Question

a) Describe an experiment that points to the particle property of light.

b) Describe an experiment that describes the wave property of light.

Answer 1

Part a) Photo-electric effect :- The photoelectric effect or photoemission is the production of electrons when light falls upon a material especially metal surfaces.

The Photoelectric Effect

APPARATUS:-

• Photodiode with amplifier
• Batteries to operate amplifier and provide reverse voltage
• Digital voltmeter to read reverse voltage
• Source of monochromatic light beams to irradiate photocathode
• Neutral filter to vary light intensity

INTRODUCTION:-

The energy quantization of electromagnetic radiation in general, and of light in particular, is expressed in the famous relation

  where is frequency of the incident light.

Specifically:

1. No photoelectrons are emitted from the metal when the incident light is below a minimum frequency called threshold frequency , regardless of its intensity. (The value of the minimum frequency is unique to each metal.)

2. Photoelectrons are emitted from the metal when the incident light is above a threshold frequency. Thekinetic energy of the emitted photoelectrons increases with the frequency of the light.

3. The number of emitted photoelectrons increases with the intensity of the incident light. However, the kinetic energy of these electrons is independent of the light intensity.

4. Photoemission is effectively instantaneous.

THEORY:- Consider the conduction electrons in a metal to be bound in a well-defined potential. The energy required to release an electron is called the work function W₀ of the metal. In the classical model, a photoelectron could be released if the incident light had sufficient intensity. Then using law of conservation of energy; Kinetic energy of ejected electron will be

When the incident light intensity is increased, more photons are available for the release of electrons, and the magnitude of the photoelectric current increases.

The photoelectric current in a typical setup is extremely small, and making a precise measurement is difficult. Normally the electrons will reach the anode of the photodiode, and their number can be measured from the (minute) anode current. However, we can apply a reverse voltage to the anode; this reverse voltage repels the electrons and prevents them from reaching the anode. The minimum required voltage is called thestopping potential Vs, and the “stopping energy” of each electron is therefore eVs. Thus,

So if we plot a graph between V_s and then h/e will give the slope of the plot and -W₀/e be the vertical intercept.

And plot will be look like this

PROCEDURE : DEPENDENCE OF THE STOPPING POTENTIAL ON THE FREQUENCY OF LIGHT

You can see five colors in the mercury light spectrum. The diffraction grating has two usable orders for deflection on one side of the center.

1. Adjust the photodiode-housing assembly so that only one color from the first-order diffraction pattern on one side of the center falls on the collimator.

2. For each color in the first order, record the photodiode output voltage reading on the DVM.

3. For each color in the second order, record the photodiode output voltage reading on the DVM.

4. Plot a graph of the stopping potential as a function of frequency, and determine the slope and the y-intercept of the graph. From this data, calculate W₀ and h. Compare this value of hh with that provided in the “Introduction” section of this experiment.

Part B) Diffraction experiment :-

Diffraction experiment shows that light also shows wave nature.

Apparatus

Laser monochromatic light, black paper sheet, a blade , glass slide and screen

Theory:

When a wave train strikes an obstacle, the light ray will bend at the corners and edges of it, which causes the spreading of light waves into the geometrical shadow of the obstacle. This phenomenon is termed as diffraction.

Procedure:-

1. Paste a black paper sheet on the glass slide.

2. Make two small slits on the black paper with the help of blade, slits should be very fine and not away from each other.

3. Mount the glass slide on a stand. Also mount the laser light on second stand.

4. Allign both the stand, and let the laser light falls on the slide.

5. Adjust the screen distance to get a diffraction pattern on the screen.