CHEMISTRY COMMUNITY

Why does energy decrease when wavelength increase? My TA explained the concept in discussion but I don't think I fully got it.

When wavelength increases, the EM radiation's frequency decreases. Since energy of a photon is E = hv, energy of the photon must decrease since frequency v decreased.

Energy = [(Planck's Constant) x (Speed of light)] / [wavelength]

Because wavelength is in the denominator, as the wavelength increases, energy decreases.

I guess it's the conservation of energy, if the speed changed then this would require an acceleration which would need energy to be put in or taken out of the wave.

It is not conservation of energy.

Increasing the wavelength decreases the kinetic energy of ejected electrons because when you have E=hv, and you substitute in the frequency (v) as c/(lambda) you get.. E= h * c/(lambda).

So when you increase the wavelength, lambda, it makes the fraction c/(lambda) a smaller number overall which means the energy decreases.

I hope the way I explained it makes sense and helps.

It is good to think of it with the equation E=(Planck's constant)(frequency) and speed of light=(wavelength)(frequency). An important skill is linking these two equations together.
Let's try solving for frequency with the equation speed of light=(wavelength)(frequency) which the equation would end up being frequency=(speed of light)/(wavelength). Since E=(Planck's constant)(frequency) you can substitute frequency with the equation we just manipulated which would up being E=((Planck's constant)(speed of light))/(wavelength).

Energy increases as wavelength decrease because of the idea of these two variables being inversely proportional as demonstrated by the equation we just manipulated.

I hope this helps, correct me if I am wrong in some places.

It is important to note that when the wavelength increases, the electro magnetic radiation's frequency decreases. Since energy of a photon is expressed through the formula E = hv, energy of the photon must decrease since frequency decreased.

Because E=hv and if the wavelength increases that means the frequency would decrease.

Because E=hc/lambda wavelength is the denominator and the greater the wavelength means the less energy you have since the variables h and c are constants.

Energy decreases when wavelength increases because as wavelength increases, the wave frequency decreases (due to the fact that they are inversely proportional) because larger wavelength means fewer waves. Wave frequency gives off energy so if wavelength increases=frequency decreases, then energy also decreases.

conceptually, I think it's because since the wavelength is longer there are fewer cycles per second so it is moving slower and has less energy.