Presentation on theme: "Grade 8 Science Unit 2: Optics Chapter 6: Lenses refract light to form images."— 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.
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. (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)
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.
Determining Focal Length... 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 Microscopes Uses two convex lenses with relatively short focal points to magnify Magnified twice to increase enlargement
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.