Grade 8 Science

Unit 2: Optics

Chapter 6:

Lenses refract light to form images.

Lenses

A curved piece of

transparent material that

refracts light in a

predictable way.

Usually made from glass or

plastic.

Lense

s

There are two types of lenses:

1. Convex

Centre of the lens bulges out

Causes light rays to bend toward each other (converge)

2. Concave

Centre of the lens is curved

in

Causes light rays to bend

away from each other

(diverge)

Conve

x v

s. C

onca

ve L

ense

s

Convex Lenses

Can 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 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 Lenses

The Eye (pg. 229)

Vis

ion

Near-sighted Vision

See objects up close but

not at a distance.

Concave lenses are used to

correct this vision.

Near

- si

ghte

d V

isio

n

Far-sighted Vision

See objects at a distance

but not up close.

Convex lenses are used to

correct this vision.

Far

-sig

hte

d V

isio

n

Dete

rmin

ing

Foca

l Lengt

h...

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 Lenses

Object 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 Lenses

The above is true for an object in all

positions.

CORE

STSE:

“Fibre

Optics” Fibre Cable

Optical Technologies...

1. Microscopes

Uses two convex lenses

with relatively short focal

points to magnify

Magnified twice to

increase enlargement

The M

icro

scope

2. Telescopes

The objective lens has a

longer focal length than the

microscope.

Can have either a refracting

or reflecting telescope

Refracting Telescope (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 (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 shields

6. Magnifying glasses

7. Contact lenses

8. Flashlights

9. Eye glasses

The Nature of Science

The 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...