Light and Reflection

1. Light and Reflection

2. Electromagnetic Waves • All EM waves are essentially the same. The only difference is frequency and wavelength. • c=3.0x108m/s=speed of light in a vacuum

3. Sample Problems • What is the wavelength range for the FM radio band (88 MHz – 108 MHz)? • What is the frequency of an electromagnetic wave if it has a wavelength of 1.0 km? • What is the frequency of 560 nm light (yellow-green)? • What is the frequency of ultraviolet radiation that has a wavelength of 125 nm?

4. Light • The portion of the EM spectrum which can be perceived by the eye. • Ranges from 400nm (purple) to 700nm (red) • Light waves are transverse waves. • For geometric optics, light waves are represented as rays showing the direction of propagation.

5. Law of Reflection • The angle of incidence equals the angle of reflection. • Both angles are measured from the normal, or an imaginary line perpendicular to the surface.

6. Reflection • Some things emit their own light; most things reflect light from something else. • Specular reflection-Reflection from a uniform surface which produces an image. • Rays maintain their orientation upon reflection • Diffuse reflection-reflection from a bumpy surface which results in no image. • Rays are randomly oriented after reflection,

7. Categories of Objects • Opaque-most light is reflected. Light that does transmit is absorbed by the material and wasted in heat. • Transparent-most light is transmitted and follows a relatively straight line path allowing one to see through the object. • Translucent-most light is transmitted, however, it is scattered in the process and a clear image of the other side is not viewable.

8. Polarizers • Allow only light in a certain orientation, not light perpendicular. • If light is at an angle other than zero or 90 degrees, the component that is parallel will pass through.

9. Polarization by Reflection • At certain angles, all reflected light is polarized the same way, parallel to the surface. • Light that has the opposite orientation is transmitted. • Polarized sunglasses have their transmission axis vertical so that glare will not pass through.

10. Ray Diagrams • Object-the thing that is acting as the source of the light. • You, when you are looking in the mirror. • do=distance from mirror to object • Image-the convergence or apparent convergence of light. • Your reflection, when you are looking in the mirror. • di=distance from image to the mirror

11. Ray Diagrams • Ray diagrams are visual models of light’s behavior which allow us to predict where images occur. • We use the law of reflection to determine how light will move after hitting the mirror. • The image occurs where the light either converges or appears to converge.

12. Image Types • Virtual images-images where light does not converge. If one were to stand where the image was apparently located, nothing would be there. Flat mirrors produce these • Real images-images that occur when light actually does converge. Real images can be projected on a screen or appear to float. The pig demo shows this. • One can determine whether a real or virtual image will be produced by either ray diagrams or equations.

13. Mirror Terminology • We deal only with flat mirrors and spherical mirrors. Most mirrors used for imaging are either spherical or almost spherical. • Concave mirrors-mirrors where the shiny side caves in. • Convex mirrors-mirrors where the shiny side bubbles out.

14. Curved Mirror Terminology • C=the center of curvature • Principle axis=the line that goes from C to the center of the mirror. • F=the focal point of the mirror; the point where all light that is parallel to the principle axis converges upon reflection. • R=the radius of the mirror • f=the focal length=½R

15. Ray Diagram Mirror Rules • Incident lines which are parallel to the principle axis will reflect through f. • Incident rays which pass through f will reflect parallel to the principle axis.

16. Our First Ray Diagram • Incident lines which are parallel to the principle axis will reflect through f. • Incident rays which pass through f will reflect parallel to the principle axis.

17. Object on C • Incident lines which are parallel to the principle axis will reflect through f. • Incident rays which pass through f will reflect parallel to the principle axis.

18. Object Between C and F • Incident lines which are parallel to the principle axis will reflect through f. • Incident rays which pass through f will reflect parallel to the principle axis.

19. Object on F • Incident lines which are parallel to the principle axis will reflect through f. • Incident rays which pass through f will reflect parallel to the principle axis.

20. Object Between F and Mirror • Incident lines which are parallel to the principle axis will reflect through f. • Incident rays which pass through f will reflect parallel to the principle axis.

21. Convex Mirror • Incident lines which are parallel to the principle axis will reflect through f. • Incident rays which pass through f will reflect parallel to the principle axis.

22. The Mirror Equation

23. Practice Problems • A candle is reflected in a mirror with a magnification of -3.33. The image is formed 7.2cm from the mirror’s surface. • Is the image upright or inverted? • What is the distance of the object to the mirror? • What is the focal length of the mirror? • Is the image virtual or real? • A convex security mirror shows the reflection of a thief. The thief is 2.5m from the mirror and 1.8m tall. If the image of the thief is 3cm tall, what is the distance of the image to the mirror and the focal length of the mirror?

24. Practice Problems • A concave spherical mirror has a focal length of 10.0 cm. Locate the image of a pencil and find the magnification, when the pencil is placed: • 30.0 cm from the mirror • 10.0 cm from the mirror • 5.00 cm from the mirror • The image of a crayon appears to be 23.0 cm behind the surface of a convex mirror and is 1.70 cm tall. If the mirror’s focal length is 46.0 cm, how far in front of the mirror is the crayon positioned? What is the magnification of the image? Is the image virtual or real? Is the image inverted or upright? How tall is the actual crayon? • A convex mirror with a radius of curvature of 0.550 m is placed above the aisles in a store. Determine the image distance and magnification of a customer lying on the floor 3.1 m below the mirror. Is the image virtual or real? Is the image inverted or upright?

25. Additive Color Mixing • Additive color mixing is using various colors of light to make different, intermediate colors. • White light is how we perceive all of the colors together. • Primary colors of light are red, green, and blue. • Mixing different primary colors in different combinations makes us perceive the intermediate colors.

26. Additive Primaries • Blue + Green = Cyan • Blue + Red = Magenta • Green + Red = Yellow • The colors cyan, magenta, and yellow are called the secondary colors of light. • Complementary colors are two colors that add together to form white light • Blue + Yellow • Green + Magenta • Red + Cyan

27. Subtractive Color Mixing • In subtractive color mixing, white light is shown on a pigment or dye and that substance absorbs certain colors and reflects others. We see the reflected colors. • Primary pigments are those that absorb one primary color of light • Cyan • Magenta • Yellow

28. Color Perception By the Eye • Rods-can’t distinguish between color but are activated at lower light levels. • Cones-can see color but are operational only at upper light levels. • Three types; red, green, blue • Color blindness results from missing one or more types of cones. • Color Blindness Test

29. Color Mixing by the Eye • Each type of cone can perceive a range of frequencies, however, only fires one way. • In between colors will set off more than one type of cone. • That’s why it doesn’t matter if a light is emitting yellow wavelengths or a combo of green and red. Our eyes can’t perceive the difference.