The Telescope

(Reflecting & Refracting)by: Caryl Lou Cotejo

The Telescope (definition)

• It is an instrument which helps the human eye see objects not ordinarily visible• It also increases the size of an object’s image on the retina.

Types of Telescopes

Optical telescopes may be divided into two general categories:• ( ) refracting telescopes that use

lenses to gather and focus light, and• ( ) reflecting telescopes that use

mirrors to accomplish the same purpose.

Refracting Telescope

As we learned earlier , the direction of light propagation is changed at the boundary of glass and air by refraction. By designing lenses having the right curvature, this principle can be used to gather and focus light. The following figure illustrates the use of a lens to gather and focus light, and the use of two lenses to make a simple refracting telescope.

Refracting Telescope

Two types of lenses are used: Objective Lens – a very large lens that

collects a large amount of light from a faraway object. It also has a long focal length

This forms the object’s first real, inverted and diminished image.

Refracting Telescope

Eyepiece lens – it has a short focal length. It acts like a magnifying lens

for the image cast by the objective lens.

is closer to the eye. It forms the final virtual, enlarged image of the object.

Refracting Telescope

Reflecting Telescope

The following figure illustrates the principle of reflection. The right side of the figure illustrates the use of a mirror to make a reflecting telescope.

Refracting Telescope

• • Many large astronomical • telescopes use concave mirrors • instead of objective lenses.

WHY?This because large perfect lenses are more

difficult to make than large mirrors.

Telescopes of this type are called REFLECTING TELESCOPES

Ray Diagrammi

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Reflecting Telescope

Refracting Telescope