1. Page 124: 5, Q + 5 A from today’s class

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The Particle Nature of Light The wave model of light cannot explain why heated objects emits only certain frequencies of light at a given temperature, or why some metals emit electrons when colored light of a specific frequency shines on then. A new model of light was needed to address these phenomena!

1. The Quantum Concept The Glowing Light

The Wave Model could not explain the emission of these different wavelength of light at different temperature.

Max Planck ( ) He study the light emitted by hot objects. Conclusion: Matter can gain and lose energy only in small, specific amount called Quanta. That is, a quantum is the minimum amount of energy that can be gained or lost by an atom.

Radiation is emitted in small packets, he called them quanta (or photons).

Where: ν = frequency h = x J s (Planck’s constant) E = energy Planck then went further and demonstrated mathematically that the energy of a quantum is related to the frequency of the emitted radiation by the equation:

2. The Photoelectric Effect 1905 Einstein extended Planck’s work by saying that light remains in its photon packets He used this to explain the photoelectric effect. Nobel Prize 1921

In the photoelectric effect, electrons are emitted from a metal surface when it is exposed to light

The discovery of the photoelectric effect could not be explained by the electromagnetic theory of light. Albert Einstein developed the quantum theory of light in 1905.

Albert Einstein proposed in 1905 that.. Electromagnetic radiation has both wavelike and particle like natures. A beam of light has many wavelike characteristics, it also can be thought of as a stream of tiny particles, or bundles of energy, called photons. Thus, a photon is a particle of electromagnetic radiation with no mass that carries a quantum of energy.

3. Einstein put forward a theory: Light energy is quantised. Light consists of a stream of particles called photons. The energy of each photon ( E ) depends on the frequency ( ν ) of the light.

So is light a wave or a particle ? On macroscopic scales, we can treat a large number of photons as a wave. When dealing with subatomic phenomenon, we are often dealing with a single photon, or a few. In this case, you cannot use the wave description of light. It doesn’t work !

What is light? Light exhibits either wave characteristics or particle (photon) characteristics, but never both at the same time. The wave theory of light and the quantum theory of light are both needed to explain the nature of light and therefore complement each other.

Applications of photoelectric effect Mainly used as sensors or switches. Intrusion alarms. Street lights. Automatic doors. Sound tracks for films. Others ( extra )

Tiny water drops in the air disperse the white light of the sun into a rainbow. What is the energy of a photon front the violet portion of the rainbow if it has a frequency of 7.23 x s -1 Data: V= 7.23 x s -1 H= x J  s -1 E photon = x Formula: E photon = hv

E photon = hV E photon = (6.626 x J s -1 ) (7.23 x s -1 ) E photon = 4.79 x J The energy of photon of violet light is 4.79 x J

In the following examples, which has the greatest amplitude? B A C B

The amplitude of this diagram is: 2

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Remember, Wavelength tells you the type of light And, Amplitude tells you about the intensity of the light

1.The wavelength of the wave in the diagram above is given by letter ______. Answer: A The wavelength is the distance from crest to crest (or from trough to trough) (or between any two corresponding points on adjacent waves).

2. The amplitude of the wave in the diagram above is given by letter _____. Answer: D The amplitude is the distance from rest to crest or from rest to trough.

3. Indicate the interval that represents one full wavelength. a. A to C b. B to D c. A to G d. C to G 3. Answer: D The wavelength is the distance from crest to crest, trough to trough, or from a point on one wave cycle to the corresponding point on the next adjacent wave cycle.

Microwaves are used to transmit information. What is the wavelength of a microwave having a frequency of 3.44 x 10 9 Hz? c = λ ν λ = c ν Data C = 3.00 x 10 8 m/s λ = ? ν = 3.44 x 10 9 Hz or 3.44 x 10 9 sec -1

λ = c ν Data λ = ? C = 3.00 x 10 8 m/s ν = 3.44 x 10 9 Hz or 3.44 x 10 9 sec -1 λ = 3.00 x 10 8 m/sec 3.44 x 10 9 sec -1 λ = 8.72 x m

Radio station WGBB on Long Island, New York, broadcast its AM signal, a form of electromagnetic radiation, at a frequency of 1240 KHz. What is the wavelength of these radio waves in meters? λ = ? C = 3.00 x 10 8 m/s ν = 1240 x 10 3 sec KHz: K = 10 3 Hz = sec -1

λ = c ν λ = 3.00 x 10 8 m/sec 1240 x 10 3 sec -1 λ = 240 m

A certain shade of green light has a wavelength of 550 nm. What is the frequency of this light in hertz? c = λ ν ν = c λ 550 nm x m nm = 550 x m

A certain shade of green light has a wavelength of 550 nm. What is the frequency of this light in hertz? c = λ ν ν = c λ ν = __3.00 x 10 8 m/sec___ 550 x m ν = 5.45 x s -1 ν = 5.5 x Hz

Photons  Quantum theory describes light as a particle called a photon  According to quantum theory, a photon has an energy given by E = h = hc/ h = 6.6x [J s] Planck’s constant, after the scientist Max Planck.  The energy of the light is proportional to the frequency (inversely proportional to the wavelength) ! The higher the frequency (lower wavelength) the higher the energy of the photon.  10 photons have an energy equal to ten times a single photon.  Quantum theory describes experiments to astonishing precision, whereas the classical wave description cannot.

The Electromagnetic Spectrum Shortest wavelengths (Most energetic photons) Shortest wavelengths (Most energetic photons) Longest wavelengths (Least energetic photons) Longest wavelengths (Least energetic photons) E = h = hc/ h = 6.6x [J*sec] (Planck’s constant)

Since frequency and period are exact inverses of each other, there is a very basic pair of formulas you can use to calculate one if you know the other… It is very easy to do these calculations on calculators using the x -1 button.

What does it mean? If there were no light wave at all, these graphs would be flat, like a string before it is plucked. Call that the zero position*. When you pluck a string, you pull it away from zero position by a certain distance, called the amplitude, using a certain amount of energy. Afterwards, as the string vibrates, its wave peaks go back to (but not beyond) that original size, until the wave starts to lose energy (die down). That distance from the zero position to the top of every wave peak always tells how much energy is left in the wave. In the same way, the amplitude of a light wave is also a measure of how much energy the wave carries. Why only half? Scientists like to call the middle point zero, so that the bottom half of the wave has a negative value, like -0.7 or -1. But energy is something positive, so the measure from zero to the peak, which is always positive, is what we use. The bottom half shows the same amount of energy, and the distance is the same. Since we always use the distance from zero to the peak, we can always compare amplitudes of different waves without getting confused about plus or minus. * A scientist would call it "equilibrium," which means a state of balance.

When you measure the amplitude of a wave, you are really looking at the energy of the wave. It takes more energy to make a bigger amplitude wave. Anytime you need to remember this, just think of a home stereo’s amplifier… it makes the amplitude of the waves bigger by using more electrical energy.