Light and Energy

Chemistry IChemistry I

Classical description of light

Light is an electromagnetic wave. Light consists of elementary

particles called photons. A photon is a little packet of

energy which can carry electromagnetic radiation.

Wave Nature of Light

Electromagnetic (EM) Spectrum: (page 120 in textbook) list of all forms of radiation.

Radiation is a form of energy that exhibits wavelike behavior as it travels through space.

Light as a Wave Wavelength (λ)

Frequency (v)

Amplitude

All light travels at the same speed(3 x 108 m/s, symbol = c)

Frequency and wavelength are inversely proportional

C = (λ)(v)

Visible Light

Visible light is a very small part of the EM spectrum and can be separated into a continuous spectrum we call a rainbow.

Red light has the lowest frequency and least energy.

Violet light has the highest frequency and most amount of energy.

Spectral Lines make up a bright line spectrum

Spectral lines are produced by colored light passing through a prism. The prism breaks down the colored light into specific colors that make up the colored light we see.

Electromagnetic Spectrum

Electromagnetic Spectrum

Type of Wave Wavelength

Radio Waves 1 m – 1 km

Microwaves 1 cm

Infra-red 0.01 mm

Visible 400-700 nm

Ultraviolet 100 nm

X-Ray 1 nm

Gamma Ray 0.01 nm

Electromagnetic Spectrum

Rabbits Radio

Meet Microwave

In Infra-red

Very Visible

Unusual Ultraviolet

eXpensive X-Rays

Gardens Gamma

Calculations Find the wavelength of blue light if

its frequency is known to be 6.4 x 1014/s? (any number per second is also a unit of frequency).

The answer is 4.7 x 10-7 m

Substitute into your equation:

3.0 x 108 m/s = * 6.4 x 1014/s

Light as a Particle

The energy of a photon can also be calculated.

E=hv says that the energy of a photon is related to its’ frequency.

h is Planck’s constant(named for Max Planck)h = 6.626 x 10-34 J•s

Light as a Particle c=λν

speed of light = wavelength times frequency

E=hvenergy = Planck’s constant times frequency

E=hc/λWe can combine the two equations as well to find the energy, if only the wavelength is known.

Calculations

Calculations

What is the energy of x- radiation with a 1 x 10-6m wavelength?

E=hv and c=λv so E=hc/λ

E=(6.626 x 10-34 Js)(3x108 m/s)/(1x10-

6m) E=1.99 x 10-19 J

Electrons move between levels of energy

When an atom aborbs heat or electricity, its electrons move to a higher level of energy. This is called the excited state. The original level of energy is called the ground state.

Excited state electron = unstable electron

Electrons in an excited state are unstable and quickly fall back to ground state. This causes a release of energy in the form of light.

The color of light emitted depends upon the falling distance of the electron. If the falling distance was large, much energy is released.

Falling distance is related to energy released

If the falling distance is small, then little energy is released.

Questions 1. What is the difference between

continuous and bright line spectrum? How are each produced?

2. What lead to the development of the Bohr model?

3. What are spectral lines?

Questions 4. What is the relationship between

frequency and wavelength? 5. What is the difference between

ground and excited state electrons? 6. How is light produced? 7. Why is light of different color? 8. What color is high frequency

light? Low frequency?

Questions