Grade 8 Science Unit 2: Optics Chapter 6: Lenses refract light to form images.

Grade 8 ScienceGrade 8 ScienceUnit 2: OpticsUnit 2: Optics

Chapter 6:Lenses refract light to form images.

LensesLensesA curved piece of transparent material that refracts light in a predictable way.

Usually made from glass or plastic.

Len

ses

There are two types of lenses:

1. ConvexCentre of the lens bulges out

Causes light rays to bend toward each other (converge)

2. ConcaveCentre of the lens is curved in

Causes light rays to bend away from each other (diverge)

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Convex LensesConvex LensesCan act as a magnifying glass

Each lens has its own focal length (the distance from the centre of the lens to the focal point)

The greater the curvature of the lens, the shorter the focal length. (pg. 217)

Lenses have focal points on either side because light shines through either side.

The line through the centre of the lens is called the principle axis.

Optical centre is where the principle axis meets the lens centre.

Principle Axis

Optical Centre

Concave LensesConcave Lenses

To find the focal point, you must extend the refracted rays back.

Lenses with the greater curvature have the shortest focal length. (pg. 221)

Corrective LensesCorrective Lenses

The Eye (pg. 229)

Vis

ion

Near-sighted VisionNear-sighted Vision

See objects up close but not at a distance.

Concave lenses are used to correct this vision.

Near-

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Vis

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Far-sighted VisionFar-sighted Vision

See objects at a distance but not up close.

Convex lenses are used to correct this vision.

Far-

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.You must include the following incident rays:1.Travelling parallel to the p.a.2.Travelling through the o.c.3.Travelling through the F.

Ray Diagrams: Convex Ray Diagrams: Convex LensesLensesObject between the lens and focal point.

Object is between the focal point and 2X the focal length.

Object is more than 2X the focal length.

Ray Diagrams: Concave Ray Diagrams: Concave LensesLenses

The above is true for an object in all positions.

CORESTSE:“Fibre Optics”

Fibre Cable

Optical Technologies...Optical Technologies...

1. Microscopes Uses two convex

lenses with relatively short focal points to magnify

Magnified twice to increase enlargement

Th

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2. TelescopesThe objective lens has a longer focal length than the microscope.

Can have either a refracting or reflecting telescope

Refracting Telescope Refracting Telescope (pg. 245)(pg. 245)The lenses bend light to focus it.

The objective lens must be as large as possible to view distant galaxies (makes it heavy)

Reflecting Telescope Reflecting Telescope (pg. 246)(pg. 246)Uses a concave mirror, plane mirror and a convex lens to collect and focus light from objects at a great distance.

Most large telescopes are this type.

The Hubble Space Telescope

Launched in 1990

The Reflecting Telescope

3. Cameras (pg. 248)4. Binoculars (pg. 247)5. Face shields6. Magnifying glasses7. Contact lenses8. Flashlights9. Eye glasses

The Nature of ScienceThe Nature of ScienceThe development of new technologies involve many individuals and groups of people.

These technologies can alter what we know about the nature of science.

1602

1672

Individuals...

Galileo

Newton

Groups...