23_LectureOutline.pptx
- 1. L4: Lenses Thin lenses are those whose thickness is small compared to their radius of curvature. They may be either converging (a) or diverging (b).
- 2. Converging Lenses; Ray Tracing Parallel rays are brought to a focus by a converging lens (one that is thicker in the center than it is at the edge).
- 3. Diverging Lenses; Ray Tracing A diverging lens (thicker at the edge than in the center) make parallel light diverge; the focal point is that point where the diverging rays would converge if projected back.
- 4. Thin Lenses; Ray Tracing © 2014 Pearson Education, Inc.
- 5. Thin Lenses; Ray Tracing For a diverging lens, we can use the same three rays; the image is upright and virtual. © 2014 Pearson Education, Inc.
- 6. Ray Diagrams for Thin Lenses • Three rays are drawn – 1st ray: parallel to the principle axis and then passes through (or appears to come from) one of the focal points F – 2nd ray is drawn through the center of the lens C and continues in a straight line. – 3rd ray is drawn through the other focal point F and emerges from the lens parallel to the principle axis (not shown) There are an infinite number of rays, these are convenient
- 7. Ray Diagram Examples • Note the changes in the image as the object moves through the focal point
- 8. Ray Diagram for Converging Lens, p > f • The image is real • The image is inverted • The image is on the back side of the lens
- 9. Ray Diagram for Converging Lens, p < f • The image is virtual • The image is upright • The image is on the front side of the lens
- 10. Ray Diagram for Diverging Lens • The image is virtual • The image is upright • The image is on the front side of the lens
- 11. 23-8 The Thin Lens Equation The thin lens equation is the same as the mirror equation: © 2014 Pearson Education, Inc. (23-8)
- 12. 23-8 The Thin Lens Equation The sign conventions are slightly different: 1. The focal length is positive for converging lenses and negative for diverging. 2. The object distance is positive when the object is on the same side as the light entering the lens (not an issue except in compound systems); otherwise it is negative. 3. The image distance is positive if the image is on the opposite side from the light entering the lens; otherwise it is negative. 4. The height of the image is positive if the image is upright and negative otherwise. © 2014 Pearson Education, Inc.
- 13. 23-8 The Thin Lens Equation The magnification formula is also the same as that for a mirror: The power of a lens is positive if it is converging and negative if it is diverging. © 2014 Pearson Education, Inc. (23-9)
- 14. 23-8 The Thin Lens Equation Problem Solving: Thin Lenses 1. Draw a ray diagram. The image is located where the key rays intersect. 2. Solve for unknowns. 3. Follow the sign conventions. 4. Check that your answers are consistent with the ray diagram. © 2014 Pearson Education, Inc.
- 15. Summary • A diverging lens spreads incoming rays so that they appear to come from a point • Power of a lens: • Thin lens equation: • Magnification: © 2014 Pearson Education, Inc.