Simple Telescope and astronomical telescope, Construction working, uses, ray diagram
Simple telescope
A simple working telescope requires nothing more than a pair of lenses mounted in a tube. The lens in front, known as the objective, focuses an image; the lens in back, known as the eyepiece, magnifies the image. Although it may seem like a crude device, a simple telescope nicely illustrates the basic working principles of more powerful astronomical instruments.
LENSES
Light normally moves in straight lines, but there are situations in which this is not true. You are already familiar with some: for example, the distortions you see looking through the surface of the ocean occur because light bends as it passes from the water into the air. Long before we understood why light bends as it passes from one transparent material to another, people had used this effect to create lenses: optical devices which can gather light together or spread it apart.
In order to understand how a lens works, you need to know a little about how light behaves in passing from one material to another. Imagine a tank of water on the table in front of you; the surface of the water should be perfectly flat and horizontal. If you shine a ray of light straight down from above, it will pass through the surface of the water without bending. But if you shine the light in at an angle, it will bend as it passes through the surface. Fig. 1 illustrates two important facts about this effect. First, in passing from air to water, the light always bends into the water. Second, the smaller the angle between the light ray and the surface, the more it bends in passing through. The same rules would also apply if the tank of water was replaced with a block of glass.
To create a lens which can focus many parallel rays of light to a single point, the idea is to curve the surface of the glass so that all these rays, after passing through, come together at the same place. It’s a bit tricky to do this right, but we don’t need to worry about the details. The simplest kind of lens is a `plano-convex’ lens; one side is flat, while the other bulges out at the middle. Fig. 2 shows how such a lens focuses light. The optical axis of the lens is the thick line which passes right through the middle of the lens; a ray of light traveling along the optical axis is not bent at all. Rays which pass through the top of the lens are bent downward, while rays which pass through the bottom of the lens are bent upward. Thus all these light rays are bent toward the optical axis. If the lens is well-made, all rays meet at the same focal point. The distance between the lens and the focal point, measured along the optical axis, is called the focal length.
A simple lens in operation. Parallel light rays come from the right, pass through the lens, and meet at the focal point on the left. The thick line through the middle of the lens is the optical axis; the distance F is the focal length.
Image formation
A lens which could only focus light rays striking the glass head-on (as in Fig. 2) would be fairly useless for astronomy. Fortunately, most lenses can also accept rays which come in at a slight angle to the optical axis, and bring them to a focus as well. This focal point is not the same as the focal point for rays which are parallel to the optical axis; depending on the angle of the incoming rays, their focus lies on one side or the other of the optical axis, as shown in figure. But if the lens is well-made, all these focal points will lie on a plane which is parallel to the face of the lens; this is called the focal plane.
A simple lens forming an image. The red rays arrive with an downward slant, and come to a focus below the optical axis, while the blue rays arrive with a upward slant, and come to a focus above the optical axis. The vertical dotted line at left represents the focal plane.
There’s one slightly subtle consequence of this image-formation process: the image is upside-down! Fig. 4 shows why: rays from the lower part of the subject (on the right) come together at the upper part of the image (left), and vice versa. This is also true of a camera; of course, you turn the prints right way up when you get them back from the store, so you’re probably not aware that the image is upside down inside your camera.
The image formed by a simple lens is upside-down with respect to the subject. Here the subject (right) is an arrow with a red tip pointing upward; its image (left, at the focal plane) points down.
EYEPIECES AND MAGNIFICATION
To make a telescope you can actually look through, you’ll need to add another lens. This eyepiece lens magnifies the image formed by the large objective lens and directs the light to your eye. Basically, the eyepiece works a lot like a magnifying glass; it enables your eye to focus much more closely than it normally can. The eyepiece on a typical telescope allows you to inspect the image formed by the objective lens from a distance of an inch or less. Fig. 5 shows how the objective lens and eyepiece work together in a simple telescope.
The magnification of a telescope is easy to calculate once you know the focal lengths F and f of the objective lens and eyepiece, respectively. The formula for the magnification M is
M = F ÷ f
Here you can use any units for F and f, as long as you use the same units for both. For example, if you measure F in millimeters, you should also measure f in millimeters. Using the values for F and f you measured above, calculate the expected magnification of your telescope.
To measure the magnification of your telescope directly, we will set up a target – basically a picture of a ruler with marks a unit distance apart. From the other end of the room, focus your telescope on the target. Now look through the telescope while keeping both eyes open; you should see a double image, where one image is magnified and the other is not. Compare the two images; how many of the unmagnified units fit within one magnified unit? The answer is a direct measurement of your telescope’s magnification; record it in your notebook and compare it to the magnification you calculated using the formula above.
Astronomical telescope
An astronomical telescope is an optical instrument which is used to see the magnified image of distant heavenly bodies like stars, planets, satellites and galaxies etc. The final image formed by an astronomical telescope is always virtual, inverted and magnified.
Principle of Astronomical Telescope
An astronomical telescope works on the principle that when an object to be magnified is placed at a large distance from the objective lens of telescope, a virtual, inverted and magnified image of the object is formed at the least distance of distinct vision from the eye held close to the eye piece.
Construction of Astronomical Telescope
An astronomical telescope consists of two convex lenses : an objective lens O and an eye piece E. the focal length fo of the objective lens of astronomical telescope is large as compared to the focal length fe of the eye piece. And the aperture of objective lens O is large as compared to that of eye piece, so that it can receive more light from the distant object and form a bright image of the distant object. Both the objective lens and the eye piece are fitted at the free ends of two sliding tubes, at a suitable distance from each other.
Working of Astronomical telescope
The ray diagram to show the working of the astronomical telescope is shown in figure. A parallel beam of light from a heavenly body such as stars, planets or satellites fall on the objective lens of the telescope. The objective lens forms a real, inverted and diminished image A’B’ of the heavenly body. This image (A’B’) now acts as an object for the eye piece E, whose position is adjusted so that the image lies between the focus fe’ and the optical centre C2 of the eye piece. Now the eye piece forms a virtual, inverted and highly magnified image of object at infinity. When the final image of an object is formed at infinity, the telescope is said to be in ‘normal adjustment’.
It should be noted that, the final image of object (such as stars, planets or satellites) formed by an astronomical telescope is always inverted with respect to the object. But it does not matter whether the image formed by an astronomical telescope is inverted or not, as all the heavenly bodies are usually spherical is shape.
Magnifying Power of an Astronomical Telescope
The Magnifying Power of a telescope is given by:
m = 
Magnifying Power of an Astronomical Telescope
Where, fo = Focal length of the objective lens
fe = Focal length of the eye-piece lens
And the length (L) of the tube of telescope is equal to the sum of the focal lengths of the objective lens and the eye piece. Thus, L = fo + fe
,
A telescope is an optical instrument that makes distant objects appear closer. Telescopes are used for a variety of purposes, including astronomy, navigation, and bird watching.
There are two main types of telescopes: refracting telescopes and reflecting telescopes. Refracting telescopes use lenses to bend and focus light, while reflecting telescopes use mirrors.
The first telescopes were invented in the early 17th century by Hans Lippershey and Galileo Galilei. These early telescopes were refracting telescopes, and they were used to observe the planets and stars.
In the late 17th century, Isaac Newton invented the reflecting telescope. Reflecting telescopes are more powerful than refracting telescopes, and they are now used for most astronomical observations.
Telescopes are used for a variety of purposes. Astronomy is the most common use for telescopes, and they are used to observe objects in space, such as planets, stars, and galaxies. Telescopes are also used for navigation, and they are used to help ships and airplanes find their way. Telescopes are also used for bird watching, and they are used to observe birds in their natural habitat.
Telescopes are a valuable tool for scientists and hobbyists alike. They allow us to see the world in a new way, and they help us to understand the universe around us.
Simple Telescope
A simple telescope is a device that uses two convex lenses to magnify distant objects. The objective lens is the larger lens, and it is located at the front of the telescope. The eyepiece is the smaller lens, and it is located at the back of the telescope.
Light from a distant object enters the objective lens and is focused to a point in front of the eyepiece. The eyepiece then magnifies this image. The magnification of a telescope is the ratio of the size of the image of an object formed by the telescope to the size of the object itself.
Simple telescopes are often used for bird watching and astronomy. They are also used for navigation and other purposes.
Astronomical Telescope
An astronomical telescope is a type of telescope that is used to observe objects in space. Astronomical telescopes are typically much larger than simple telescopes, and they have more powerful lenses.
Astronomical telescopes are used to observe planets, stars, galaxies, and other objects in space. They are also used to study The Solar System and the universe.
Astronomical telescopes are an important tool for astronomers. They allow astronomers to study objects in space in great detail.
Ray Diagram
A ray diagram is a diagram that shows how light travels through an optical system. Ray diagrams are used to understand how lenses and mirrors work.
In a ray diagram, light rays are represented by straight lines. The intersection of two or more light rays is called a focus. The focal length of a lens is the distance from the lens to its focus. The focal length of a mirror is the distance from the mirror to its focus.
The principal axis of a lens or mirror is a line that passes through the center of the lens or mirror and is perpendicular to the surface of the lens or mirror. The optical axis of a telescope is the line that passes through the centers of the objective lens and the eyepiece.
The field of view of a telescope is the angle subtended by the image of a distant object at the eyepiece. The magnification of a telescope is the ratio of the size of the image of an object formed by the telescope to the size of the object itself.
Ray diagrams are a valuable tool for understanding how optical systems work. They can be used to design new optical systems and to troubleshoot problems with existing optical systems.
Simple Telescope
A simple telescope is a device that uses lenses to make distant objects appear closer. It is made up of two lenses: an objective lens and an eyepiece lens. The objective lens is the larger lens, and it is located at the front of the telescope. The eyepiece lens is the smaller lens, and it is located at the back of the telescope.
When light from a distant object passes through the objective lens, it is bent and focused to form an image inside the telescope. This image is then magnified by the eyepiece lens, making the distant object appear closer.
Simple telescopes are often used for bird watching, stargazing, and other activities where it is helpful to see distant objects in more detail.
Astronomical Telescope
An astronomical telescope is a device that uses lenses or mirrors to make distant objects appear closer. It is made up of two main parts: the objective and the eyepiece. The objective is the larger lens or mirror at the front of the telescope, and the eyepiece is the smaller lens or mirror at the back.
The objective lens or mirror collects light from the distant object and focuses it to a point in front of the eyepiece. The eyepiece then magnifies this image, making the distant object appear closer.
Astronomical telescopes are used for a variety of purposes, including astronomy, bird watching, and nature watching.
Construction
A simple telescope is made up of two lenses: an objective lens and an eyepiece lens. The objective lens is the larger lens, and it is located at the front of the telescope. The eyepiece lens is the smaller lens, and it is located at the back of the telescope.
The objective lens is usually made of glass, and it has a convex shape. This means that the center of the lens is thicker than the edges. The convex shape of the objective lens causes light rays to converge, or come together, at a point in front of the lens. This point is called the focal point.
The eyepiece lens is also made of glass, and it has a concave shape. This means that the center of the lens is thinner than the edges. The concave shape of the eyepiece lens causes light rays to diverge, or spread apart, after they pass through the lens.
When light from a distant object passes through the objective lens, it is bent and focused to a point in front of the lens. This point is called the focal point. The light rays then pass through the eyepiece lens, and they are bent again. This time, the light rays are bent so that they appear to come from a point that is behind the eyepiece lens. This point is called the virtual image.
The virtual image is magnified, or made larger, than the original object. This is because the eyepiece lens is closer to the virtual image than the objective lens is. The closer the eyepiece lens is to the virtual image, the larger the image will be.
Working
A simple telescope works by using lenses to bend and focus light. The objective lens collects light from a distant object and focuses it to a point in front of the telescope. The eyepiece lens then magnifies this image, making the distant object appear closer.
The amount of magnification that a telescope provides is determined by the focal length of the objective lens and the focal length of the eyepiece lens. The focal length of a lens is the distance from the lens to the point where light rays are focused. The longer the focal length of a lens, the more magnification it provides.
Uses
Simple telescopes are often used for bird watching, stargazing, and other activities where it is helpful to see distant objects in more detail. They can also be used for educational purposes, such as teaching students about The Solar System.
Ray Diagram
A ray diagram is a diagram that shows how light rays travel through a lens or mirror. Ray diagrams can be used to explain how a telescope works.
To draw a ray diagram for a telescope, start by drawing a straight line to represent the path of light from a distant object. Then, draw a line to represent the objective lens. The objective lens will bend the light rays so that they converge at a point in front of the lens. This point is called the focal point.
Next, draw a line to represent the eyepiece lens. The eyepiece lens will bend the light rays so that they diverge after they pass through the lens. The light rays will appear to come from a point behind the eyepiece lens. This point is called the virtual image.
The virtual image will be magnified, or made larger, than the original object. This is because the eyepiece lens is closer to the virtual image than the objective lens is. The closer the eyepiece lens is to the virtual image, the larger the image will be
- A simple telescope consists of two lenses: an objective lens and an eyepiece lens. The objective lens is larger than the eyepiece lens, and it collects light from distant objects. The eyepiece lens magnifies the image formed by the objective lens.
- A simple telescope can be used to see objects that are too far away to be seen with the naked eye. It can also be used to see objects that are very faint.
- The uses of a simple telescope include astronomy, bird watching, and nature watching.
- The ray diagram for a simple telescope is shown below. The light from a distant object is collected by the objective lens and focused at a point behind the lens. The eyepiece lens then magnifies the image formed by the objective lens.
[asy]
unitsize(1 cm);
draw((0,0)–(10,0));
draw((0,2)–(10,2));
draw((0,4)–(10,4));
draw((0,6)–(10,6));
draw((0,8)–(10,8));
draw((0,10)–(10,10));
draw((0,0)–(2,0));
draw((4,0)–(6,0));
draw((8,0)–(10,0));
draw((0,2)–(2,2));
draw((4,2)–(6,2));
draw((8,2)–(10,2));
draw((0,4)–(2,4));
draw((4,4)–(6,4));
draw((8,4)–(10,4));
draw((0,6)–(2,6));
draw((4,6)–(6,6));
draw((8,6)–(10,6));
draw((0,8)–(2,8));
draw((4,8)–(6,8));
draw((8,8)–(10,8));
draw((0,10)–(2,10));
draw((4,10)–(6,10));
draw((8,10)–(10,10));
label(“$O$”, (0,0), S);
label(“$I$”, (10,0), S);
label(“$F_o$”, (5,0), S);
label(“$F_e$”, (5,2), S);
label(“$A$”, (5,4), S);
label(“$B$”, (5,6), S);
label(“$C$”, (5,8), S);
label(“$D$”, (5,10), S);
[/asy]
The image formed by the objective lens is inverted and real. The eyepiece lens magnifies the image, but it does not invert it. Therefore, the final image formed by the telescope is inverted and magnified.
The magnification of a telescope is given by the formula $M = \frac{f_o}{f_e}$, where $f_o$ is the focal length of the objective lens and $f_e$ is the focal length of the eyepiece lens.
The resolving power of a telescope is its ability to distinguish between two closely spaced objects. The resolving power of a telescope is given by the formula $\lambda/D$, where $\lambda$ is the wavelength of light and $D$ is the diameter of the objective lens.
The light gathering power of a telescope is its ability to collect light from distant objects. The light gathering power of a telescope is proportional to the square of the diameter of the objective lens.
The field of view of a telescope is the angular extent of the sky that can be seen through the telescope. The field of view of a telescope is inversely proportional to the focal length of the eyepiece lens.
The exit pupil of a telescope is the image of the objective lens formed by the eyepiece lens. The exit pupil is important because it determines the amount of light that enters the eye. The diameter of the exit pupil is equal to the focal length of the eyepiece lens divided by the magnification of the telescope.