Let there be light Most of what we know about the Universe comes from information that has been carried to us by light. We will begin by examining the properties of light and then we will discuss the instruments that are used to detect it (telescopes).t.

Light Basics Light is radiant energy Radiant energy is energy that can travel through space from one point to another without the need of a direct physical link In empty space light travels at a constant speed, 299,792.458 kilometers per second (this is a constant and is denoted by “c”). Fast enough to circle the Earth over 7 times in one second! When light travels through matter, glass, water, gasses, etc. it slows down. Light can be thought of one of two ways: 1. As an electromagnetic wave – a mix of electric and magnetic energy 2. As photons – photons are packets of energy Both models have one thing in common – they classify light as being energy

Light As A Wave Parts of an electromagnetic wave: Frequency, which is the number of cycles (wavelengths) that pass in a fixed point in a given time. Parts of an electromagnetic wave: Crest – the high point of a wave Trough – the low point of a wave Wavelength () – the distance of two wavecrests or two wavetroughs

Looking At Waves Through Sound We can’t see light waves, but we can look at sound waves to help us understand wave structure and wave energy. Which one of these wave models has the bigger wavelength? Which one of these models has the greater frequency?

The Difference Between Light and Sound Waves We use sound waves to hear. Even though sound has the same wave structure as light they are fundamentally different. Sound energy relies on the vibration of sound waves through matter. In other words, sound is transferred from one place to another via direct contact Light is a form of radiation It is not dependant on matter to get form one place to another.

Light is Radiation!!!! Oh No!!!! Yes, light is radiation. It is one of many types of radiation. All this means is that it can travel through space without the need for a direct transfer of energy. Sound was the opposite. It moves because of a direct transfer of energy from one particle to another

Types of Radiation Visible Light – The light we see with our eyes Infrared radiation – Heat sensitive radiation Radio Waves – NOT the same as sound waves Ultraviolet Light X-Ray Radiation Gamma Radiation

The Electromagnetic Spectrum Composed of a bunch of different types of energy emitted from waves (radiation). The different types of radiation in the spectrum include: Gamma Rays X-Rays Ultra Violet Visible Light Infrared Microwaves Radio Waves Increasing Energy

Wave Frequency = Energy So now the BIG question, what separates one type of radiation from another? They are all related All have wave properties Crest, trough, and wavelength What separates one from another however is frequency Wave Frequency = Energy

High Frequency Low Frequency

What does this have to do with anything? Well, since we already know that there are different types of radiation, we should now investigate why one different from another Each of the energies we discussed is radiant energy (radioactive energy). What separates one from another is the frequency at which they radiate. Higher frequency = ? Energy = ? Wavelength Lower frequency = ? Energy = ? Wavelength

So….. Why is this boy wearing these fancy glasses? Is this dangerous? Why is this man looking so spiffy? So….. Why is this a bad idea?

How do we know this model is correct? We can use what we already know and some simple concepts to help us out. We know that every form of radiation behaves as a wave We also know that each type of radiation moves at a constant speed through empty space (“c”) Furthermore, we know that some waves have a higher frequency than others Lastly we know that different radiations different wavelengths

Looking at it another way… Wavelength (λ) x Frequency (f) = The Speed of Light (c) Or λ x f = c When we look at this we can notice a couple of things The speed of light (c) is a constant (3.0 x 108 meters/sec) High frequency = short wavelength Low frequency = large wavelength Wavelength and frequency are inversely proportional

So how do we use this? The numbers on a radio dial for FM stations are their frequencies in megahertz (MHz), or millions of hertz. If you listen to 93.1 WXRT, it broadcasts radio waves with a frequency of 93.1 million cycles per second (1/sec). What is the wavelength of these radio waves? We know two things The speed of light = 3 x 108 m/sec The frequency = 9.31 x 107 1/sec Now its just plug and chug

The Solution λ x f = c 3 x 108 m/sec 9.31 x 107 1/sec λ = c/f λ = Solve for λ λ = c/f Then solve for λ: 3 x 108 m/sec 9.31 x 107 1/sec λ = λ = 3.2 meters Now you try it for 97.1 The Drive, but before you do, will the wave length be bigger or smaller than XRT’s? Did you get 3.09 meters?

Wait, what about energy? How is energy related to λ = c/f ? To help us with this lets take a small trip to this place and learn about a guy named Max Planck. Now we have Planck’s constant (h) Planck said that Energy (E) = h (constant) x frequency (f) If we take λ x f = c and solve for frequency(f), than we get f = c/ λ If we plug this into E = h x f we then have E = hc λ