Lenses This Presentation was used for Year 12 students. Note: This PowerPoint is one of over 60 that were developed and used at John Paul College, Rotorua between 1996 and 2002 by D Bradley. They were primary designed as teacher directed resources.

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

Lenses Diverging Lens Or Double Concave lens Converging Lens Or Virtual Focal point! Diverging Lens Or Double Concave lens Converging Lens Or Double Convex lens F F f f

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 x2

Light passes through a lens Ray Tracing for Lenses Light passes through a lens 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

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

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

Converging Lens Practical Activity Equipment: meter ruler candle Converging lens (f ~ 20 cm ???) Object location do > 2f do = 2f 2f > do > 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) do = f do < f Investigate the image produced for

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?

Converging Thin Lens: do < f Double Convex Converging Thin Lens: do < f f f ‘ Virtual Upright Enlarged R1 R2 do di

Example: Magnifying Glass Web Link: Ray tracing

Convex thin lens: do < f Similar Triangles ho 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 image f object 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 image f object Smaller and “sharper”. Smaller and “fuzzier”. Larger and “sharper”. Larger and “fuzzier”. Would disappear.

Finally, Jennifer blocks half the lens, as shown, with a piece of paper. What happens to the image? object 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.

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