Light & Telescopes Ch. 3

! What is light? ! How do we focus light? ! How do telescopes work? ! Different types of telescopes

Light is a Wave! Light is an Electro-Magnetic wave ! Electric charges and magnets make electric & magnetic

fields.

Magnetic FieldElectric Field

! When an electric or magnetic field changes, an Electro-Magnetic wave is created.

! The wave moves out like a ripple in a pond.

Electric Field

Magnetic Field

Waves of Light

Light has wave-like properties, such as diffraction. (bending around an opening)

Diffraction in Water Waves Diffraction of Light Waves

Wave-Particle Duality

Note: Light can also behave like a particle.

A particle of light is called a photon

Also, all particles (e.g. electrons) behave like waves.

In fact, everything can be seen as a particle OR a wave!

…a “wavicle”

This curiously profound observation is called the: “wave-particle duality of nature”

All Waves Have 3 Characteristics! Speed

! c = 300,000 km/s = 3*108 m/s = speed of light

! Wavelength: The distance between wave crests

! We use the Greek letter λ (lambda) to represent wavelength

! Frequency: the number of waves passing by per second

! f = frequency

c = f x λ! c, f and λ are related through:

f = c/λ

Wavelength and Frequency

! Wavelength and Frequency are inversely related ! High frequency waves have short wavelengths ! High frequency waves also have more energy.

In Music, Wavelength (λ) Is Pitch

Low Pitch: Long Waves High Pitch: Short Waves

In light, Wavelength (λ) Is Color

The length of light waves determines their color Red Light: Long Waves Blue Light: Short Waves

Electromagnetic Spectrum

! There are many different kinds of light (besides visible) ! Each type has a different wavelength ! Together, we call them the Electromagnetic Spectrum

What’s your favorite color?

What’s your favorite radio station?

Which (EM) wavelength do you like best?

The range of human perception (sight & sound) is very limited.

In the grand scheme of things, we’re all pretty much blind and deaf.

! Visible light has wavelengths between 400-700 nm ! To detect other types of light, astronomers launch telescopes into space

on satellites

1 nm = 10-9 m

We normally measure wavelengths (λ) using nanometers (nm)

Increasing wavelengths (λ)

Measuring Light waves

400-700 nm

•9. A.) How many waves of red light are there in a penny’s thickness (1.5 mm)?

Homework Help

•tpenny = ______ m

•λred = ____________ m

! Objects in space give off every kind of light. ! However only visible light and radio waves reach Earth’s surface ! (some Ultra-violet light also reaches Earth)

Wavelengths (λ)Remote Demo

The main purpose of a telescope is to gather light.

To do this we need to bend the light and focus it to a point.

There are two ways to do this: lenses & mirrors

How Do Telescopes Work?

Focusing with Lenses

The Focus is the point at which the light converges.

Focal length = distance from the lens to the focus

focus

focus

Refractors

• Galileo’s telescope used lenses to bend, or refract, and focus the light.

• Telescopes using lenses are called refractors.• The largest refractors use 40-inch lenses.• But refractors are ultimately limited in size.

Lick Observatory, CA

Focusing with Mirrors: Reflectors• To bring parallel light to a focus, we can use a mirror with a special

dish-like shape*.

Telescopes that use mirrorsAre called reflectors.

Today, all research telescopes are reflectors.

Focus

(*called a paraboloid)

Refracting vs Reflecting TelescopesRefracting Telescope:

Lens focuses light

Reflecting Telescope:

Mirror focuses light

Telescope Types Newtonian

• Isaac Newton invented a reflecting telescope• Light reflects off its curved primary mirror …• Then is sent to an eyepiece by a flat secondary mirror• This design is still used by amateurs

Primary Mirror

Telescope Types Cassegrain

A Cassegrain telescope has a hole in the primary mirror.The light comes to a focus behind the primary mirror.

Primary Mirror

The primary mirror at Keck Observatory is 10 meters in diameter.

1. Light Gathering Power: The ability to collect light

2. Resolving Power: The ability to see fine details

3. Magnifying Power: The ability to make objects look bigger

Telescopes have Three Powers

Pizza Time!!!

You and your friends are buying Pizza. Pizzas with a larger diameter cost more. Which is a better deal?

A. 8 inch pizza for $10.00

B. 16 inch pizza for $25.00

C. They are the same.

One 16-inch pizza = FOUR 8-inch pizzas

Light-gathering power

Light-gathering power determines how faint a star a telescope can see.

It depends on the area (A) of the primary lens or mirror:

Area = π (r)2

D

A = π (D/2)2

The Keck telescope in Hawaii has a 10 meter primary mirror, while a telescope at McDonald observatory in Texas has a 1 meter telescope.

Question: Keck can see fainter stars, but how many times fainter?

Answer: The greater a telescope’s Light Gathering Power, the fainter an object it can see.

Light Gathering power depends on Area: A = π(D/2)2 [D=diameter]

DKeck = 10 m ; DMc = 1 mAKeck = π ( DKeck/2)2 ; AMc = π (DMc/2)2

Akeck / Amc = [π (DKeck/2)2 ] / [π (DMc/2)2] = (Dkeck/DMc)2 = (10/1)2 = 100

Keck can see stars which are 100 times fainter!

Light Gathering Power: Example