Particle Properties of

Waves

LIGHT

WAVES vs PARTICLE

The Corpuscular Theory

• Newton: light consists of streams of tiny particles, which he called “corpuscles”.

• Rectilinear Propagation• Reflection• Refraction

The Wave Theory

• Christian Huygens : the wave nature of light was firmly established

• Interference

• Diffraction

Rectilinear Propagation

Wave fronts - The portions of water surface whose particles that are all in the same phase of motion.

*The direction of propagation of the advancing straight wave is perpendicular to the wave front.

Reflection

A wave is turned back, or reflected, when it encounters a barrier that is boundary of the medium in which the wave is traveling.

Reflection

I and r is 0 degree when the incident wave approaches the barrier along a line perpendicular to it.

Reflection

Law of Reflection:i = r

When a wave disturbance is reflected at the boundary of a transmitting medium, the angle of incidence is equal to the angle of reflection.

Reflection

Refraction

The bending of the path of a wave disturbance as it passes obliquely from one medium into another of different propagation speed.

Refraction

Water waves travel faster on the surface of deep waterthan they do on shallow water. The change in speed of the wave will cause refraction. The slower wave in the shallow water has a smaller wavelength.

Refraction

Diffraction

Spreading of a wave disturbance beyond the edge of a barrier.

Set-up: Place two straight barriers across the tray on a line parallel with the straight way generator. An aperture, or opening, is left between them approximately equal to the wavelength of the wave to be used. As a segment of each wave crest passes through the aperture, it clearly spreads into the region beyond the barriers.

Diffraction

The diffraction of a periodic straight wave as it passes through a small aperture. Observe the decrease in the diffraction effect as the wavelength of the disturbance sent against the barrier is shortened.

The Superposition Principle

 When two or more waves travel simultaneously through the same medium, (I) each wave proceeds independently as though no other waves were present and (2) the resultant displacement of any particle is the vector sum of the displacements that the individual waves acting alone would give it.

The Superposition Principle

In effect, the displacement of any particle of the medium by one wave at any instant is superimposed on the displacement of that particle by the other wave at that instant. The action of each wave on a particle is independent of the action of the other, and the particle displacement is the resultant of both wave action.

Y1- black solid line

Y2- black dashed line

Y- red line

Interference

The general term interference is used to describe the effects produced by two or more waves that superpose while passing through a given region.

Interference

Constructive Interference-suppose the displacement of a particular particle caused by one wave at any instant is in the same direction as that caused by the other wave. Then the total displacement of that particle at that instant is the sum of the separate displacements (superposition principle). The resultant displacement is greater than either wave would have caused separately.

Interference

Destructive Interference-if the displacement effects of the two waves on the particle are in opposite directions, they tend to cancel one another. The resultant displacement of that particle at that instant is the difference of the two separate displacements and is in the direction of the larger (superposition principle). The resultant displacement is less than one of the waves would have caused separately.

Interference

Complete destructive interferenceIf two such opposite displacements are equal in magnitude, the resultant displacement is zero. The destructive interference is complete. The particle is not displaced at all but is in it’s equilibrium position at that instant.

Electromagnetic Waves

A periodic disturbance involving electric and magnetic force. They are all the same kind of wavy disturbance that repeats itself over a distance called the wavelength.

Electromagnetic waves

Electromagnetic Waves

Electromagnetic waves

The ELECTROMAGNETIC SPECTRUM is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object.

Electromagnetic waves

The Photoelectric Effect

The emission of electrons by a substance when illuminated by

electromagnetic radiation.

The Photoelectric Effect

The photoelectric effect was accidentally discovered by Heinrich Hertz in 1887 during the course of the experiment that discovered radio waves.

Observation: when a negatively charged body was illuminated with light, its charge was diminished.

The Photoelectric Effect

J.J. Thomson and P. Lenard determined the ratio e/m for the particles emitted by the body under illumination – the same as for electrons.

The effect remained unexplained until 1905 when Albert Einstein postulated the existence of quanta of light -- photons -- which, when absorbed by an electron near the surface of a material, could give the electron enough energy to escape from the material.

The Photoelectric Effect

Robert Milliken carried out a careful set of experiments, extending over ten years, that verified the predictions of Einstein’s photon theory of light.

The Photoelectric Effect

The Photoelectric Effect

Observations:

• For a given material of the cathode, the “stopping” voltage does not depend on the light intensity – the energy of photons is determined by the light frequency, not intensity

• The saturation current is proportional to the intensity of light at f =const – the saturation current is proportional to the number of photons, thus to the light intensity

• Material-specific “red boundary” f0 exists: no photocurrent at f < f0 – at f < f0 (hf < W) the photon energy is insufficient to extract an electron from metal

The Photoelectric Effect

It takes a certain amount of energy for an electron to escape from the metal. Electrons absorb this energy from the light

Light is made up of photons with a certain amount of energy given by

E = hf

h = planck’s constant (6.63x10^-34)

f = frequency

The Photoelectric Effect

Energy of the photon goes into:

1. work function – work to free the electron

2. kinetic energy of the electron

The Photoelectric Effect

Sample Problem:

Radiation with a wavelength of 200 nm strikes a metal surface in a vacuum. Ejected electrons have a maximum speed of 7.22x10^5 m/s. What is the work function of the metal in eV?

The Photoelectric Effect

Given:

h (planck’s constant) = 6.63 x 10^-34

wavelength= 200 x 10^-9 m

Speed (v) = 7.22 x 10^5 m/s

m (mass of electron) = 9.1 x 10^-31 kg

The Photoelectric Effect

f= speed of light

wavelength

f = 3 x 10^8 m/s

200 x 10^-9 m

= 1.5 x 10^15 Hz

The Photoelectric Effect

(6.63 x 10^-34 J.s) 1.5 x 10^15 Hz = W + ½ (9.1 x 10^-31 kg)(7.22 x 10^5 m/s)^2

9.94 x10^-19 J = W + 2.37 x 10^-19 J

(9.94 x 10^-19 J) – (2.37 x 10^-19 J) = W

W = 7.57 x 10^ -19 J

(7.57 x 10^-19 J) x 1ev = 4.73 eV

1.6 x 10^-19 J

Prepared by:

Maria Criselda V. dela CruzBs Bio 2A