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

DoubleConvex

DoubleConcave

Concave Meniscus

PlanoConvex

PlanoConcave

Convex Meniscus

Farsighted people use lenses similar to these.Nearsighted people use lenses similar to these.

A Convex Lens Converges Light Rays

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A Concave Lens Diverges Light Rays

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Imaging with a Convex Lens

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Convex Lens

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Principal Axis

Arrow asObject 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

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Convex Lens

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Arrow asObjectA 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.

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Imaging with a Concave Lens

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Concave Lens

Arrow asObject 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 anAstronomical 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)

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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 aTerrestrial 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 aTerrestrial Telescope

Inverter(Convex Lens)

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The image formed by the objective becomes the object for the inverter.

The inverter simply inverts the image.

Galilean Telescopeor 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)

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Light from the objective

would form an image hereif 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

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objectivetheoflengthf ocalionMagnifi cat

eyepiecetheoflengthf ocal

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)