 # Modern Physics formulae for NEET

Modern physics is a branch of physics which deals with the post-Newtonian concepts in the world of physics. It is based on the two major breakthroughs of the twentieth century: Relativity and Quantum Mechanics.

1. The work function is minimum for caesium (1.9 eV)

2. Work function W = hv0 = hc/λ0

3. Photo effect is directly proportional to the incident radiation

4. Photoelectrons ejected from metal have kinetic energies ranging from 0 to KEmax

Here KEmax = eVs

5. Stopping potential (Vs ) is independent of the intensity of light

6. The intensity in terms of an electric field is given by

I = ½ Є0E2C

7. The momentum of one photon is h/λ

8. Einstein equation for the photoelectric effect

hv = w0 + KEmax

9. De Broglie wavelength

λ = h/mv = h/p

10. Radius and speed of the electron in hydrogen-like atoms

rn = (n2/Z )a0

a0 = 0.529 Å

vn = (Z/n) v0

v0 = 2.19 x 106 m/s

11. Energy in nth orbit

En = E1.(Z2/n2)

E1 = -13.6 eV

12. Wavelength corresponding to spectral lines

1/λ = R [1/n12 – 1/n22]

For layman series n1 = 1 n2= 2,3,4…..

Balmer series n1 = 2 n2= 3,4, 5…..

Paschen n1= 3 n2 = 4,5,6….

The Lyman series is in ultraviolet and Paschen, Brackett, Pfund series in the infrared region

13. If the effect of nucleus motion is considered

rn=(0.529 Å) (n2/Z) (m/μ)

En= (-13.6 eV) (Z2/n2)(μ/m)

μ is the reduced mass

14. Minimum wavelength for x-rays

λmin = hc/eV0 = 12400/V0 Å

15. Moseley’s Law

√v = a(z – b)

a and b are positive constants

16. Average radius of the nucleus may be written as

R = R0A1/3

R0 = 1.1 x 10-15 M

A – mass number

17. Binding energy of nucleus of mass M is given by

B = (ZMp + NMN – M)C2

18. Alpha decay process

zXA ➡️ z-2 Y A-4 + 2He4

Q value = [m(zXA) – m(z-2 Y A-4) -m(2He4)]C2

19. Beta-minus decay

zXA ➡️ z+1 Y A + β + v

Q value = [m(zXA) – m(z+1 Y A)]C2

Beta plus decay

zXA ➡️ z-1 Y A + β+ + v

Q value = [m(zXA) – m(z-1 Y A) – 2me)]C2

20. Electron capture

zXA + e ➡️ z-1 Y A + v

Q value = [m(zXA) – m(z-1 Y A)]c2

21. In radioactive decay, the number of nuclei at an instant is given by N = N0 e-λt

22. Activity of sample A = A0e-λt

23. Activity per unit mass are called specific activity

24. Half life T1/2 = 0.693/λ

25. Average life Tav= T1/2 /0.693

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