# Waves and Light

Part I: Waves
Light is an example of a wave, and there are many types of waves in nature.  Examples of familiar waves are water waves, sound waves, and light waves (and don’t forget gravity waves!).  Waves are characterized by three parameters: wavelength, frequency (waves per second), and speed, and these properties are related by a simple expression.
Speed = Wavelength  x  Frequency

Speed is measured as distance traveled per second.
Wavelength is the length of a wave from peak to peak.
Frequency is the number of waves per second that pass a given point.

1)  What is the wavelength of a typical sound wave?  The frequency of, say, middle C is about 256-278 cycles per second depending on the scale, and sound travels at about 340 meters per second.
2) A tsunami wave travels at a speed of about 0.2 meters/second, with a wavelength of about 50,000 m.  What will be the time between peaks of a tsunami wave coming ashore on the beach?
3)  Electromagnetic waves travel with speed of about 300,000 km per second (3 x 108 meters per second).  What is the wavelength of an electromagnetic wave with a frequency of one billion cycles per second (109 cycles per second).  What kind of light is this?
4)  Gravity waves also travel at the speed of light (3 x 108 meters per second).  Two neutron stars, each with mass equal to two solar masses, orbiting each other with a separation of 0.63 light seconds and a period of 750 seconds (frequency = 0.0013 per second), will emit gravity waves.  What is the wavelength of these gravity waves?
Part II: Light
Astronomers use light to study celestial objects, but light comes in many forms.  The most familiar form is visible light, the form of electromagnetic radiation that we can see.  The full spectrum of electromagnetic radiation extends to much longer and to much shorter wavelengths that we can see, however.  Radio waves, microwaves, infrared, visible, and ultraviolet light, x-rays, and gamma rays are all forms of electromagnetic radiation.
For each form of light, determine the wavelength and frequency of the waves of electromagnetic radiation, and identify a common object that is about the same size.
Wavelength __________________
Frequency __________________
Object of similar size ___________
Microwaves
Wavelength __________________
Frequency __________________
Object of similar size ___________
Infrared light
Wavelength __________________
Frequency __________________
Object of similar size ___________
Visible light
Wavelength__________________
Frequency __________________
Object of similar size ___________
Ultraviolet light
Wavelength __________________
Frequency __________________
Object of similar size ___________
X-rays
Wavelength _________________
Frequency ______________
Object of similar size _____________
Gamma Rays
Wavelength __________________
Frequency __________________
Object of similar size ___________
Part III: Multiwavelength Astronomy
This lab will teach you to make pretty astronomical images from true astronomical data. Our eyes are wavelength detectors: within the visual spectrum (roughly 390 to 700 nm), we perceive light at different wavelength as light of different color. Most astronomical images that are published in the media are in color, however when we look through a telescope, mostly we just see white light, and images that are often less spectacular than the published images. Astronomers usually release images in false or enhanced colors. Let’s look at this popular image: it is a false color image of Eta Carinae (left panel), a spectacular nebula in the southern sky:
However if you were to look at Eta Carinae with a telescopes you would see basically only white light. If you took a photo of Eta Carinae through the telescope you would see at best, the diversity of colors of the image on the right panel, dominated by red colors due to strong hydrogen emission (656 nm, see the rightmost plot). However with telescopes, as you know, we can collect light that is invisible to the human eye. Basically what astronomers do is compressing a broader spectrum range, say, for example from NIR to soft UV, 50 to 100nm (but really whichever range we want to probe!) into the visible spectrum: the longest wavelength will be represented by a red image, the shortest by blue image. Because of the way our eyes work, almost any color can be simulated by combining red, green, and blue light. So when we put together a red, a blue and a green image our eye perceive a beautiful diversity of colors. However, not necessarily the colors our eyes would perceive when looking at this particular astronomical object.
The astronomical images we see on the web and in the media are usually ‘refined’ or ‘processed’ as compared to the raw data that the astronomers work on with their computers. In ‘pretty pictures’ all artifacts coming from the telescope or the detectors are for instance removed as they do not say anything about the objects themselves. It is very rare that images are taken with the sole intention of producing a ‘pretty’ color picture. Most ‘pretty pictures’ are constructed from data that was acquired to study some physical process, and the astronomer herself probably never bothered to assemble the greyscale images to a color image.
Natural color images
It is possible to create color images that are close to “true-color” if three wide band exposures exist, and if the filters are close to the r, g and b receptors in our eyes. Images that approximate what a fictitious space traveller would see if he or she actually travelled to the object are called “natural color” images. To make a natural color image the order of the colors assigned to the different exposures should be in “chromatic order”, i.e. the lowest wavelength should be given a blue hue, the middle wavelength a green hue and the highest wavelength should be red.
Representative color images
If one or more of the images in a data set is taken through a filter that allows radiation that lies outside the human vision span to pass – i.e. it records radiation invisible to us – it is of course not possible to make a natural color image. But it is still possible to make a color image that shows important information about the object. This type of image is called a representative color image. Normally one would assign colors to these exposures in chromatic order with blue assigned to the shortest wavelength, and red to the longest. In this way it is possible to make color images from electromagnetic radiation far from the human vision area, for example x-rays. Most often it is either infrared or ultraviolet radiation that is used.
Enhanced color images
Sometimes there are reasons to not use a chromatic order for an image. Often these reasons are purely aesthetic, as is seen in the example below. This type of color image is called an enhanced color image.

Figure 12: An example of an enhanced color image (not in chromatic order).
Sometimes it is necessary to break the ‘rules’ for image processing. Here the Hydrogen-alpha filter is colored blue instead of the red color it is in nature. This is an example of a so-called false-color image, where the blue was chosen for aesthetic reasons.
You are the judge

When processing raw science images one of the biggest problems is that, to a large degree, you are ‘creating’ the image and this means a colossal freedom within a huge parameter space. There are literally thousands of sliders, numbers, dials, curves etc. to twist and turn. Speaking of right and wrong, there really are no wrong or right images. There are some fundamental scientific principles that should normally be observed, but the rest is a matter of aesthetics — taste. Chromatic ordering of the exposures is one of the important scientific principles.

Now, it’s your turn to make your own scientific images! Use this link (Links to an external site.) to select an object, choose which wavelengths go to which filters, and fill out the questions below.

Pick at least 2 images and fill out the following questions:
[Insert a screenshot of your astronomical image .]
Name :
What type of object is it?
What filters are used?
Describe the final image:
What color dominates the final image?
Guess what the color tell us about the object and anything you find noticeable.

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