, mainly metals when they are exposed to electromagnetic radiations (such as X-rays, light rays, UV rays, gamma rays) is called Photoelectric Effect. The electrons emitted are called Photoelectrons.

Eg., Alkali metals like Lithium, sodium, potassium emit photo electrons when exposed to visible light.

Zinc, magnesium emit photo electrons when exposed to Ultraviolet radiation.

Caesium emits photo electrons when exposed to Infrared radiation.


Laws of photoelectric emission

The following are some of the laws of photoelectric emission which have been postulated after experimental study.

Photoelectric emission is an instantaneous phenomenon. There is almost no time-lag between the incidence of radiation and emission of photoelectrons.

For every photosensitive material, there is a minimum frequency below which no photoelectric emission takes place. This frequency is called Threshold frequency. The threshold frequency is different for different metals.

[Threshold frequency is independent of the intensity of radiations.]

For a frequency greater than the threshold frequency, the number of photoelectrons emitted is directly proportional to the intensity of incident radiations.

The kinetic energy of photoelectrons emitted increases with the increase in frequency of incident radiation. The kinetic energy of photoelectrons is independent of the intensity of incident radiations and the duration of time of exposure.


Einstein's explanation of Photoelectric effect

Einstein explained the photoelectric effect by extending Planck's Quantum hypothesis. Einstein assumed that:

i. Light consists of tiny energy particles called photons.

ii. Photons propagate through space and interact with matter.

iii. Photons travel in vacuum with the velocity of light.

iv. Radiations are emitted and absorbed as photons.

v. On encountering an electron, photon interacts with it and transfers its energy completely to the electron.

vi. Electron is bound to the material. A certain amount of work has to be done to release an electron from the material surface. This work done is by the incident photon.

vii. When the frequency of incident radiation is less than the threshold frequency, the energy is insufficient to release an electron. There is no photoelectric effect.

viii. When the frequency of incident radiation is greater than the threshold frequency, a part of energy is used to liberate the electrons from the material surface. The rest of the energy is utilized in raising the kinetic energy of electrons.


Applications of Photoelectric effect

The principle of photoelectric effect is used in photoelectric cells or photocells to convert light energy into electrical energy.

Uses of Photocells:

i] Burglar Alarm: Ultraviolet rays are made to fall on a photocell, which produces current. When a person intercepts the rays, the radiation and current is cutoff thereby closing a circuit containing a bell that starts ringing.

ii] Automatic fire alarm: A photoelectric cell is located in industrial areas where there is a risk of fire accidents. In case the fire breaks out, it illuminates the cathode of the photoelectric cell. This in turn automatically rings the fire alarm which is connected in the circuit.

iii] Temperature control in furnaces: When the temperature exceeds the desired value, the radiation incident on the photocell increases and the current is cut off automatically, thus reducing the temperature of the furnace. It switches on again when the temperature falls below to the desired value.

iv] Light meters & exposure meters: Highly intense light incident on a photoelectric cell generates photoelectric current which is read on a suitable scale & the correct exposure for camera is set automatically or manually. This setup is used in TV scanning system.

v] Astronomy: The light from stars is too feeble to be detected and analysed. But this light is good enough to activate the cathode of a photoelectric cell. The cathode emits electrons whose intensities and energies are interpreted as the intensity of the stars and their temperatures.

vi] Automatic switching on and off of street lights: The photoelectric cell setup is kept facing the sky. The moment the sky is overcast with dense clouds or when it gets dark in the evening, light is cut off, causing the photoelectric current to decrease; thus in turn switching on the street lights automatically. During the day the switch is automatically put off by daylight


Properties of a Laser beam

  1. A Laser is a device which produces a highly intense beam.
  2. The light produced by a Laser is nearly monochromatic light.
  3. Laser light can travel long distances without spreading.
  4. It can be focused to give an enormous power density as high as 108 W/cm2.
  5. Laser is a coherent source of light with all waves in phase with each other.

Principle of a Laser

· The process of supplying energy from an external source, to achieve population inversion is called Optical pumping.

· Once population inversion has occurred to a sufficient extent, Laser process is started by a stray photon of suitable energy. The number of photons formed increases (photon amplification) and the photons emer­ge as a narrow beam.

· In a Laser, solid, liquid or gas can be used.

· Solid state Laser - Ruby Laser

· Gas Laser - Helium Neon Laser

Helium Neon Laser

· The Laser tube (discharge tube) contains Helium-Neon mixture in the ratio 5:1 at a definite pressure.

· The tube has two parallel mirrors, one of them partly transparent and the other perfectly reflecting each fixed at each end of the tube.

· By passing direct current through the mixture contained in the discharge tube, the mixture is ionized. This provides the energy required for pumping and therefore population inversion occurs.

· The helium atoms obtain the required energy and collides with the neon atoms, making the neon atoms to produce light.

· During stimulated emissions, photons which travel perpendicular to the mirror surfaces, move to and fro, after being reflected by the mirrors. Due to multiple reflections, the intensity of light increases. When it reaches a certain level, light streams out continuously through the partially reflecting mirror as a strong narrow beam of monochromatic light.


Uses of Lasers

  1. The distance between two objects can be found accurately using laser reflectors. This technique is called 'Laser Ranging'.
  2. A laser reflector was left behind on the moon by the astronauts of Apollo-II. A laser light is sent from the earth to the moon. Light reflected by the reflector on the moon is received. By knowing the time taken for the to and fro journey of the LASER, the distance between the earth and moon is calculated.
  3. Laser light is used in Raman spectroscopy to understand the molecular structure of a material.
  4. Laser has a wide range of use in industry in cutting, drilling and welding applications.
  5. Laser is used in optical surgery to fix a detached or a loose retina, back into place to cure a common form of partial blindness.
  6. Laser is used in a technique called holography where complete three dimensional images of an object can be seen or taken.