How does telescope magnify

The distance from the lens to the focal point is called the focal length of the lens. If a lens is concave or diverging, it takes parallel rays and bends them so that they spread out. The rays will then appear to originate from a point in front of the lens. This point is also called the focal point, and its distance is measured in negative units.

The earliest telescopes, as well as many amateur telescopes today, use lenses to gather more light than the human eye could collect on its own. They focus the light and make distant objects appear brighter, clearer and magnified. This type of telescope is called a refracting telescope. Most refracting telescopes use two main lenses. The largest lens is called the objective lens, and the smaller lens used for viewing is called the eyepiece lens.

The size of an image produced by a lens is proportional to the focal length of the lens. The longer the focal length, the larger the image. The brightness of an image from a telescope depends partly on how much light is collected by the telescope.

The light-gathering power of a telescope is directly proportional to the area of the objective lens. The larger the lens, the more light the telescope can gather. Doubling the diameter of the lens increases the light gathering power by a factor of 4. Brightness of images also depends on how big an area the image light is spread over.

The smaller the area, the brighter the image. The magnifying power of a telescope is the ratio of an object's angular diameter to its naked eye diameter. This depends on the focal length of both lenses. Magnification might seem like the most important aspect of a telescope, but there are limits to how sharp an image a telescope can produce because of the blurring effects of the Earth's atmosphere. 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. A lens which could only focus light rays striking the glass head-on as in the illustration above 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 the diagram below.

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! The next diagram 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 any camera you may own; of course, you turn the prints right way up when you get them from the photo store, so you're probably not aware of the orientation of the image 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. Your simple telescope kit includes a large objective lens which you will use to study image formation. Take the large lens and mount it at one end of the larger cardboard tube; slide the smaller tube into the other end of the larger one, and use a rubber band to hold a sheet of tracing paper over the open end of the smaller tube.

Now point the tubes at the subject we've set up in the lab, and slide the smaller tube in and out until you focus a sharp image of the subject on the tracing paper.

Record your observations and measurements in your lab notebook. Include a sketch the image. Most of the time, professional astronomers use telescopes to take pictures of astronomical objects. The instrument you've just built is a crude model of a professional photographic telescope; if the tracing paper was replaced by a piece of photographic film, you could use this equipment to take a picture in much the same way the pros do.

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 you 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.

The first image formed by a telescope objective as seen in Figure 1b will not be large compared with what you might see by looking at the object directly. For example, the spot formed by sunlight focused on a piece of paper by a magnifying glass is the image of the Sun, and it is small. The telescope eyepiece like the microscope eyepiece magnifies this first image. The distance between the eyepiece and the objective lens is made slightly less than the sum of their focal lengths so that the first image is closer to the eyepiece than its focal length.

It can be shown that the angular magnification of a telescope is related to the focal lengths of the objective and eyepiece; and is given by. The minus sign indicates the image is inverted. To obtain the greatest angular magnification, it is best to have a long focal length objective and a short focal length eyepiece. The greater the angular magnification M , the larger an object will appear when viewed through a telescope, making more details visible.

Limits to observable details are imposed by many factors, including lens quality and atmospheric disturbance. The image in most telescopes is inverted, which is unimportant for observing the stars but a real problem for other applications, such as telescopes on ships or telescopic gun sights.

But a more common arrangement is to use a third convex lens as an eyepiece, increasing the distance between the first two and inverting the image once again as seen in Figure 2. Figure 2. This arrangement of three lenses in a telescope produces an upright final image.

The first two lenses are far enough apart that the second lens inverts the image of the first one more time. The third lens acts as a magnifier and keeps the image upright and in a location that is easy to view. Figure 3. A two-element telescope composed of a mirror as the objective and a lens for the eyepiece is shown. This telescope forms an image in the same manner as the two-convex-lens telescope already discussed, but it does not suffer from chromatic aberrations.

Such telescopes can gather more light, since larger mirrors than lenses can be constructed. A telescope can also be made with a concave mirror as its first element or objective, since a concave mirror acts like a convex lens as seen in Figure 3. Flat mirrors are often employed in optical instruments to make them more compact or to send light to cameras and other sensing devices. There are many advantages to using mirrors rather than lenses for telescope objectives. Mirrors can be constructed much larger than lenses and can, thus, gather large amounts of light, as needed to view distant galaxies, for example.

Large and relatively flat mirrors have very long focal lengths, so that great angular magnification is possible. Telescopes, like microscopes, can utilize a range of frequencies from the electromagnetic spectrum.