Important Notes on Modern Physics for JEE Main

Check important Modern Physics notes for JEE Main. Modern Physics is an important topic of Physics Section in JEE Main. The chapter covers at least 4-8 marks weightage in the entire JEE Main Physics Section. Below are the important points, Notes and formulae for the topic. 

Modern Physics Notes for JEE Main

Prepare the entire Modern Physics with the short notes explained below:-

Modern Physics:

• Work function  λ-threshold wavelength. v₀ – threshold frequency.
• Cesium has the minimum work function which is equal to 1.9eV
• Work function is minimum for cesium (1.9 eV)
• Photoelectric current is directly proportional to intensity of incident radiation. (v-constant)
  I = n hv : where n = number of photons per unit area per unit time.
• Photoelectrons ejected from the metal have kinetic energies which have range from 0 to KE_max
  Here KE_max = ev_s V_s – stopping potential
  So, the Stopping potential is independent of intensity of light used as V (v-constant)
• Momentum  is for one photon
  The photoelectric Einstein equation is

• Energy is given by, 
• The Force due to the radiation (Photon) (no transmission)

When light is incident perpendicularly

(1) 

(2) 

(3) Partial reflection – partial absorption

When light is incident at an angle q

(a) a = 1, r = 0

(b) r = 1, a = 0

(c) 0 < r < 1, a + r = 1

• The expression of De Broglie wavelength is -

 where V → potential difference

H-atom

• Radius and speed of electron in hydrogen like atoms in the different cases is .

(i) 

(ii) ; Z → atomic number

;n → orbit number

(iii) 

(iv) 

• Energy in the nth orbit of an atom is given by-.

• Corresponding to spectral lines the wavelength is given by

 R = Rydberg constant = 1.097 x 10⁷ m^-1

for Lyman series n₁ = 1 n₂ = 2, 3, 4 ……………

Balmer n₁ = 2 n₂ = 3, 4, 5 ……………

Paschen n₁ = 3 n₂ = 4, 5, 6 ……………

• Total number of possible transitions is  (from the nth state) [for gas having sufficient atom]
• The Lymen series is an ultraviolet, and Paschen, Brackett and Pfund series are in the infrared region. Balmer is in visible region.
• If effect of nucleus motion is considered,

Here m-reduced mass

 M-mass of nucleus

X-ray spectrum

• Minimum wavelength for x-rays

• Moseley's Lay

Where a and b are the positive numbers(constants) for one type of x-rays (independent of Z)

• Average radius of nucleus may be written as

R = R₀A^1/3, R₀ = 1.1 x 10^-15 M

A – mass number

• Binding energy of nucleus of  mass M, is given by B = (ZM_p + NM_N – M)C² and the mass defect 
• For nuclear reaction A + B → C + Q

The energy of reaction Q is given by

• α^- decay process

Q-value is

Kinetic energy of a-particle is given by

• β^- decay process

• β^- decay

Where  are atomic masses.

• Electron capture: where atomic electron is captured, X-ray are emitted.

Where  are atomic masses.

• In radioactive decay, number of nuclei at instant t is given by N = N₀ e^-lt, l-decay constant.
• Activity of sample: 
• Activity per unit mass is called specific activity.
• The Half life of an nuclei is f=given by: 
• Number of nuclei present after n half lives i.e. after a time t = n t_1/2

• A radioactive nucleus can be decayed by two different phenimenas having half lives t₁ and t₂ respectively. Effective half-life of nucleus is given by 
• The expression for the Activity: 
• The Expression after n half lives is given by: 

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