According to Max Planck Constant velocity of Light:

According to Max Planck the velocity of electromagnetic waves equals to one Planck length per Planck time; the limit to which photon can travel. 

Planck time ~ 5.39×10^−44 s.

Planck length ~ 1.61626×10^−35 m. 

Therefore, speed of electromagnetic wave is constant through the equations of Max Planck and Combined with Albert Einstein. 

c = ~3x10^8 m/s = fλ = √E/m.

Planck units are a set of units of measurement defined exclusively in terms of four universal physical constants. Originally proposed by the German physicist Max Planck in 1899, these units are a system of natural units because their definition is based on properties of nature. It may be mentioned here that Einstein first published his special theory of relativity in 1905, which describes his revolutionary ideas about light, time and energy.

The four universal constants, by definition, have a numerical value of 1 when expressed in these units:

1. Speed of light in vacuum, c,

2. Gravitational constant, G,

3. Reduced Planck constant, ħ, and

4. Boltzmann constant, kB.

• Planck length = ℓP = L ≈ 1.61626 × 10^−35 m; 

• Planck time = tP = T ≈ 5.391247 × 10^−44 s; 

• ℓP/tP is the ratio of the Planck length to the Planck time;

Since, ℓP/tP = (1.61626 × 10^−35 m) / (5.391247 × 10^−44 s);

1. To divide two numbers in scientific notation, we subtract the exponents of the 10 and divide the coefficients:

2. Coefficient: (1.61626) / (5.391247) ≈ 0.299792458

3. Exponent: (10^(-35)) / (10^(-44)) = 10^(-35 - (-44)) = 10^9

4. So the simplified value is approximately:

5. 0.299792458 × 10^9 m/s

6. Now, we recognize that this is the speed of light in a vacuum, which is denoted by 'c':

7. c ≈ 2.99792458 × 10^8 m/s

8. So, the simplified expression is:

9. (1.61626 × 10^−35 m) / (5.391247 × 10^−44 s) ≈ 2.99792458 × 10^8 m/s;

The ratio of the Planck length to the Planck time (ℓP/tP) yields a value to the speed of light in a vacuum, c;

This is a fundamental constant in physics and is denoted by 'c'.