CLASS 8-SCIENCE Q&A..LIGHT

1.Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?
ANS: Regular reflection takes place from a smooth or a regular surface. In regular reflection, all reflected rays are parallel to each other for parallel incident rays. Irregular or diffused reflection takes place from an irregular surface. In diffused reflection, the reflected rays are not parallel to each other for parallel incident rays.

2.State the laws of reflection.
Ans:(i) The angle of reflection is always equal to the angle of iincidence.

(ii) The incident ray, the reflected ray and the normal to the reflective surface at the point of incidence all lie in the same plane.

3.Draw a labelled sketch of the human eye.

4.Why Laser light is harmful for the human eyes

Ans: Laser light is harmful for the human eyes, because its intensity is very high. It can cause damage to the retina and lead to blindness. Hence, it is advisable not to look at a laser beam directly.

Chemistry --structure of atom

Bohr’s model for hydrogen atom:-

Postulates for Bohr’s model:

 1.ectron in hydrogen atom move around nucleus in circular path of fixed radius and energy. these paths are called orbits.  

Energy of e does not change with time.

However, when electron move from lower to higher stationary state it absorbed sub amount of energy and energy release when it comes back.

3.     Frequency of radiations emitted or absorbed when transition of e occur is given by

Where, e1 & e2 is lower & higher energy state.

4.     Angular momentum of n electron in given stationary state is given by 

[Where n =1,2,3.....]

Limitation of Bohr’s model:-

1.     Bohr model fail to explain finer detail of hydrogen atom spectrum observed by spectroscopic, techniques.

2.     It fails to explain spectrum of other atom except hydrogen atom.

3.     It fails to explain splitting of the spectral lines in presence of electric (stark effect) or magnetic field ( Zeeman effect )

4.     Fell to explain formation of molecules from atoms by chemical bonding.

Dual behaviour of matter :-De-broglie explained that matter also behave like radiation and exhibit dual behavior means both like particle and wave like properties .

Relation

   Where  l =  wavelength.

             m = mass of particle ,

             v = velocity

p = momentum

Heisenberg’s uncertainty principle:- He explain that it is impossible to determine simultaneously the exact positive and exact momentum (or velocity) of an electron

Mathematical explanation(see text)

Aufbau principle   :- 

                              According to this principle in the ground state of the atoms the orbital’s are filled in order of their increasing energies means electrons enter higher energy orbital’s  so order in which orbital’s are filled is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p.

Pauli exclusion principle  :-

Two electrons in an atoms can’t have same set of 4-quantum no.

·        Only two electrons may exist in same orbital and these electrons must have opposite spin.

Hund’s  rule of maximum multiplicity :-.According to this rule pairing of  electron in the orbital’s belonging to the same sub-shell (p, d or f)

 Does not take place until each orbital belonging to that sub-shell has got one electron each i.e. it is singly occupied.

CHIMISTRY-STRUCTURE OF ATOM

Cathode ray discharge tube experiments: - 

1.     Cathode rays start from cathode and move toward anode.

2.     These rays are not visible but there behaviour can be observed with fluorescent or phosphorus sent material.

3.     In the absence of magnetic or electric field these travels in strait lines

4.     In the presence or magnetic or electric field the behaviour of cathode.

Cathode rays (electrons) do not depend on  the martial of the electrode and  nature of the gas tin the tube so electro us are basic constituent of all atoms.

 

Mass of electron

 From charge on e^- & e/m_e

 We get,                  

             M_e = 9.1094 ´ 10^-31 kG

DISCOVERY OF NEUTRON:

Discovered by Chadwick (1932).

  By bombarding a thin sheet of beryllium by alpha particles.

  Electrically neutral particles were emitted known as neutrons.

Rutherford’s nuclear model of atom

When beam of high energy α- particles was directed at gold foil then tiny flash of light observed at photographic plate.

Rutherford observed that-

1)    Most of the α-  practical  passed  through gold foil undeflected :

2)    A small fraction of α- particles was deflected by small angles.

3)    A very few α- particles (about 1 in 20000) bounced back means deflected by nearly 180⁰

^{CONCLUSION :}

1)    Most of space in atom is empty because most of α- particles passed 

undeflected.

2)    Few +ve charged α- particles were deflected.

Because + ve charge of the atom present in center in very small volume that repelled & deflected the +ve charged α- particles.

3)    Volume of nucleus is negligible as compared to total volume of atom

On the basis of observation &  conclusion Rutherford proposed model:

1)    +ve charge & most of mass present in the center of atom known as nucleus.

2)    Electrons moves around nucleus with very high speed in circular paths known as orbits.

3)    Electrons and nucleus (protons) are held together by electrostatic force of attraction 

Atomic number (Z) = no of protons in the nucleus of an atom

                                       = no of electrons in a neutral atom

Mass number (A) = number of protons (z) + number of electron (n)

Isobars :- 

              These are atoms with same mass number but different atomic no.

Isotopes: - 

               These are atoms with same atomic number but  different atomic mass no.

Planck’s Quantum theory:

Energy (E) of  quantum of  radiation is directly proportional to its frequency(n)

 i.e.            E=hn

Where,      h = planks constant = 6.626× 10^-34 js

Photo electric effect:-When a beam of light strike a metal surface then electrons were ejected. This phenomena is known as photo electric effect.

Number of electron ejected is directly proportional to intensity (or brightness) of light

     There is characteristic minimum frequency (n₀threshold frequency) below which photoelectric effect is not observed.

    If n > n₀ then electrons comes out with kinetic energy which increases with increase in frequency of light.

Kinetic energy of ejected electrons is given by-

h n = h n₀+ ½(m_eV²)