Chapter 7

Light as a wave  carries energy but doesn’t actually move  Think of a cork on water and surfer. The above diagram is a typical way to visualize a wave. One wave is the curve from A to C.

There are several terms referring to waves that we need to learn and we will use this diagram to help us define these terms. Wavelength ( ) – The direct distance between the same point on neighboring waves. In our diagram, it would be the horizontal distance from B to D. Frequency ( ) – The number of waves that appear to pass one point in one second. Referred to as cycles per second. Units are sec -1 or herz (Hz)

Since waves don’t actually move from point to point, but carry energy from point to point, they appear to move. Therefore we refer to the speed of waves. Actually we are referring to the speed at which energy is moving along the waves. speed = The speed of any particular type of wave; sound, light etc. will be constant. Therefore if frequency is high then wavelength has to be low (short) and vice-versa.

Electromagnetic waves - These waves all travel at the speed of light (symbolized as c) and = 3.00 x 10 8 m/ sec. c = Electromagnetic spectrum

We are most interested right now in the visible region. Note that violet is on the extreme left while red is on the extreme right. On this diagram, the left has high frequency and short wavelength. The decreases as you go from left to right, therefore the increases. For each, in the visible region there is a specific color. The colors gradually change as the gets longer. Remember that = c/. There is a specific color for each wavelength and frequency.

Max Planck and Albert Einstein discovered that electromagnetic radiation has some particle-like behavior. Particles of light are called quanta (singular is quantum) (It is the smallest amount of energy that can be emitted or absorbed in the form of electromagnetic energy. E = h h = Planck’s constant = x j-s

According to their theories, energy is always emitted in multiples of h, but never fractional amounts. Therefore every wave has a specific energy associated with it (carried by it). The higher the, the higher the energy. High energy waves are on the left of the spectrum, with energy decreasing as you move to the right.

Albert Einstein, in 1905, solved a phenomenon called the photoelectric effect, in which it was known that electrons are emitted from the surface of certain metals when exposed to light of a certain minimum frequency, called the threshold frequency. Below, none were emitted, but above the # of electrons emitted depended on the intensity of the light. Wave theory couldn’t explain this. Einstein proposed a revolutionary idea; that a beam of light was actually composed of particles of light, called photons. Each photon has an energy described by E = h.

This was a problem. Einstein’s proposal forced scientist to consider light as waves (associated with energy) and as particles (associated with matter). Up to this point energy and matter were consider to be 2 different components of the Universe, and not interchangeable. Now we had to look at light as having a dual nature.

If an element is exposed to very high energy, it will eventually emit electromagnetic radiation, including some visible radiation for most elements. If you look through a special instrument, called a spectroscope (like a very good prism) you will actually see discreet wavelengths of light. This is called the Emission Spectrum for that element. It will always be identical, no matter what form the element is in.