Differences

This shows you the differences between two versions of the page.

--- courses:phy100:3 [2023/06/21 01:22] – [2. Color] asad
+++ courses:phy100:3 [2023/06/22 11:41] (current) – [5. Seeing the invisible] asad
@@ Line 2: / Line 2: @@
 ===== - Light =====
-Light can be described both as a **particle** and a **wave**. Light is made of particles called **photons** and waves called **electromagnetic waves**. Let us try to understand light as a wave first.
+Light can be described both as a **particle** and a **wave**. Light is made of particles called **photons** and (at the same freaking time) waves called **electromagnetic waves**. Let us try to understand light as a wave first.
 The best way to visualize a wave is to use the example of a //water wave//. In the following video, you see the largest replica of an ocean created inside a huge dome by US Navy. They use this indoor ocean the size of a football field for experimenting with the effect of waves on oceangoing ships.
@@ Line 60: / Line 60: @@
 This is the 'spectrum' of sunlight, meaning the light of the sun at different colors. Note that this spectrum is not the same as the spectrum you saw in the beginning of this section. The previous spectrum was smooth, that did not have any black lines and showed all the shades of all the seven colors. But some shades of some of the colors are missing in this spectrum. There are hundreds of such 'missing colors' and only a few prominent missing colors (called '**absorption lines**') have been identified here using the letters A, B, C, D, E, F, G, H and K.
-Why are some colors (light at some wavelengths or frequencies) missing? The sun emits smooth light, meaning it emits light at all colors from violet to red. But the sun has an atmosphere where there are many different chemical elements. The elements **absorb** light of specific colors. Each element can only absorb the light of one specific color. Each line correspond to the absorption of light at a specific color by a specific element. So the lines directly tell us which elements are present in the atmosphere of the sun.
+Why are some colors (light at some wavelengths or frequencies) missing? The sun emits smooth light, meaning it emits light at all colors from violet to red. But the sun has an atmosphere where there are many different [[un:chemical element]]s. The elements **absorb** light of specific colors. Each element can only absorb the light of one specific color. Each line correspond to the absorption of light at a specific color by a specific element. So the lines directly tell us which elements are present in the atmosphere of the sun.
 A and B lines are created by oxygen molecules, C lines by hydrogen atoms, D by sodium, E by iron, and G, H and K by calcium. So the dark 'absorption lines' or the 'missing colors' give away the chemical composition of stars.
 ===== - Telescopes =====
-{{https://upload.wikimedia.org/wikipedia/commons/3/3d/Schmidt-Newton.png?nolink}}
+A telescope works almost the way our eyes work. Our eyes are made of lenses that can bend light to focus them around a single point as shown below.
-{{https://upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Chromatic_aberration_lens_diagram.svg/640px-Chromatic_aberration_lens_diagram.svg.png?nolink}}
+{{https://askabiologist.asu.edu/sites/default/files/resources/articles/seecolor/eye-anatomy-1000.jpg?nolink&500}}
-{{https://upload.wikimedia.org/wikipedia/commons/thumb/2/2a/Focus-balanced_parabolic_reflector.svg/640px-Focus-balanced_parabolic_reflector.svg.png?nolink}}
+The lens produced an inverted image of the leaf on the back of the eye in a plane called the retina. There are receptors in retina that convert the photons of light to chemical signals and then send them to the brain via the optic nerve as electrical signals. That means our vision has 3 components: the **collector** (lens), the **sensor** (retina) and the **processor** (brain). A modern telescope also has exactly these 3 components.
+In case of modern telescopes, a mirror is used as the collector for focusing light. The sensor is made of an electronic device called the [[un:CCD]] that can convert photons to electrons. The electrons are then sent from the sensor to a computer that works as the processor. So the mirror is like the lens of our eyes, the CCD is like the retina and the computer is like the brain. Let us see how the collector (or reflector) works.
+{{:courses:phy100:newtonian.jpg?nolink&800|}}
+This is a typical 'optical tube' of a small amateur telescope of the Schmidt-Newtonian type. It has two mirrors: the bigger 'primary' mirror and the smaller 'secondary' mirror. Astronomical objects are so far away from us that their light enters our optical tubes always parallel to each other. Here you see two such lightrays (red dashed parallel lines) entering the tube from the left. They are reflected from the primary mirror and sent to the secondary mirror which in turn reflects them to the eyepiece. If you look through the eyepiece, you see an enlarged 'image' of the 'real' object out there in the sky.
+Modern telescopes do not have eyepieces. Astronomers do not look through an eyepiece anymore. Instead, they take pictures of the sky using sensors, receivers or detectors which will discuss in the next section. This section is dedicated to the collector.
+The initial collectors or reflecting mirrors were spherical, but large **spherical** mirrors cannot focus light properly and we do need large mirrors to collect as much light as possible. To solve this problem, **parabolic** mirrors were invented.
+{{:courses:phy100:parabolic.jpg?nolink|}}
+Small spherical mirrors can focus all parallel rays onto a single point  $F$  at a distance  $f$  from the mirror. But in case of large spherical mirrors, different rays meet at different points. If we cannot focus all the rays onto a single point, we cannot detect properly using a sensor which cannot be at multiple places at the same time. If all light are focused at a single point, a sensor located there would be able to receive all of them. This is why parabolic mirrors were made in the 17th and 18th centuries. They are more curved then a spherical mirror (left panel) and their shape is not described by a circle but a parabola. All modern telescopes use parabolic reflectors.
 {{https://upload.wikimedia.org/wikipedia/commons/c/c5/Comparison_optical_telescope_primary_mirrors.svg?nolink}}
+Here is a comparison of the largest current and planned visible-light telescopes. Extremely large mirrors are now made using multiple smaller pieces. For example, the mirror of the recently-sent James Webb Space Telescope (lower left) is made of 18 hexagonal segments.
 ===== - eVscope and eQuinox =====
+In this course, you will use an eQuinox telescope made by Unistellar for your final project. Unistellar makes two models: eVscope and eQuinox. The eVscope has a live projection system in place of an eyepiece so that people can have a sneak-peek of what the telescope is observing. The eQuinox does not have any eyepiece or live projection system. Both models work in a similar way. The telescope creates a WiFi network that you connect to using your phone. Then you can open the 'Unistellar' app on your phone and control and move the telescope using the app. The field of view of the telescope is always visible on the app and you can capture an image whenever you want.
 {{:courses:phy100:evscope.jpg?nolink|}}
+The telescope has the same 3 parts mentioned above: collector, sensor, processor. The collector, however, is made of a single mirror. There is no secondary mirror at the focal point of this mirror, but the sensor is placed there instead. The sensor converts all light reflected by the mirror into electrons. The electrons are sent to the 'onboard computer' (the processor) located at beside the optical tube. This is a diagram of eVscope.
 {{:courses:phy100:equinox-tech.jpg?nolink|}}
+The parts are more clearly identified in this diagram of an eQuinox. The integrated battery located at the bottom of the tube can give backup for almost 10 hours, that is one night of observing time.
+This telescope does not enlarge the image but rather, like all modern telescopes, take high-**resolution** pictures of various objects of the sky. If it has high resolution, you can always zoom in using a computer and see more details of the object. The 'zooming in' is almost like the modern equivalent of the optical 'magnification' used in the olden days.
+The '**field of view**' (FoV) of the telescope is around 0.5 degrees, that is the size of an image taken by eVscope or eQuinox is around 0.5 degrees, almost same as the size of the sun or the moon as viewed from earth. You can hide the moon completely using your pinkie, that means **0.5 degrees (30 arcminutes)** is the size of your pinkie, the little finger.
 ===== - Seeing the invisible =====
+There are light waves beyond ultraviolet at the high-frequency end and beyond infrared at the low-frequency end. Heinrich Hertz detected such a wave for the first time and named it 'x-ray' as it was unknown then. Later many more '**invisible**' light waves have been found. We use microwave light for heating our food and watching TV and we use radio waves for all kinds of wireless communication and the all-important Global Positioning System (GPS).
 {{:courses:phy100:waves.jpg?nolink&500|}}
+X-rays and gamma-rays are shorter (higher frequency) than visible light and radio and microwaves are longer (lower frequency) than visible light as shown above. There are telescopes custom-made for detecting light at all these wavelengths or frequencies. The ultraviolet and infrared wave are not shown here, but we have telescopes for detecting those light as well.
+{{https://upload.wikimedia.org/wikipedia/commons/thumb/3/34/Atmospheric_electromagnetic_opacity.svg/1024px-Atmospheric_electromagnetic_opacity.svg.png?nolink}}
+This figure shows the opacity (opposite of transparency) of Earth's atmosphere at different wavelengths ranging from radio waves all the way to gamma rays. Earth's atmosphere is 100% opaque to x-rays and gamma-rays, so we have to send telescopes to space for observing those waves coming from astronomical objects. Visible light and the near-infrared part can be seen from the surface, but the far-infrared part of the spectrum can be observed only from space. The atmosphere is almost completely transparent to radio waves. So radio astronomers are pretty happy on earth.
+And radio astronomy is having a huge boost similar to some other branches of astronomy. Around the year 2030, the largest scientific facility in the world will be a radio telescope so large that it spans two continents: Africa and Oceania. The telescope, named the **[[https://www.skao.int/en|Square Kilometer Array (SKA)]]**, will be an array thousands of antennas scattered throughout the globe. The higher frequency antennas will be in Southern Africa and the lower frequency parts will be in Australia and New Zealand. But many countries are working together to make this dream come true as you see in the promotional video below.
+{{youtube>iHQLVvAygcE?large}}
+\\
+I did my postdoctoral research with the SKA and continue to collaborate with the SKA community from IUB.