Telescopes

Introduction  A telescope is designed to form on the retina of the eye a larger image of an object than would be created if the object were viewed with the naked eye.  In this experiment three different types of telescopes will be constructed.

The three telescopes are:  Astronomical  Terrestrial  Galilean (opera glass)  To understand how they operate one should first see how an image is created by a single lens.

Types of Lenses Double Convex Double Concave Concave Meniscus Plano Convex Plano Concave Convex Meniscus Farsighted people use lenses similar to these.Nearsighted people use lenses similar to these.

A Convex Lens Converges Light Rays f

A Concave Lens Diverges Light Rays f

Imaging with a Convex Lens sees an image here. Convex Lens f Principal Axis Arrow as Object A ray parallel to the principal axis is bent upon entering the lens. Upon exiting the lens it is bent again and passes through a point called the focal point. A ray passing through the center of the lens is basically undeflected. An eye placed here This arrangement produces an inverted, real, diminished image. Image descriptions follow.

Image Descriptions Inverted image means that the image is up-side-down compared to the object. Real image means that the image can be viewed by the reflection from a screen placed at the image’s location. The light rays forming the image actually pass through the physical location of the image. Diminished image means that it is reduced in size compared to the size of the object.

More Imaging With a Convex Lens sees an image here. Convex Lens f Arrow as Object A ray parallel to the principal axis is bent upon entering the lens. Upon exiting the lens it is bent again and passes through a point called the focal point. A ray passing through the center of the lens is basically undeflected. An eye placed here This arrangement produces an upright, virtual, magnified image. It is a simple magnifying glass.

Image Descriptions Upright image means that the image is in the same up or down orientation as the object. Virtual image means that the image cannot be formed on a screen. The light rays forming the image only appear to pass through the physical location of the image. Magnified or enlarged image means that it is larger in size compared to the size of the object.

f Imaging with a Concave Lens sees an image here. Concave Lens Arrow as Object A ray parallel to the principal axis such that is appears to have come from a point called the focal point. is bent upon entering the lens. Upon exiting the lens it is bent again A ray passing through the center of the lens is basically undeflected. An eye placed here This arrangement produces an upright, virtual, diminished image.

Astronomical Telescope  The astronomical telescope is composed of an objective lens and an eyepiece lens.  The objective lens produces a diminished, real inverted image of an object being viewed.  For distant objects this image is just outside the focal point of the objective lens.

The Objective Lens of an Astronomical Telescope Objective (Convex Lens) The rays from a distant object (arrow) This real image becomes the object for the eyepiece. form this image.

Eyepiece of an Astronomical Telescope  The eyepiece lens is used as a simple magnifying glass to observe the image formed by the objective lens.  This image formed by the eyepiece is a virtual image of the real image produced by the objective.  The observer will see an inverted image with this type of telescope.

The Eyepiece Lens of an Astronomical Telescope Eyepiece (Convex Lens) f The image formed by the objective becomes the object for the eyepiece.

 An astronomical telescope can serve as a microscope if the eyepiece and objective are swapped.

Inverter Lens for a Terrestrial Telescope  A third lens can be introduced between the objective lens and the eyepiece lens of an astronomical telescope in such a way as to produce an upright image for the observer.  This lens effectively transforms the astronomical telescope into a terrestrial telescope.

The Inverter Lens of a Terrestrial Telescope Inverter (Convex Lens) f The image formed by the objective becomes the object for the inverter. The inverter simply inverts the image.

Galilean Telescope or Opera Glass  The objective lens is convex and the eyepiece lens is concave.  The resultant image is upright, magnified, and virtual.  The field of view is quite limited, which is okay for operas but is not very useful for watching football games for example.

The Eyepiece lens of a Galilean Telescope (Opera Glass) Eyepiece (Concave Lens) f Light from the objective would form an image here if the eyepiece were not present. An eye placed here The parallel ray actually bends like this. would see an image here.

Magnification of a Telescope

Telescope Summary Follows

The Astronomical Telescope Eyepiece (Convex Lens) (Shorter focal length) Objective (Convex Lens) (Longer focal length)

The Terrestrial Telescope Eyepiece (Convex Lens) (Shorter focal length) Objective (Convex Lens) (Longer focal length) Inverter (Convex Lens) (Shorter focal length)

The Galilean Telescope (Opera Glass) Eyepiece (Concave Lens) (Shorter focal length) Objective (Convex Lens) (Longer focal length)