1. Lenses
Chapter 30

2. Converging and Diverging Lenses
Lens – a piece of glass which bends parallel rays so that they cross and form an image
Converging Lens – a lens which is thicker in the middle, makes parallel light rays converge
Diverging Lens – a lens which is thinner in the middle, makes parallel light rays diverge
Principle Axis – line joining the centers of curvature of a lens’ surface
Focal Point – the point at which a beam of light converges
Incident parallel beams that are not parallel to the principle axis focus at points above or below the focal point
Focal Plane – all possible points from incident beams
A lens has two focal points and two focal planes

3. Converging and Diverging Lenses

4. Image Formation by a Lens
Magnification occurs when an image is observed through a larger angle with the use of a lens than with out the lens, and allows more detail to be seen
A converging lens will only magnify when the object is between the focal point and the lens
Virtual Image – an image formed through reflection or refraction that can be seen by an observer but cannot be projected on a screen because light from the object does not actually come to a focus
Real Image – an image that is formed by converging light rays and that can be displayed on a screen
When a diverging lens is used alone, the image is always virtual, right-side up, and smaller than the object

5. Image Formation by a Lens

6. Constructing Images Through Ray Diagrams
Ray Diagrams – show the principle rays that can be used to determine that size and location of an image
One end of the object is always placed right on the principle axis
3 useful rays in ray diagrams
Ray parallel to the principle axis that passes through the focal point after refraction by the lens
A ray through the center of the lens that does not change direction
A ray through the focal point in front of the lens that emerges parallel to the principle axis after refraction by the lens

7. Constructing Images Through Ray Diagrams

8. Image Formation Summarized
If an object is one focal length away from a converging lens, the image is virtual, magnified, and right-side up
If an object is beyond one focal length, a converging lens produces a real, inverted image
If the object is close to the focal point, the image appears far away; if it is far from the focal point, the image appears nearer
When an object is viewed through a diverging lens, the image is virtual, reduced, and right-side up

9. Some Common Optical Instruments
The Camera – consists of a lens and sensitive film mounted in a lighttight box, the amount of light which gets to the film is regulated by a shutter and a diaphragm (which varies the size of the opening)
The Telescope – a simple telescope uses a lens to form a real image of a distant object
The Compound Microscope – uses two converging lenses of short focal length, forming a real and a virtual image (so we see the object right-side up)
The Projector – An arrangement of converging lenses is used

10. Some Common Optical Instruments

11. The Eye
The amount of light that enters is regulated by the iris, the colored part of the eye that surrounds the opening called the pupil
Light enters through the transparent covering called the cornea, passes through the pupil and lens, and is focused on a layer of tissue at the back of the eye—the retina—extremely sensitive to light
In both the camera and the eye, the image is upside down, our brain flips the image right-side up for us

12. The Eye

13. Some Defects in Vision
Farsighted – person’s eyes form images behind the retina, the eyeball is too short
Nearsighted – person’s eyes form images in front of the retina, the eyeball is too long
Astigmatism – the cornea is curved more in one direction than the other, the eye does not form sharp images

14. Some Defects in Vision

15. Some Defects in Lenses Aberrations – the distortions in an image
By combining lenses in certain ways, aberrations can be minimized
Spherical aberration results when light passes through the edges of a lens and focuses at a slightly different place from light passing through the center of the lens
Chromatic aberration is the result of the different speeds of light of various colors and hence the different refractions they undergo

16. Some Defects in Lenses

17. Assignment
Read Chapter 30 (pg )
Do #15-34 (pg )