1. Lenses

2. Lenses in General Lenses work by refraction Lens Terminology
2 refractions, one when light enters lens and one when it leaves lens.
Light is bent due to it having a lesser speed in the glass or plastic
Lens Terminology
Principal axis= axis through center of lens
Focal point = point on principal axis where rays appear to focus, f.l. = distance from lens to f.p.
Object distance= distance between object and center of lens.
Questions: Do lenses work through reflection or refraction? How many times does a lens bend a light ray? Draw a converging lens and indicate the focal point, focal length, principal axis and object distance.
Activities:
Show Chp. 28 Video from Phy Sci Laser disk again.
State that lenses work due to refraction and then draw a lens and show the two refractions that occur.
Draw lens shapes first as prisms and rectangular glass and then smooth out shapes to show the middle aged man lens and the WOMAN lens.

3. Image Terminology
Real images form from converging light rays and can be projected while virtual images form from diverging rays and cannot be projected
Magnified images are larger than the object, reduced are smaller.
Erect images are right-side-up while inverse images are upside-down.
Primary rays are rays that always bend the same for a type of lens.
Questions: Distinguish between real and virtual images, magnified and reduced images and erect and inverted images.
Activities:
1) Discuss difference between objects and images. Objects produce or reflect their own light while images are simply where that light appears to converge.
Define real and virtual images, including which are projectable and which can be seen by the human eye.
Define magnified and reduced images
Define erect and inverted images. Mention that real images are always inverted while virtual images are always erect.

4. Converging Lens
Causes parallel rays to come together; shape is thickest in middle
Primary Rays
Ray that is parallel to principal axis is refracted through far focal point
Ray that goes through near focal point is refracted parallel to principal axis
Ray that hits center of lens refracts straight on through
Images and Ray diagrams
Real, reduced and inverted for D0 > 2 f.l.
Real, slightly magnified and inverted for 1 f.l<D0 <2 f.l.
Virtual, magnified and erect for D0 < 1 f.l.

5. Diverging Lens
Lens that causes parallel rays to spread out; shape is one where middle is thinnest.
Primary rays
Ray parallel to principal axis refracts as if it came from near false focal point
Ray that would go through far focal point refracts parallel to principal axis
Ray through center refracts straight through
Image: Reduced, Erect and Virtual

6. Lens Aberrations (Flaws)
Spherical Aberration occurs when rays from different parts of the lens focus to different focal points
Can be limited by reducing amount of lens used
Front surface mirror avoids the problem
Chromatic aberration occurs when different colors of rays focus to different focal points.
Achromatic lens can eliminate this.
Questions: Define spherical and chromatic aberration for lenses. What are the fixes for each?
Activities:
Draw converging lens and note how rays hitting the lens at different locations must go through different amounts of glass and therefore will cross the principal axis at different locations. This causes more than one image to be formed fuzzing out the full image. This aberration or flaw is called spherical aberration.
Reducing part of lens in use with a diaphragm or iris can limit the spherical aberration though not completely fix it.
In cases where you do not want any spherical aberration a front surface mirror is used. Since light never passes through glass, no aberration occurs.
Draw picture showing different colors coming to focus at different distances from the lens. This causes images of different colors to focus at different distances= chromatic aberration.
Chromatic aberration is fixed by attaching a second diverging lens forming an achromatic lens.

7. Ray Tracing
Process allows quick determination of what type of image forms.
Diverging lens always gives same image (Reduced, Erect and Virtual)
Converging Lens has 1 of 3 images depending on object distance.
Use principal rays to determine type of image
Use 1/f= 1/p + 1/q and M=-q/p to find location and magnification. Also cc/2 = f
Positive side of lens is opposite object
Question: List the 3 principal rays of a converging lens. Show a ray diagram of an object less than one focal length from the lens. Describe the image produced. What type of image is produced when the object is more than two focal lengths away? How about between 1 and 2 focal lengths away.
Activities:
Mention idea of ray tracing as method of determining what type of image forms and why.
Refer students to three primary rays of converging lens on handout.
Do 3 ray diagrams w/ students for converging lens. Demo each image with lens on desk.
Have students look at image through diverging lens.
Show slide.

8. Primary Rays Principal Ray Converging Lens Diverging Lens
A ray parallel to the principal axis will be refracted
Through the focal point
As if it came from the near false focal point

9. Lens Terminology
Focal point: where rays parallel to the principal axis would meet after passing through converging lens
Focal Length: Distance between focal point and center of lens
False focal points: points on principal axis of diverging lens where rays appear to come together.
Question: Draw a converging lens and label its principal axis, focal points and focal length. Do the same for a diverging lens.
Activity
Draw converging lens and label parts as shown above.