Science Focus 8
Light and Optical Systems
Topic 2 Topic 2 – Reflection REFLECTION is the process in which light strikes a surface and bounces back off that surface. But what determines the difference in how you see the reflection? * Turn to page 188 and 189 of your textbook and read the two introductory pages to Topic 2. The difference between seeing your own image in a mirror and seeing a printed page is determined by the surface from which light reflects. How the light bounces off the surface depends on the LAW OF REFLECTION . If the light hits a rough surface (like the page in your textbook) the light will be scattered . If the light hits a smooth surface , the light reflects at an opposite angle to the angle it hits . The light ray coming from a light source is called an INCIDENT RAY and the light ray that bounces off the surface is called a REFLECTED RAY . A line that is perpendicular (90 o with the surface) to the plane mirror is called the NORMAL line (the thick lines are the NORMAL lines). The angle between the incident ray and the normal line is called the ANGLE OF INCIDENCE , i . The angle between the reflected ray and the normal line is called the ANGLE OF REFLECTION , r . Science Focus 8
Light and Optical Systems
Topic 2 Forming an Image The LAW OF REFLECTION states that: “ the angle of incidence equals the angle of reflection ” Another important relationship is that the incident ray, the normal line and the reflected ray lie in the same plane (an imaginary flat surface). This is displayed well in Figure 3.17 on page 194. Your brain “knows” that light travels in straight lines, therefore it interprets the image as though it is behind the mirror. In the space below, draw Figure 3.17 that is located on page 194 of your textbook. Another important feature of images in mirrors is demonstrated in Figure 3.19 on page 195. The figure helps explain why an image in a mirror is the same size as the object and appears to be the same distance from the mirror as the object. These results are only true for plane (flat) mirrors. * Turn to page 194 and 195 of your textbook and read “Forming an Image”. Curved Mirrors Mirrors that bulge out are called CONVEX mirrors , while mirrors that cave in are called CONCAVE mirrors. CONVEX mirrors form images that appear much smaller and farther away than the object , but they can reflect light from a large area , making them useful as security mirrors in stores. They are also placed in side mirrors of cars. Science Focus 8
Light and Optical Systems
Topic 2 CONCAVE mirrors reflect light inward to one focal point, therefore they are used to focus light. Unlike convex mirrors, concave mirrors show different image types depending on the distance between the object and the mirror. * Read “Curved Mirrors” on page 195 and 196 of your textbook. QUESTION 1 : Label the following mirrors as either convex or concave. MIRROR 1: CONCAVE
MIRROR 2: CONVEX Rough Surfaces Smooth surfaces reflect light uniformly, while rough surfaces appear to reflect light randomly, but this seemingly scattered light creates the image of the print that you see on this page. * Go back to page one of these notes to see the difference between rough surface reflection and smooth surface reflection. Note the NORMAL lines. Light hits the white paper of these notes (a rough surface) and reflects it in all directions (some of it reaching your eye). Since there is no pattern, your eye just sees white light. On the other hand, the ink on this paper absorbs the light and no light from the ink reaches your eye. Therefore your eye sees the letters in black ink. * Turn to page 197 of your textbook and read “Rough Surfaces”. Then turn to page 198 and 199 and read “Using Reflections”. QUESTION 2 : What do rough surfaces do to light? Scatters light QUESTION 3 : Name TWO different ways in which you can use reflections. Pool, bicycle reflectors Science Focus 8
Light and Optical Systems
Topic 2 QUESTION 4 : Turn to page 199 of your textbook and complete the following questions from Topic 2 Review: Page 199 #