Light and Matter Chapter 2

enLIGHTened Objectives Models of light light is a wave light is a particle Absorption and radiation of light by atoms Measuring light to know composition of stars temperature of stars motion of stars

Models of light So which one is right? light is a wave light is a particle So which one is right? They are both right...and they are both wrong. That’s called wave-particle duality In some experiments, the wave model works best. In other experiments, the particle model works best. Thus, we use both.

Light is a wave propagating wave of oscillating electric and magnetic fields described by wavelength, , and frequency, f. f = v where v is the speed of the wave. In a vacuum, v = c = 3.00 x 108 m/s. large wavelength corresponds to small frequency and small wavelength corresponds to large frequency. synonyms for “light” are electromagnetic wave electromagnetic radiation radiation visible light is light that our eyes are sensitive to; however, that is not the only type of electromagnetic radiation

Wavelength and Frequency

Light comes in many wavelengths When white light passes through a glass prism (or a diffraction grating), it separates into colors. These colors have different wavelengths. This group of wavelengths is the visible part of the electromagnetic spectrum. When you “see” the entire spectrum with no thin dark bands, it is a continuous spectrum.

Electromagnetic spectrum

Wavelength and frequency

Practice What is the wavelength of white light? Which color of light has a longer wavelength purple or red? Suppose that a certain medical treatment requires exposing certain tissues to high frequency radiation. Would that radiation likely be gamma rays or radio waves?

Light is a particle Albert Einstein proposed that light consisted of photons. A photon is a “particle” or “packet” of energy. A photon has an energy of E=hf where h is called Planck’s constant and f is frequency. High frequency (low wavelength) photons have high energy; low frequency (high wavelength) photons have low energy.

Absorption and emission How is light absorbed and emitted by atoms in interstellar gases or stars?

Bohr model (I hope it’s not bohring) The Bohr model is a planetary model, where the electron orbits the nucleus like a planet orbits the Sun. An electron is only allowed in DISCRETE orbits (n=1, n=2, n=3, etc.) The higher the orbit, the higher the energy of the electron.

Modern view of hydrogen Now, we know that the electron has discrete energy levels, but it does not orbit the nucleus at fixed distances from the nucleus. In fact, it may be found anywhere in certain allowed regions called orbitals. Each orbital corresponds to a certain energy of the electron.

Absorption, emission, and energy When an atom absorbs a photon, it gains energy. When an atom loses energy, it emits a photon. An atom can only absorb photons or emit photons of just the right energy. Those “right energies” correspond to the DIFFERENCES in energy between the allowed energy levels. photon Absorption photon Emission

Hydrogen only certain energies are allowed the change in the energy between two levels corresponds to a certain color photon absorbed or emitted by the atom the lowest energy level is the ground state higher energy levels are called excited states energy level energy n=5 n=4 n=3 n=2 n=1 -0.544 eV -0.850 eV -1.51 eV -3.40 eV -13.6 eV

Absorption If light of a continuous spectrum is incident on a gas of hydrogen atoms, then electrons will absorb some of the light. As a result, bands of the spectrum are missing; these are called absorption lines. By the way, these same atoms emit the same colors in an emission spectrum!

Emission If excited hydrogen atoms fall to lower energy states, photons will be emitted. The emitted photons will be detected as light of certain bands of frequencies (i.e. colors). The collection of bands (or lines) forms an emission spectrum.

What’s so EXCITING? Sure, electrons get excited when they change energy levels, by why do we get so excited? Each element absorbs and emits a different set of spectra. By measuring the spectral lines, we can know what element a gas is made of. Now, we have way of determining what elements stars like the Sun are composed of Here are spectra for the most abundant elements that compose the Sun.

Clouds of gas (nebulae) emit light, some by absorption and some by emission

emission nebula

Practice See the “Spectrum” handout If an atom is in the ground state (n=1) and is excited to n=3, what energy photon was absorbed? What part of the spectrum does this correspond to? If a hydrogen atom is in the state n=4, to what level must it “fall” in order to emit a blue photon? energy level energy n=5 n=4 n=3 n=2 n=1 -0.544 eV -0.850 eV -1.51 eV -3.40 eV -13.6 eV

Practice If an atom absorbs a photon, does the atom’s energy increase, decrease, or remain constant? Suppose that a gas of 4 hydrogen atoms has an atom in each of the 4 lowest energy levels. How many distinct photons can be emitted by this gas? Suppose that a particular gas will only emit a red photon and a yellow photon. What colors will it absorb if visible light is incident on the gas with many of its atoms in the ground state?

the temperature of stars Using light to know the temperature of stars

Blackbody radiation A perfect absorber of light is a blackbody. A blackbody is also a perfect emitter. The emission spectrum of a blackbody is continuous and depends on temperature. T ~ 4000 K

Blackbody curves

Temperature and brightness As T increases, the wavelength for peak brightness decreases (i.e. shifts toward the violet and ultraviolet wavelenghs). As T increases, the brightness increases.

the motion of stars, planets, and other objects in the sky Using light to know the motion of stars, planets, and other objects in the sky

Doppler shift As a star approaches you, the frequencies of the absorption lines increase (so the wavelengths decrease). They are blueshifted. As a star recedes away from you, the frequencies of the absorption lines decrease (so the wavelengths increase). They are redshifted.

Blueshift

Redshift

Light tells us what something is made of what temperature it is by analyzing emission and absorption spectral lines what temperature it is blackbody curve how fast it is moving toward us or away from us doppler shift of spectral lines