# Calculate the energy of the laser photons

1. Mar 11, 2007

### CathyLou

Hiya.

I'm really stuck on the following questions and so any help would be really appeciated.

(a) A laser produces light of wavelength 632nm. Calculate the energy of the laser photons.

Speed = Wavelength x Frequency

3 x 108 = 632 x 10^-9 x f

f = 4.75 x 10^14 Hz

E = hf

E = 6.6 x 10^-34 x 4.75 x 10^14

E = 3.14 x 10^-19 J

(b) The laser has a power output of 100Mw. Calculate the number of photons released each second.

Thank you.

Cathy

2. Mar 11, 2007

### H_man

A Megawatt is a million watts.

A million watts is a million Joules per second.

You know how much energy each photon has...........

3. Mar 11, 2007

### CathyLou

Okay. So, is the answer 3.18 x 10^24 photons per second?

Cathy

4. Mar 11, 2007

### CathyLou

The photoelectric work function of a metal is 3eV and light with energy 5eV is shone onto the metal surface.

(a) Calculate the wavelength of a photon with energy 5eV.

(b) Calculate the maximum kinetic energy of the emitted photoelectrons, giving your answer in joules.

Thank you.

Cathy

5. Mar 11, 2007

### H_man

The answer you calculated to your first question looks fine though I don't have a calculator to hand.

1)For part (a), perhaps you can tell me what is an electron volt?

2)For part (b)... what is the work function a measure of?

I know from the previous question that you already have the equations and knowledge to solve this if you can answer my 2 questions.

6. Mar 11, 2007

### CathyLou

1 eV is the energy transferred when an electron moves between two points separated by a p.d. of 1 V.

So, would I use E = hf to find the frequency, (where E = 5), and then use Speed = Wavelength x Frequency to find the wavelength?

The work function is the least amount of energy needed for an electron to escape from the surface of the metal.

Would I use hf = work function + Ek for this part?

Cathy

7. Mar 11, 2007

### Staff: Mentor

E = 5 eV. (Don't leave off the units!) You may have to convert to standard energy units--Joules--when you calculate the frequency. 1 eV = ? Joules? (Look it up!)

Otherwise: Looks good!

You got it.

8. Mar 11, 2007

### CathyLou

Okay. Thanks very much.

Cathy

9. Mar 11, 2007

### CathyLou

I'm also really struggling with the following questions and so would really appreciate any help.

The minimum wavelength of light that causes emission of photoelectron from a metal surface is 5 x 10-7.

(a) Calculate a value for the work function of the metal in joules and electron volts.

(b) Calculate the maximum kinetic energy of the photoelectrons released from the same metal surface if light of 4.5 x 10-7m were shone onto it.

The work functions of potassium, sodium and zinc are 1.81Ev, 2.28eV and 4.31eV respectively.

(a) Which of these metals when irradiated by incident light of frequency 6 x 1014 Hz would emit photoelectrons? Justify your answer.

(b) Calculate the maximum kinetic energies of the electrons emitted.

(c) Calculate the speed of the electrons emitted ignoring relativistic effects. What % of the speed of light is this?

Thank you.

Cathy

10. Mar 11, 2007

### Staff: Mentor

You've already used the key relationship that applies here (in post #6), so give these a shot.

11. Mar 12, 2007

### CathyLou

I've figured out the anwers for the other questions but I'm still really stuck on this one. Any help would be really appreciated.

4. The work functions of potassium, sodium and zinc are 1.81Ev, 2.28eV and 4.31eV respectively.

(a) Which of these metals when irradiated by incident light of frequency 6 x 1014 Hz would emit photoelectrons? Justify your answer.

(b) Calculate the maximum kinetic energies of the electrons emitted.

(c) Calculate the speed of the electrons emitted ignoring relativistic effects. What % of the speed of light is this?

Thank you.

Cathy

12. Mar 12, 2007

### Staff: Mentor

Again, it's the same relationship that you used before (at least for parts a and b). Start by figuring out the energy of a 6e14 Hz photon. Then compare that to the given work functions.

13. Mar 12, 2007