Modern Physics

Q1. Define Radioactivity:- The property possessed by heavy elements like uranium, thorium, radium etc, of spontaneously emitting highly penetrating and invisible rays is called radioactivity. Q. 2 . Answer the following questions. 1. What is radioactive substance? Give two examples of radioactive substance. Ans. A substance possessing the property of spontaneously emitting highly penetrating and invisible rays (radiation) is called radioactive substance. Example : Uranium, Thorium, Radium etc. 2. Describe an experiment to show that the radiations emitted by a radioactive substance are of three types. Ans. 1. A radioactive substance is kept in the narrow cavity drilled in a thick lead block. 2. The lead absorbs all the radiation emitted by the substance, except those emerging through the cavity in the upward direction. 3. The radiation emerging from the cavity of the lead block pass through a strong electric field applied between plates P1 and P2 and then fall the photographic plate p. 4. The entire apparatus is enclosed in an evacuated chamber kept in a dark room. 5. When the photographic plate is developed after exposing it to the radiation for some time, three distinct spots are observed on the plate. 6. This shows that the radiations are split up into three parts in the presence of an electric field. 7. The radiation deflected towards the negative plate is called α – radiation. 8. The radiation deflected towards the positive plate is called β – radiation. 9. The radiation which is not deflected by the electric field, is called Y- radiation.

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3. State the nature of α – particle, β – particle and Y – radiation. Ans. 1. An α – particle is the nucleus of helium. 2. A β – particle is an electron. 3. Y – radiation consists of electromagnetic waves of very short wavelength of the order of 10-10m. 4. State the laws of radioactive transformation. Ans. Laws of radioactive transformation are: 1. When an atom of radioactive element emits an α – Particle, its mass number decreases by 4 and atomic number decreases by 2. 2. When an atom of radioactive element emits a β – Particle, its mass number remains the same but atomic number increases by 1. 3. When an atom of a radioactive element emits Y – radiation, its mass number and atomic number remain unchanged. 5. State the four properties of α – Particles. Ans. 1. An α – Particle is a positively charged particle. 2. It is the nucleus of helium. 3. α – Particle have velocities ranging from 1/10th to 1/100th of the velocity of light. 4. Their ionizing power is very high. 5. Their penetrating power is small. 6. They can only pass through very thin sheets of paper. 7. They effect photographic plates. 8. They produce fluorescence in florescent substance like zinc sulphide. 6. State the four properties of β – particles. Ans. 1. An β – particle is a negatively charged particle. 2. A β – particle is an electron. 3. β – particles have very high velocities. 4. Some β – particles have velocity equal to 99% of velocity of light. 5. Their ionizing power is small compared to that of α – particles. 6. Their penetrating power is greater that that of α – particles, 7. β – particles can pass through thin sheets of metals. 8. They effect photographic plates. 9. They produce fluorescence in fluorescent substance lime zinc sulphide. 7. State the four properties of Y – radiations. Ans. 1. Y – Radiations are electromagnetic waves of very short wavelength of the order of 10-10m. 2. Being electromagnetic waves, their velocity is the same as the velocity of light. 3. Their ionizing power is very small compared to the other two particles.

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4. Their penetrating power is very high compared to the other two particles. 5. They affect photographic plates. 6. They produce fluorescence in fluorescent substance like zinc sulphide. 8. State the four properties of cathode rays. Ans. 1. Cathode rays are emitted in a direction normal to the surface of the cathode and this direction is independent of the position of the anode. 2. They travel in a straight line. 3. They exert mechanical force on an object in their path. 4. They are deflected by a magnetic field. 5. They produce fluorescence in many substance like zinc sulphide. 6. When an obstacle stops them, X – rays are produced. 9. State the four properties of X – rays Ans. 1. X – rays are electromagnetic waves of very short wavelength of the order 10-6m. 2. They posses all properties of visible light such as rectilinear propagation, reflection, refraction etc. 3. Being electromagnetic waves they travel in air or vacuum with the speed of light. 4. They are not deflected by electric or magnetic field. 5. They posses very high energy. Hence, they can penetrate through dense material such as iron, lead etc. 10. State the uses of cathode rays. Ans. Uses of cathode rays are 1. Cathode rays are used to produce X – rays. 2. Cathode rays produce fluorescence in fluorescent substances. 3. Cathode rays are used in the picture tube of T.V. receiver. 11. State the uses of X – rays Ans. Uses of X – rays are _ 1. To detect fracture of bones. 2. To detect the presence of foreign materials in a human body. 3. To detect diseases of lungs, kidneys etc. 4. To cure skin diseases and malignant tumor hidden deep within the body. 5. To distinguish real diamonds from artificial ones. 6. To detect explosives and drugs hidden in bags or packages. 7. To study the structure of crystals.

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12. Describe in brief the experiment to show that cathode rays are emitted in a direction normal to the surface of the cathode and this direction is independent of the position of the anode. Ans. In a discharge tube, even if the anode is fitted on one side, the fluorescent spot is observed in front of the cathode as shown in the figure. This shows that cathode rays are emitted in a direction normal to the position of the anode.

13. Describe in brief the experiment to show that cathode rays travel in a straight line. Ans. If an object is placed in front of the cathode in a discharge tube, a sharp shadow of the object if formed in the fluorescent region of the tube as shown in figure. This shows that cathode rays travel in a straight line.

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14. Describe in brief the experiment to show that cathode rays exert mechanical force on an object in their path. Ans. A paddle wheel free to roll on horizontal rails is kept inside the discharge tube in such a way that the cathode rays are incident only on the upper part of the wheel. When tube is operated by switching on the D.C. supply, the cathode rays fall on the wheel and hence, the wheel rolls away from the cathode. This shows that cathode rays exert mechanical force on an object in their path.

15. Describe in brief the experiment to show that cathode rays are deflected in an electric field. Ans. A uniform electric field is produced in the region between the plated P1 and P2. When the cathode rays pass through the electric field, they are deflected towards the positive plate indicating that the cathode particles are negatively charged as shown in the figure.

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16. Describe in brief the experiment to show that cathode rays are effected in a magnetic field. Ans. The experimental arrangement used to study the effect of a magnetic field on cathode rays. A strong horse shoe magnet is used to produce a magnetic field at right angles to the plane of the paper and directed inward. When the cathode rays pass through the magnetic field, they are deflected downward. Hence, the fluorescent sport is displaced downward. The direction of the deflections shows that the cathode particles are negatively charged.

Q3. Give scientific reasons. 1. X – rays are used for detection of fracture of bones. Ans. X – rays can pass through skin and flesh, but they cannot pass through bones. Hence, X – rays photographs are used for detection of fracture of bones. 2. While using X – rays for curing skin diseases or other diseases, long exposure to the X- rays should be avoided. Ans. 1. X rays kill living cells. 2. They kill healthy tissues in the part exposed to X – rays. Hence, while using X – rays for curing skin diseases or other diseases, long exposure to X – rays should be avoided. 3. X – rays are invisible. Ans. 1. X – rays are electromagnetic waves having very short wavelengths of the order 10-6 meters. 2. The only difference between X – rays and visible light is that X – rays have short wavelengths. Therefore X – rays are invisible. 4. X – rays are used to detect smuggling of gold and silver. Ans. 1. X – rays cannot penetrate through gold and silver.

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2. X – rays photograph reveals the presence of hidden metal in bags and packages of the passengers. Hence, X- rays are used to detect smuggling of gold and silver. Q4. Distinguish between1. β – particles and Cathode rays. β – Particles 1. These are electrons emitted by a radioactive atom. 2. When a tungsten plate stops β – Particles we do not get X – rays. 3. When β – Particles emitted, the atomic number of the atom increases by one unit.

Cathode rays 1. These are also electrons that come from cathode of an evacuated cathode ray tube. 2. When a tungsten plate suddenly stops cathode rays, we get X – rays. 3. Cathode rays are free electrons emitted from the cathode. Hence, there is no change in atomic number.

2. α – Particles and β – particles α – Particles 1. The α – Particles consists of two protons and two neutrons. Hence, we say it the nucleus of Helium atom. 2. α – Particles have velocity ranging from 1/10th to 1/100th of that of light. 3. α – particles have very low penetrating power. 4. α – Particles are deflected towards negative plate. 5. When a α – particles are emitted the atomic number decreases by two and atomic mass number decreases by four. 3. X – Rays and Y—Rays. X – rays 1. These are electromagnetic rays of very short wave length produced when the path of cathode rays are obstructed. 2. When x – rays fall on a metal plate, the place emits electrons. 3. X – rays have the velocity of light. 4. X – rays have the properties of light rays; they can penetrate through muscles, tissues but not through bones.

β – Particles 1. β – Particles are electrons. 2. β – Particles have very high velocity up to 99% of the velocity of light. 3. β – Particles can penetrate 100 times more than α – particles. 4. β – Particles are deflected towards anode plate. 5. When β – Particles are emitted atomic number increases by one unit; mass number does not change.

Y - rays 1. These are also electromagnetic rays of very short wave length but emitted by an atom the nucleus of an radioactive atom. 2. Y – rays do not knock out electrons from metal plate. 3. Y – rays also has the velocity of light. 4. Y – rays do not have all the properties of light but they can penetrate even bones.

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