Lenses and the formation of images

1. SNC2D - OPTICS Lenses and the formation of images

2. What kinds of lenses do you use in everyday life? What is the function of a lens?

3. Lenses & The Formation of Images Roman emperor, Nero used an emerald to watch gladiators fight, which is believed to have been a “lens” to improve sight.

4. Reading stones (magnified print on the page) used in the middle ages by scholars, monks and nuns Other uses

5. Lenses & the formation of images • Lens • A lens is a transparent object with at least one curved surface that causes light to refract • Usually made of glass, plastic or living tissue • The word “lens” comes from the word “lentil”!

6. Two basic Lens shapes • Convex or Converging Lens • a lens that is thickest in the middle & causes incident parallel light rays to converge through a single point after refraction • The lens shown below is biconvex (both sides convex)

7. Two basic lens shapes • 2) Concave or Diverging Lens • a lens that is thinnest in the middle & causes incident parallel light rays to diverge (spread apart) after refraction. • The lens shown is biconcave (2 concave surfaces)

8. We will only be concerning ourselves with biconcave and biconvex lenses.

9. Simplifying the path of light rays • There are always 2 refractions when light passes through a lens. 1. When the ray goes from air  glass. (towards normal) • 2. When the ray goes from glass  air (away from normal) • To simplify drawing, a dashed vertical line through the centre of the lens as a reference point shows light being refracted only once. This line is called the Lens Axis (or Axis of Symmetry)

10. Lens Terminology Converging Lens Light can pass through the lens in either direction so there are 2 Fs. Optical centre (O) = point at the exact centre of the lens Principal Focus (F) = point on the principal axis of a lens where light rays parallel to the principal axis converge after refraction. Focal length (f) = distance from F to O

11. Lens terminology Diverging Lens • Parallel light rays will diverge. If you project diverging rays backward, it looks like they originate from a virtualfocus

12. One key difference - In the converging lens, F is on the opposite side of the lens as the incoming light rays. - In a diverging lens F is on the same side of the lens as the incoming light.

13. F’ F A ray that passes through the optical centre (O) is not refracted at all ray diagrams – CONVERGING lensES

14. F F’ A ray parallel to the principal axis will refract through the principal focus (F) on the other side of the lens. ray diagrams – CONVERGING LENS

15. F’ F A light ray directed through F’ (the secondary focal point) will refract parallel to the principal axis ray diagrams – CONVERGING lenses

16. F’ F The image will be at the convergence of all the refracted rays (note – you only need 2 rays) Ray diagram (convex lensES) This is a REAL image (real light rays meet)

17. F F’ A ray that passes through the optical centre (O) is not refracted at all ray diagrams – DIVERGING lenses

18. F F’ A ray parallel to the principal axis refracts away from the principal focus (F) – same side as the object. ray diagrams – DIVERGING lenses

19. F F’ A light ray directed at F’ (other side of the lens) will refract parallel to the principal axis ray diagrams – DIVERGING lenses

20. F F’ The refracted rays do not meet so you must extend them backward (the same side as the object). The image is where these meet. ray diagrams – DIVERGING lenses What type of image is this? This is a Virtual image (it is not real light rays which meet)

21. image type • Images are REAL when they are found at the meeting of real (!) light rays (on the opposite side of the lens as the object) • Images are VIRTUALwhen they are found at extensions back in front of the lens (same side as the object). It is not real light that meets!