Basic Wavelength/Energy Calculations

I have a question regarding wavelength calculations. I've got a question that reads:

- An electromagnetic photon hits a metallic surface (Technetium-99), inducing emittance of an electron. The electron has a speed of 3.6E3 kilometres/s.

(a) What's the wavelength of the electron?

λ = h/mass x speed
λ = 2.02E-10 metres

- Below the frequency of 2.5E16 Hz, no emittance occurs.

(b) How much energy is needed to induce emittance?

E = hv
E = 1.6E-17J

(c) What is the wavelength of the photon that induced emittance?

This is where I am seemingly having problems. The book gives the answer of 8.8E-9m however I am getting:

E = hν
c = λν
E = hc/λ
λ = hc/E
λ = (6.626E-34 J.s x 299,792,458m/s) / 1.6E-17J
λ = 1.2E-8m

Am I doing something wrong?

Thank you!

- J

Reply 1

Nebula

Yes, the question says that a photon of a frequency 2.5 e16 Hz is the minimum to cause emission. The electron in the question isn't just emitted, it's emitted at 3.6 e3 km s^-1. So the energy you are using is too low.

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