What are the characteristics of waves?


The following parameters are used to characterize these waves:

(i) Wavelength: It is defined as the distance between two adjacent crests or troughs of the wave. It is denoted by lambda (λ). It is generally expressed in meters; Angstroms (Å) or nanometres (nm).
1 Å = 10-10m
1 nm = 10-9 m

(ii) Frequency: It is defined as number of waves which pass through a given point in one second. It is denoted by v (nu) and is measured in cycles per second (cps) or simply reciprocal seconds (sec-1). It is also expressed in terms of Hertz (Hz).



It is inversely proportional to the wavelength.



(iii) Velocity: The distance travelled by the wave in one second is called the velocity of the wave. It is denoted by c and is equal to the product of wavelength and frequency.
c =vλ

The units of velocity are ms-1 or cm s-1.
Light and all other types of electromagnetic radiations travel through space with the same velocity i.e. 3 x 10-8 ms-1.

(iv) Amplitude: The height of the crest or the depth of the trough or distance of maximum displacement is called amplitude. It is denoted by a. It determines the density or brightness of a beam of light.

(v) Wave number: It is defined as the number of wavelengths per unit of length and is equal to the inverse of wavelength expressed in centimeters. It is denoted by ⊽ cm-1 or in-1.
                                                    ...(1)
But                          straight c space equals vλ
or                                                ...(2)
therefore  From (1) and (2),
                            
or            

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How many neutrons and protons arc there in the following nuclei:
 space left parenthesis straight i right parenthesis space straight C presubscript 06 presuperscript 12 space space space space space space left parenthesis ii right parenthesis space straight O presubscript 08 presuperscript 16 space space space space space space left parenthesis iii right parenthesis space Mg presubscript 12 presuperscript 24 space space space left parenthesis iv right parenthesis space Fe presubscript 26 presuperscript 56 space space space space space space space space space left parenthesis straight v right parenthesis space Sr presubscript 38 presuperscript 88


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What are atomic number and mass number? How are they related to each other?


Atomic number (Z): In 1913, Mosley devised an experiment to find the positive charge on the nucleus of an atom. Atomic number of an element is equal to the number of unit positive charges or protons present in the nucleus of an atom of that element.

An atom is electrically neutral, therefore, in an atom,
Number of protons = No. of electrons
Atomic number = No. of protons in the nucleus of the atom
= No. of electrons present outside the nucleus of the same atom.
Atomic number is a fundamental property of the atom and is generally denoted by Z.

Mass number (A):The mass of an atom is mainly concentrated in the nucleus. In the nucleus, there are protons and neutrons, these are collectively called nucleons. Mass number of an element is the sum of the number of protons and neutrons present in its atom. It is generally denoted by A.
Mass No. (A) = No. of protons + No. of neutrons = No. of nucleons.

Relation between mass number and atomic number:

We know,

Mass number (A) = No. of protons + No. of neutrons
= At. No. (Z) + No. of neutrons.
No. of neutrons = Mass No. (A) - Atomic No. (Z)

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How was neutron discovered? Describe briefly.


It was Rutherford who showed that almost the entire mass of an atom is concentrated in the nucleus. However, he could not explain the total known mass of the atom in terms of the number of protons in the nucleus. For example, helium nucleus has a charge of 2+ which is furnished by 2 protons, also contributes approximately 2 units of mass. As the mass of helium atom is 4 a.m.u., therefore 2 units of mass are still to be accounted for.


Rutherford postulated the existence of a neutral particle to account for the missing mass. Chadwick in 1932, actually discovered this neutral particle by bombardment of beryllium or boron by alpha particles. It has almost exactly the same mass as of a proton but carried no electric charge. These neutral particles were called neutrons.
Thus, a neutron is a neutral particle having a mass (1.675 x 10-27 kg) which is almost exactly the same as that of a proton. On the a.m.u. scale, the mass of the neutron is 1.00867.

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Describe the nature of light and characteristics of radiant energy.

Light is an important form of energy, According to Newton's corpuscular theory,the light was considered as a stream of particles called corpuscles of light. This theory could explain the phenomenon of reflection and refraction, but failed to explain the phenomenon of diffraction and interference. Therefore, the corpuscular theory was discarded and replaced by the wave theory of light. According to this theory, light is transmitted in the form of electromagnetic waves. These waves are associated with the oscillating electric field and magnetic fields.

The energy emitted from any source in the form of radiation is called radiant energy. The most common forms of radiant energy are cosmic rays, gamma rays, X-rays, ultraviolet rays, visible rays, infrared rays, radio waves and heat. Clark Maxwell (1856) proposed that radiant energy has wave character. These waves are associated with both electric and magnetic fields, thus, these radiations are called electromagnetic radiations. All types of electromagnetic radiations travel through space with the same velocity i.e. the velocity of light (3 x 108 ms-1).

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