# Grade 8 Science Unit 2: Optics

## Presentation on theme: "Grade 8 Science Unit 2: Optics"— Presentation transcript:

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.

Lenses

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)

Convex vs. Concave Lenses

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
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.

Optical Centre Principle Axis

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

Corrective Lenses The Eye (pg. 229)

Vision

Near-sighted Vision See objects up close but not at a distance. Concave lenses are used to correct this vision.

Near- sighted Vision

Far-sighted Vision See objects at a distance but not up close. Convex lenses are used to correct this vision.

Far-sighted Vision

Determining Focal Length...
You must include the following incident rays: Travelling parallel to the p.a. Travelling through the o.c. Travelling through the F. Determining Focal Length...

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 Microscope

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
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 Individuals... 1672 Galileo Newton

Groups...