LensesReflecting and Refracting

TelescopesLenses use the idea of refraction:

When light enters a MORE DENSE medium it slows down…

You need to be able to describe what the two types of lenses do to

light• Lenses refract light.• Convex lenses and converging lenses• Concave lenses are diverging lenses

10/19/15

Ray diagrams for lenses

The rays of light are refracted INWARDS and meet at the focus, F.The image formed is REAL – in other words, it can be seen on a screen

A “distant object”

Focal length

F

You have to be able to state what is meant by a focal point and how to find

the focal length• Focal pint is where the rays of light

meet• The focal length is the distance

between the lens and the focal point.

Now lets look at the two types of telescopes you need to know

about...

• Reflecting Telescopes

• Refracting Telescopes

Refracting telescope

• They consist generally of two lenses. • The first is the objective lens and the

second is the eye piece.• The objective lens bends (refracts)

the light so that it converges onto a focal point.

What do the 2 lenses do?

• Objective lens – collects light creates a real image within a telescope of a distant object.

• Eyepiece lens – magnifies the image of a distant object.

Problems with the Refracting Telescope

• What problems might there be with the refracting telescope, think about reflection.

Refracting telescopes

• Because of their simple design, they are generally easy to use and more reliable as their optics are permanently fixed and aligned.

• They can be heavy, especially larger aperture telescopes, because of their need for large lenses.

• They can also have a longer body which may impact on transportation and storage.

• They may also be more expensive, as large high quality lenses are more costly to produce. Cheaper lenses are more susceptible to aberrations.

• Advantage • Disadvantages

Reflecting Telescope

• The first reflecting telescope that worked well was produced in 1668 by Sir Isaac Newton.

• Reflecting telescopes do no have the problems that refracting telescopes do.

How do Reflecting Telescopes Work?

• Reflecting telescopes use curved mirrors instead of objective lenses.

Reflecting telescope

• In a reflecting telescope the image is formed by reflection from a curved mirror.

• It is then magnified by a secondary mirror.

• Remember- There is still a lens!

Reflecting telescopes

• Fairly compact and portable which is great for transportation and storage. Generally more affordable. Mirrors can produce less optical aberrations.

• Collect more light then refracting telescopes

• Mirrors need regular re-alignment as they can easily slip out of alignment due to knocks and bumps or temperature changes. They can be susceptible to spherical aberrations.

• Advantage • Disadvantage

Any Difference

• Both have eye piece lens that magnifies the image.

• Both have an objective lens/ curved mirror that focuses the light.

• One focuses the light via reflection using a mirror.

• The other focuses light through refraction using a lens.

• Refracting telescopes are shorter.

• Similarities • Differences

Questions1. Write a short definition of refraction.

2. What does focal length mean?

3. Explain how lenses work and how they are used in refracting telescopes.

Answers• 1.Write a short definition of

‘refraction’.

• A: Light/waves change direction when they cross the boundary between two media.

Answers• 2 What does ‘focal length’ mean?

• A:The distance from a lens to the point where rays of light from a distant object meet.

Answers• 3 Explain how lenses work and how they are used

in refracting telescopes.

• A: Lenses change the direction of light entering and leaving them. The shape of convex lenses bends rays of light so that a beam of parallel rays is brought to a focus. In a refracting telescope an objective lens brings parallel rays from distant stars to a point. Another convex lens is used as an eyepiece lens, which magnifies this image.

6 mark past paper question

Answer