Light and the Electromagnetic Spectrum

Light Phenomenon Isaac Newton ( ) believed light consisted of particles. By 1900, most scientists believed that light behaved as a wave.

Wave Properties Amplitude – is a measure of how big the wave is. Wavelength - the distance between successive crests of a wave Frequency - a measurement of how many cycles can happen in a certain amount of time (Ex. cycles per second)

Frequency, Wavelength, Amplitude ce_share/vis_sim/wslm05_pg18_graph/ws lm05_pg18_graph.htmlhttp:// ce_share/vis_sim/wslm05_pg18_graph/ws lm05_pg18_graph.html

The Electromagnetic Spectrum The electromagnetic spectrum represents the range of energy of waves. Low energy, long wavelengths: Example – radio waves and microwaves High energy, small wavelengths: Example – x-rays, gamma rays

Visible light is a small portion of this spectrum. This is the only part of this energy range that our eyes can detect. R ed O range Y ellow G reen B lue I ndigo V iolet ROY G BIV

Wavelength Ranges of Visible Light Range of the visible spectrum (400 nm – 700 nm). Red light has a wavelength of about 7.0 × 10 ­7 m (700nm). Violet light, at the other end of the visible range has half the wavelength 4.0 × 10 ­7 m (400nm).

Why do we see color? Why is your shirt that color? Why are plants green? The sun provides all of the light on Earth. When objects have a certain color, they are actually reflecting that color to your eye and absorbing all other colors. Exceptions: White – Reflects all colors Black – Absorbs all colors

C = λν The frequency (v) of a wave is the number of waves to cross a point in 1 second (units are Hertz – cycles/sec or sec -1 ) λ is the wavelength- the distance from crest to crest on a wave C = speed of light (2.998 x 10 8 m/s)

Calculate the wavelength of yellow light emitted from a sodium lamp if the frequency is 5.10 x Hz (5.10 x s -1 ) List the known infoList the unknown c = 3.00 x 10 8 m/swavelength (λ) = ? cm Frequency (v) = 5.10 x s -1 C = λv λ = c v λ = 3.00 x 10 8 m/s = 5.88 x m 5.10 x s -1 PROBLEMS

YOUR TURN 1- What is the wavelength of radiation with a frequency of 1.50 x s -1 ? 2- What frequency is radiation with a wavelength of 5.00 x m? In what region of the electromagnetic spectrum is this radiation?

Atoms and Light The movement of electrons inside of atoms produces light and other electromagnetic radiation. When atoms absorb energy, their electrons move to higher energy levels. These electrons emit light when they return to lower energy levels. Each element gives off only certain frequencies of light, called spectral lines. In effect each element has its own signature of spectral lines.

Below is a picture of the spectral lines given off by hydrogen. Note there are 3 different frequencies.

Helium

Neon Argon

Albert Einstein returned to the idea that light existed as particles. He proposed that light could be described as quanta of energy that behave as if they were particles. Light quanta are called photons. While it was difficult for scientists to believe, it did explain the photoelectric effect.

A certain frequency has to be achieved or the effect does not work The photoelectric effect – When light shines on metals, electrons (photoelectrons) are ejected from their surface. Red light will not cause electrons to eject!

E = energy of a particle of light h = Planck’s Constant (6.626 x Js) v = frequency of radiation Problem: What is the energy of a photon of microwave radiation with a frequency of 3.20 x /s? Answer: 2.12 x J