isns 3371 - phenomena of nature the eye the eye consists of pupil that allows light into the eye -...

ISNS 3371 - Phenomena of NatureThe Eye

The eye consists of pupil that allows light into the eye - it controls the amount of light allowed in through the lens - acts like a simple glass lens which focuses the light on the retina - which consists of light sensitive cells that send signals to the brain via the optic nerve. An eye with perfect vision has its focus on the retina when the muscles controlling the shape of the lens are completely relaxed - when viewing an object far away - essentially at infinity.

ISNS 3371 - Phenomena of Nature

When viewing an object not at infinity, the eye muscles contract and change the shape of the lens so that the focal plane is at the retina (in an eye with perfect vision). The image is inverted as with a single lens - the brain interprets the image and rights it.


Types of Optical Telescopes


Magnification Using Two Lenses - Refracting Telescope

f1 = 0.5 mf2 = 0.1 m

f1 = 0.5 mf2 = 0.3 m

Refracting telescope - consists of two lenses - the objective and the eyepiece (ocular). Incident light rays (from the left) are refracted by the objective and the eyepiece and reach the eye of the person looking through the telescope (to the right of the eyepiece). If the focal length of the objective (f1) is bigger than the focal length of the eyepiece (f1), the refracting astronomical telescope produces an enlarged, inverted image:

magnification = f1 /f2


Refracting Telescope

Uses lens to focus light from distant object - the eyepiece contains a small lens that brings the collected light to a focus and magnifies it for an observer looking through it.


The largest refracting telescope in the world is the at the University of Chicago’s Yerkes Observatory - it is 40 inches in diameter and 63 feet long.


Reflecting Telescope

The primary mirror focuses light at the prime focus. A camera or another mirror that reflects the light into an eyepiece is placed at the prime focus.


Types of Reflecting Telescopes

Each design incorporates a small mirror just in front of the prime focus to reflect the light to a convenient location for viewing.


The Keck Telescopes

Largest in the world - on Mauna Kea in Hawaii. 36 hexagonal mirrors function as single 10-meter mirror.


The Hubble Space Telescope is 43.5 ft long and weighs 24,500 lbs. Its primary mirror is 2.4 m (7 ft 10.5 in) in diameter.

The Hubble Space Telescope


Reflecting telescopes are primary astronomical tools used for research:

1. Lens of refracting telescope very heavy - must be placed at end of telescope - difficult to stabilize and prevent from deforming

2. Light losses from passing through thick glass of refracting lens - must be very high quality and perfectly shaped on both sides

3. Refracting lenses subject to chromatic aberration

Refracting vs Reflecting Telescopes


Lens and Mirror AberrationsSPHERICAL (lens and mirror)

Light passing through different parts of a lens or reflected from different parts of a mirror comes to focus at different distances from the lens.

Result: fuzzy image

CHROMATIC (lens only)

Objective lens acts like a prism.

Light of different wavelengths (colors) comes to focus at different distances from the lens.

Result: fuzzy image


Focal point for blue light

Focal point for red light

Focal point for all light

The problem

The solution

Simple lenses suffer from the fact that different colors of light have slightly different focal lengths. This defect is corrected by adding a second lens

Chromatic Aberration in Lenses


Simple lenses suffer from the fact that light rays entering different parts of the lens have slightly difference focal lengths. As with chromatic aberration, this defect is corrected with the addition of a second lens.

One focal point for all light rays

The problem

The solution

Spherical Aberration in Lenses


Simple concave mirrors suffer from the fact that light rays reflected from different locations on the mirror have slightly different locations on the mirror have slightly different focal lengths. This defect is corrected by making sure the concave surface of the mirror is parabolic

The Problem

The Solution

All light rays converge at a single point

Spherical Aberration in Mirrors


Mirror Position and Focus Animation

Focus Inversion Animation

The image from an reflecting telescope is inverted.

The focus is adjusted by changing the secondary mirror position.


1. Imaging– use a camera to take pictures (images)– Photometry measure total amount of light from an object

2. Spectroscopy– use a spectrograph to separate the light into its different – wavelengths (colors)

3. Timing– measure how the amount of light changes with time

(sometimes in a fraction of a second)

Uses of Telescopes


Imaging

• In astronomy, filters are usually placed in front of a camera to allow only certain colors to be imaged

• Single color images are superimposed to form true color images.


Nonvisible Light

• Most light is invisible to the human eye - gamma rays, x-rays, ultraviolet, infrared, radio waves.

• Special detectors/receivers can record such light - each type of light can provide information not available from other types.

• Digital images are reconstructed using false-color coding so that we can see this light.

Chandra X-ray image of the Center of the Milky Way Galaxy

ISNS 3371 - Phenomena of Nature The Crab NebulaVisible Infrared

Radio Waves X-rays


Earth’s atmosphere causes problems for astronomers on the ground:

• Bad weather makes it impossible to observe the night sky.

• Man-made light is reflected by the atmosphere, thus making the night sky brighter.– this is called light pollution

• The atmosphere absorbs light - dependent on wavelength

• Air turbulence in the atmosphere distorts light.– That is why the stars appear to “twinkle”.– Angular resolution is degraded.

Atmospheric Effects


Atmospheric Absorption of Light

• Earth’s atmosphere absorbs most types of light.– good thing it does, or we would be dead!

• Only visible, radio, and certain IR and UV light make it through to the ground.

To observe the other wavelengths, we must put our telescopes in space!


Space Astronomy


Space Based Telescopes

Chandra X-ray Obs. Hubble Space Telescope

Compton Gamma Ray Obs. Spitzer Space Telescope (IR)

FUSE (Far UV)


Radio Telescopes

305-meter radio telescope at Arecibo, Puerto Rico

• The wavelengths of radio waves are long.• So the dishes which reflect them must be very large to achieve any

reasonable angular resolution.


Spectrum of a Low Density Cloud Animation


Kirchhoff’s Laws of Radiation

First Law. A luminous solid, liquid or gas, such as a light bulb filament, emits light of all wavelengths thus producing a continuous spectrum of thermal radiation.

Second Law. If thermal radiation passes through a thin gas that is cooler than the thermal emitter, dark absorption lines are superimposed on the continuous spectrum. The gas absorbs certain wavelengths. This is called an absorption spectrum or dark line spectrum.

Third Law. Viewed against a cold, dark background, the same gas produces an emission line spectrum. It emits light of discrete wavelengths. This is called an emission spectrum or bright line spectrum.

.


So what astronomical body has this spectrum?


The Doppler Effect


Doppler Effect for Light Animation

ISNS 3371 - Phenomena of Nature The Doppler Effect

1. Light emitted from an object moving towards you will have its wavelength shortened.

2. Light emitted from an object moving away from you will have its wavelength lengthened.

3. Light emitted from an object moving perpendicular to your line-of-sight will not change its wavelength.

BLUESHIFT

REDSHIFTREDSHIFT


Doppler Shift of Emission Lines Animation


v c

=

The amount of spectral shift tells us the velocity of the object:


The Doppler shift only tells us part of the object’s full motion - the radial part or the part directed toward or away from us.

isns 3371 - phenomena of nature the eye the eye consists of pupil that allows light into the eye -...

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