A quantum of electromagnetic energy is called a photon You can think of photons as 'light particles' - quanta of energy - discrete packages (that can't be split)...... little packets of wave energy - wavelets!

The energy of a photon is proportional to its frequency.

E = hf

The constant of proportionality is called the Planck constant. It has the symbol 'h' and is on your data sheet. It has a value of 6.63 x 10 -34 Js

Now, for all electromagnetic radiation the velocity is 'c' (the speed of light - again on your data sheet as 3.0 x 108 m/s).

Combining this equation with the wave equation we have:

(always write it this way - learn to - it saves time!)

Try this question:

A 50W lamp radiates in the visible region of the spectrun (400 to 700 nm). Estimate the number of photons produces each second by the lamp.

The average frequency of the visible light range given in the question is 550 nm

A 50 W lamp produces 50J of light energy in 1 second.

so a 550 nm photon will have energy of

6.63 x 10-34 x 3.0 x 108

550 x 10 -9

= 3.62 x 10 -19 J

Therefore 50J of light energy will contain 50/ (3.62 x 10 -19) = 1.4 x 1020

Now, Einsein's equation E = mc2 links mass and energy. Mass can be converted into energy and vice versa.

We can therefore find the mass equivalent of a photon and then work out its momentum!

E = hf = mc2


mphoton = hf/c2

and as the momentum of the photon = mc

pphoton = hf/c


f = c/l

so pphoton = h/l

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