Question

PART A

Which of these affects the energy of an electron that is emitted by a surface exposed to light?SELECT ALL THAT APPLY.

		The wavelength of the incident light
		The material that is being illuminated
		The frequency of the incident light
		The power per area of the incident light

PART B

When photons meet and reflect from a surface and are then detected after they reflect ...

Pick those that are true

		they appear to take, on the average, a path with the least travel time
		they take all possible paths, including into the material, and the resultant sum depends on the frequency of the light
		they behave like solid projectiles and bounce off the surface precisely as given by the law of reflection
		the direction in which they go is given as a probability, not a certainty

PART C

Suppose we have two sources of light, a red one with wavelength 650 nm, and a blue one with wavelength 450 nm. Both produce the same optical power, delivering 1 milliwatt (10^-3 watts) of energy per second per square centimeter to a metal target with an area of 1 cm².

1. Which color would deliver more photons each second?

All the photoelectrons that are produced are collected by another electrode and we determine the current in amperes using a circuit and an "ammeter", that is, some device that can give a precise quantitative measure of the current. Such a device might use the magnetic field generated by the current, or the voltage produced when the current flows across a resistance.

2. If the work function of the surface is 2 eV, will we see a current with both colors, one color, or neither one? If so, how much (in amperes), or if not, explain.

3. If we now increased the power in the light sources by 1000 times, how would that change your answers to part 2?

Answer 1

A.

The photon must have sufficient energy to eject the electron from the metal surface, the minimum energy is called the work function and is a property of the material.

The wavelength of the incident light - YES, The wave length if related to energy of the photon
The material that is being illuminated, YES, it is related to the work function
The frequency of the incident light - YES it is related to the energy of the photon
The power per area of the incident light - NO, it will only eject more number of electrons from the surface

PART B: , light reflect at angle to the normal of incidence such that , incident angle = reflected angle.

	they appear to take, on the average, a path with the least travel time , YES
	they take all possible paths, including into the material, and the resultant sum depends on the frequency of the light - NO
	they behave like solid projectiles and bounce off the surface precisely as given by the law of reflection - YES
	the direction in which they go is given as a probability, not a certainty - NO

PART C

PART C

Suppose we have two sources of light, a red one with wavelength 650 nm, and a blue one with wavelength 450 nm. Both produce the same optical power, delivering 1 milliwatt (10^-3 watts) of energy per second per square centimeter to a metal target with an area of 1 cm².

1. Which color would deliver more photons each second?

RED wave length is more but less photon energy. Both sources deliver the same power, hence RED source emits more photons per sec.

All the photoelectrons that are produced are collected by another electrode and we determine the current in amperes using a circuit and an "ammeter", that is, some device that can give a precise quantitative measure of the current. Such a device might use the magnetic field generated by the current, or the voltage produced when the current flows across a resistance.

2. If the work function of the surface is 2 eV, will we see a current with both colors, one color, or neither one? If so, how much (in amperes), or if not, explain.

RED photon energy is 1240/650 less than 2 ev and it cannot eject any electrons from the target. BLUE photon energy = 1240 /450 = 2.75 ev. Electrons are ejected and produces current.

Number of photons N = P/E_photon = 1.0E-3 J/ 2.75 ev /s

1 ev = 1.6E-19 J

each photon produces one electron with charge q_e = 1.60E-19

current I = N *dq/dt = 1.0/2.75 = 0.363 mA

3. If we now increased the power in the light sources by 1000 times, how would that change your answers to part 2?

The current will increase that many times with BLUE but RED light cannot produce any current