The word lens comes from the Latin word

The word “lens” comes from the Latin word for lentils.

Lenses oc F al t! n i o p l a tu r i V F f Converging Lens Or Double Convex lens F f Diverging Lens Or Double Concave lens

Thin Lenses Thin lenses are those whose thickness is small compared to their radius of curvature. They may be either converging (a) or diverging (b).

Double Convex Lens Focal Point

Lens = refraction x 2

Ray Tracing for Lenses ü Light passes through a lens ü There is a focal point on both sides of a lens Converging Lens: Ray #1: Parallel to the axis Refracts through F Ray #2: Through F Refracts parallel to axis Ray #3: Through Center of lens undeflected

Double Convex Converging Thin Lens: do > f Inverted Real Enlarged Real Image Ray diagrams

Double Convex Thin Lens: do > f do di Similar Triangles ho f f hi

Converging Lens Practical Activity Equipment: • meter ruler • candle • Converging lens (f ~ 20 cm ? ? ? ) Object location 1) do > 2 f For the 3 object locations listed ~ Sketch a ray diagram ~ state the nature of the image ~ record di (so as to be able to calculate f) Investigate the image produced for 2) do = 2 f 3) 2 f > do > f 1) do = f 2) do < f

Example: Camera

Example: Projector

See 5 Lens applet

Find the focal length of a converging lens by holding it up to a window. (See how far away from the lens you need to hold a piece of paper to focus the image on the paper. )

A camera with a focal length of 50 mm takes a photograph of a 100 m tall building from 350 m away. How tall is the image on the film?

Double Convex Converging Thin Lens: do < f f f‘ Virtual Upright Enlarged R 1 do di R 2

Example: Magnifying Glass Web Link: Ray tracing

Convex thin lens: do < f f ho f Similar Triangles hi di negative do

A lens is used to make an image; three light rays are drawn out of the infinite number coming from the arrowhead. The image given in this sketch could be seen: f object 1. 2. 3. 4. 5. image f By placing a screen at the image point. Without a screen by looking back at the lens. By both techniques (1) and (2). Only if the lens is big enough. None of the above answers is 100% correct.

A screen is placed at the position of the image, and a “sharp” image appears on the screen. Next, Jennifer moves the screen a SHORT distance TOWARD the lens. The image would appear: f object 1. 2. 3. 4. 5. f Smaller and “sharper”. Smaller and “fuzzier”. Larger and “sharper”. Larger and “fuzzier”. Would disappear. image

Finally, Jennifer blocks half the lens, as shown, with a piece of paper. What happens to the image? f object 1. 2. 3. 4. 5. image f Paper blocks half of lens It disappears. Only half of it is still seen It looks the same, but gets slightly dimmer. It gets fuzzy. It depends on what part of the lens is blocked.

Results: Ray Tracing for Converging Lenses (in each case, draw in the 3 rays for practice) Object distance > 2 f: Image is real, smaller, and inverted 2 F F F Object between f and 2 f: Image is real, larger, inverted 2 F F F Object between f and lens: Image virtual, larger, upright 2 F F F