Lecture 6Light And Telescopes

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Homework 4 – Due Monday, Feb 19Unit 22: Problem 1, Test Yourself 2Unit 28: Review Question 3, Test Yourself 3Unit 29: Problem 1, Test Yourself 3Unit 30:Review Question 2, Test Yourself 2

Test 1 grades are posted on WebCT (raw and curved).

Bonus Opportunity! The NEXT FULL MOON will feature a total

eclipse!This happens during a full moon whenever the

Moon passes through the darkest part of the Earth’s shadow, called the “umbra”.

Unfortunately, we are too far West to see totality. The moon will be passing back out into Earth’s outer shadow (the “penumbra”) when the full moon rises at about 6:40 on Saturday, March 3.

Bonus Opportunity! To get one point added to your FINAL

GRADE:Attempt to observe the moon as early as you

can the evening of Saturday, March 3. The Moon will pass completely out of the Umbra at 7:11 pm, so try to catch it low in the East before then. It will remain in the Earth’s Penumbra until 8:25.

Write a paragraph or so about your observations. Include a sketch (or photo) of what you saw.

Radiation from the Sky

A huge percentage of astronomical data is gathered from the study of light.

But what exactly is light?

What Is Light?

The nature of light has been a major question in science for centuries.

Newton believed light was composed of innumerable particles. These particles could reflect off things, and be detected by our eyes, allowing us to see. The particles came in different colors. We see a mix

of all colors as white. Some colors couldn’t be detected by our eyes. So

there is light that we cannot actually see, but can detect with scientific experiments.

What Is Light?

Huygens believed light was a wave.

Like a water wave, but instead of being made of water, Huygens believed it was composed of aether.

Color was related to the wavelength of the light wave.

What Is Light?

Light has energy: You can warm something up by shining light on it. You can move things around with light.

Light also has information. Light carries a “fingerprint” from what emitted it that tells us a lot about the object the light came from.

But what IS it?!

What Is Light? In the 19th century, a

physicist called James Clark Maxwell thought he might have a better answer than Newton or Huygens.

Maxwell worked with electricity and magnetism.

He came up with four equations that described all the ways electricity and magnetism interact with each other.

What Is Light?

One of the things Maxwell noticed is that an electric wave could create a magnetic wave, and a magnetic wave could create an electric wave.

He also noticed both waves traveled together at exactly the speed of light.

Guessed light was an electromagnetic wave.

Field Example: Gravity

According to Newton, you pull any other object with mass toward you, using the gravitational force.

One way to look at this is to see yourself as producing a gravitational field: A region of space where everything inside is pulled

toward you. The stronger the pull, the stronger the field.

The field is stronger near you than far away. The field is stronger the more mass you have.

The Electric Field

In addition to mass, some things also have an electric charge.

Objects with electric charge exert a force on each other.

So something with electric charge produces an electric field. Works similar to a gravitational field: Gets weaker with greater distance. Gets weaker with less charge.

The Magnetic Field

Likewise, magnets can attract or repel other magnets.

So a magnet generates a magnetic field.

Something Weird:

A gravitational field has to be created by something with mass.

BUT you can create an electric field WITHOUT an electric charge!

You can also create a magnetic field without a magnet!

Electromagnetic Waves

When an electric field rapidly changes strength with time in a very regular way, it creates a magnetic field that is ALSO rapidly changing strength in time in a very regular way.

Same is true for a magnetic field: a rapidly oscillating magnetic field generates a rapidly oscillating electric field.

Both fields then travel together through empty space at the speed of light. They’re called an electromagnetic wave.

Electromagnetic Waves

In any EM wave the places where the electric field is strongest are separated by where the field is weakest.

The distance between two successive strong points (or two successive weak points) is the wavelength.

Electromagnetic Waves

The human eye is an electromagnetic radiation detector.

When an electromagnetic wave hits our eyes, we see a flash of light.

Electromagnetic Waves

Our eyes can only actually detect light in a very narrow range of wavelengths: 400 nanometers to 700 nanometers 1 nanometer, or nm, is 1 billionth of a meter.

But light waves can exist at wavelengths other than this range, from billionths of a nanometer, to hundreds of kilometers.

This light is invisible to our eyes, but we can build equipment that can detect it and make pictures from it.

Wavelengths and Colors

DDiiffffeerreenntt colors of visible light correspond to different wavelengths.

The Electromagnetic Spectrum The whole range of

wavelengths light waves can have is called the electromagnetic spectrum.

We divide the spectrum up into different “zones” with different types of electromagnetic radiation.

All wavelengths travel at the same “speed of light” The speed is related to the wavelength and frequency

C = λf (speed = wavelength . frequency)

The Electromagnetic SpectrumShortest Wavelength

Highest EnergyLongest Wavelength

Lowest Energy

Our Horrible Atmosphere

Each type of EM radiation tells astronomers something about the object emitting it.

BUT, our atmosphere blocks most EM radiation.

Only visible light, some infrared, a little UV, and radio radiation makes it through.

But Did Newton LOSE?

Remember how Newton thought light was made of particles, not waves?

Did Maxwell put Newton’s idea to rest once and for all?

NO! Turns out another very famous scientist came out on Newton’s side!

Photons

In some of his early experiments, Albert Einstein proved that sometimes light behaves as a particle rather than a wave.

These particles of light are called photons.

This “dual nature” of light is called wave-particle duality.

Optical Telescopes Astronomers use telescopes to gather

more light from astronomical objects.

The larger the telescope, the more light it

gathers.

Refracting/Reflecting TelescopesRefracting Telescope:

Lens focuses light onto the focal plane

Reflecting Telescope:

Concave Mirror focuses light onto the focal

plane

Almost all modern telescopes are reflecting telescopes.

Focal length

Focal length

In reflecting telescopes: Secondary

mirror, to re-direct light path towards back or

side of incoming light

path.

Eyepiece: To view and

enlarge the small image produced in

the focal plane of the

primary optics.

Secondary Optics

Refraction

Lenses work by Refraction Occurs when light passes from one material

into another. It’s caused by the fact that the speed of light

changes in different materials.

Refraction – The “Tank” Analogy

When we draw a “ray” of light:

We’re really drawing the directions the wave fronts are traveling! The wave fronts are where the peaks of the EM wave are located:

These vertical lines are the wave fronts.

Refraction – The “Tank” Analogy When wave fronts hit a

material at an angle, one side of the wave front hits before the other.

So one side of the wave front slows down while the other side stays moving at the same rate (like the treads of a tank).

The wavefront “turns” as it passes into the material.

The result: the ray of light is bent.

Disadvantages of Refracting Telescopes• Chromatic aberration: Some wavelengths are bent more than others, so they are focused at different focal lengths (prism effect).

Can be corrected, but not eliminated by second lens out of different material.

• Difficult and expensive to produce: All surfaces must be perfectly shaped; glass must be flawless; lens can only be

supported at the edges

The Powers of a Telescope

A telescope has three different powers. In order of importance they are:Light Gathering PowerResolving PowerMagnifying Power

Light Gathering Power

Light Gathering Power (LGP) – the amount of light a telescope can collect.

You can compare the light gathering power for two telescopes with:

LGP = (D1/D2)2

D1 is the diameter of the objective lens/mirror for the 1st telescope. D2 is the diameter of the objective lens/mirror for the second telescope.

LGP is the light gathering power of the 1st telescope compared to the second.

Resolving Power

The Resolving Power of a telescope is the minimum separation in the sky (in arc-seconds) between two stars that the telescope can “see.”

Resolving Power Here’s another example: Resolving power for an

optical telescope can be estimated by this equation:

α = 11.6 / D D is the diameter of the

telescope’s objective lens/mirror in centimeters.

α is the telescope’s resolving power in seconds of arc.

Resolving Power And Seeing

Our atmosphere is like looking through 100 km of dirty glass.

Moving air currents in the atmosphere blur the image in a telescope.

Reduce the resolution of ground-based telescopes, often dramatically.

Amount depends on how turbulent the air is on the night you observe (how much the stars twinkle is a major clue).

Resolving Power And Seeing

The atmosphere’s effect on the resolving power of a telescope is called seeing.

Because of seeing, the resolving power of most telescopes is limited to: 5 or more arc-seconds on bad nights. About 2 arc-seconds on normal nights. About 0.5 arc-seconds on exceptionally clear nights.

Most of these numbers are higher than the resolving power of any observatory telescope, so most of the time, resolving power is limited by seeing, not by the size of the telescope’s objective lens/mirror.

Adaptive OpticsComputer-controlled mirror support adjusts the mirror surface (many times per second) to compensate for distortions by atmospheric turbulence

Magnification

Magnifying power is given by this equation:

M = ft / fe

M is the magnifying power for the telescope. ft is the focal length of the telescope.

fe is the focal length of the eyepiece.

The Best Location for a Telescope

Far away from civilization – to avoid light pollution

The Best Location for a Telescope

On high mountain-tops – to avoid atmospheric turbulence ( seeing) and other weather effects

Paranal Observatory (ESO), Chile

Next time… Modern ground-based and space

Telescopes.Read Units 29 and 30.