Planck's constant
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Key Questions

Planck's constant is used to relate the energy of light to its frequency, using the equation
E = h𝛎
Where E is the energy of a photon in Joules, h is Planck's constant (6.626 x
#10^(34)# J s) and 𝛎 is the frequency of light in Hertz (or /sec).OK.

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Answer:
Planck's constant is instrumental and an unavoidable constant which appears in quantum mechanics.
Explanation:
Even though it was first introduced in the Planck's law,
#u_(lamda)d(lamda) = (8pihc)/lamda^5*(dlamda)/(e^((hc)/(lamdakT))1)# Where one quantum of radiation would have an energy,
#E = (hc)/(lamda)# , the concept of quantized radiation was extended by Einstein, later by Bohr in their theories as a part of the old quantum theory.Today almost all important relationships in quantum mechanics, contain Planck's constant (or the reduced Planck's constant
#h/(2pi)# ).Examples would include,
1) de Broglie relation 
#lamda = h/p# 2) Schrodinger equation 
#(ih)/(2pi)(delpsi)/(delt) = (h^2)/(8pi^2m)(nabla)^2psi + V(vec r,t)psi# 3) Commutator of
#x# and#p_x# #[x,p_x] = (ih)/(2pi)# And so on.
It is to quantum mechanics, what the constants
#epsilon_0# and#mu_0# are to Electricity and Magnetism.