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

energy2. 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:

• Crest – the high point of a wave

• Trough – the low point of a wave

• Wavelength () – the distance of two wavecrests or two wavetroughs

• Frequency, which is the number of cycles (wavelengths) that pass in a fixed point in a given time.

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

• Infrared radiation – Heat sensitive radiation

• Radio Waves – NOT the same as sound waves

• X-Ray Radiation

• Gamma Radiation

• Visible Light – The light we see with our eyes

• Ultraviolet Light

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

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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?

Why is this man looking so spiffy?

Why is this a bad idea?

Is this dangerous?

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 = cWhen we look at this we can notice a couple of

things

1. The speed of light (c) is a constant (3.0 x 108 meters/sec)

2. High frequency = short wavelength

3. Low frequency = large wavelength

4. Wavelength and frequency are inversely proportional

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 haveE =

hc λ