Physics multiple choice...photons?

Question

A green laser and a red laser both have a power output of 1 watt and shine onto a metal surface. which of the following statements is most accurate?

a. the green laser produces more photons per second than the red laser

b. the red laser produces more photons per second than the green laser

c. the red laser and the green laser produce the same number of photons per second

d. the number of photons produced per second depends on the threshold frequency of the metal.

please explain?

Answer 1

Power  = Energy / time

The energy for individual photons is E = hf, where h is plank's constant and f is the frequency (green is low freq, red is higher freq)

In one second, the amount of energy the photons must have is

Energy = Power x Time = 1 Watt * 1 sec = 1 joule

For multiple photons, the total energy is E = nhf

We could actually calculate how many photons per second come from a 1 watt green or red laser using wavelength x frequency = speed of light.  Red is around 650 nm, and green is around 550 nm in wavelength.  But the point is the energy from the green laser must be 1 joule for the 1 second time interval, and the same must be true for the red laser.  So

n(red)*h*f(red) = n(green)*h*f(green)

n(red) / n(green) = f(green) / f(red)

Since the frequency of green is larger than the frequency of red light, the number of red photons is greater than the number of green photons.

(B)

edit... this explanation should point out that (a) and (c) are incorrect, and those of you who still cling to (d), the photoelectric effect has to do with photons causing electrons to escape a metal surface - that would be possibly true if it read "the number of ELECTRONS produced per second depends on the threshold frequency of the metal."

Answer 2

if the conditions are met and both radiations are able to produce a photoelectric effect with the metal characterized by a given work function then in my opinion c. is the correct answer.

Answer 3

?are you using green lenses for green laser>

Answer 4

From Wiki: photoelectric effect

Experimental results of the photoelectric emission

1. For a given metal and frequency of incident radiation, the rate at which photoelectrons are ejected is directly proportional to the intensity of the incident light.

2. For a given metal, there exists a certain minimum frequency of incident radiation below which no photoelectrons can be emitted. This frequency is called the threshold frequency.

3. Above the threshold frequency, the maximum kinetic energy of the emitted photoelectron is independent of the intensity of the incident light but depends on the frequency of the incident light.

So 1. eliminates a and b; 3 eliminates d; 1. supports c.

Ans: c

Answer 5

A is true.  

B is false.

C is false.

D is false.