Light and Reflectivity

Transcription

1 Light and Reflectivity What is it about objects that lets us see them? Why do we see the road, or a pen, or a best friend? If an object does not emit its own light (which accounts for most objects in the world), then how do we see them? The answer is other source of light It is this light allows us to see. This is why it is difficult to see at night. But does that mean that just because there is light around, you will be able to see everything you look at? In the following activities, we will investigate how light interacts with objects, such as a mirror, and how this interaction allows us to see the object. Physical Sciences Connection: Develop a model to describe that light reflecting from objects and entering the eye allows the object to be seen (4-PS4-2). Activity : How does light interact with a shiny object? Initial thoughts: Do you need light to see an object? If there is light, does this mean you will see everything you look at? When you see an object, does light interact with your eye? Discuss your thoughts with your group. Part : What does each person see? What you will need: Flashlight Mirror (or piece of mylar) The room should be darkened for this part. Some of the material here is taken from Physics and Everyday thinking by Fred Goldberg.

2 Put the mirror on the table. Choose one member to hold the flashlight. The other members should stand around the table with the mirror with one person on the opposite side of the person with the flashlight and one to the side of the person with a flashlight. The person holding the flashlight should turn on the flashlight and illuminate the mirror. Each group member should say out loud what he/she sees when looking at the mirror. Does the mirror appear very bright, white, gray, black or something else? In particular, pay attention to the face of anyone who says the the mirror appears very bright. Is there any illumination on that person s face? Opposite person Move your head up and down and back and forth to determine if what is seen depends on the position your eyes. What each person observed Flashlight person Side person Appearance of mirror For people who say that the mirror appears bright, does their face appear to be illuminated? For people who say that the mirror appears bright, how does it feel when you see the light? When you look at a light source or image in a mirror, does your eye interact with the light? How do you know? 2

3 What you will need: Flashlight Mirror (or piece of mylar) White sheet of paper Part 2: What happens when light strikes a shiny surface? The room should be dark for this. Hold the front of the flashlight about 30 cm (about 2 inches) above the sheet of white paper and aim the beam onto the paper. Describe the illumination on the paper: is the entire paper uniformly illuminated, or is there a circular spot of light on the paper? Turn the flashlight over and mount it on the right edge of the paper as shown. Hold the flat mirror about 0 cm (4 inches) above the flashlight. Tilt the mirror a little so you can see some illumination on the sheet of paper. Describe the illumination on the paper when you use the mirror: is the entire paper uniformly illuminated, or is there a circular spot of light on the paper? Is the size and shape of the pattern of illumination on the paper with the mirror very similar to, or very different from, the pattern without the mirror? 3

4 Now, tilt the mirror more and more and watch what happens to the spot of illumination on the paper. Does the spot of illumination stay still, or does it move? The light is going from the flashlight to the mirror, and then from the mirror to the paper. We say that the light reflects from (bounces off) the mirror. When you tilted the mirror you noticed that the spot of illumination also moved. Is the angle at which the light strikes the mirror related to the angle at which the light reflects from the mirror? It turns out that all reflected light obeys the relationship that the angle of incidence is exactly equal to the angle of reflection. This is referred to as the law of reflection. Light-ray diagrams It is often convenient and informative to draw a diagram to show in what direction the light energy travels between objects and what happens to the light when reflecting from the surface of an object. This diagram is called a light-ray diagram. Here are the general rules for constructing a light-ray diagram:. All diagrams should start with light leaving the source. This is down with a straight arrow, showing the direction light is traveling. 2. When light strikes a shiny surface, only one light ray should be drawn. Make sure the angle of incidence is (approximately) equal to the angle of reflection 3. If the observer sees the object, then the light ray should enter their eye. 4

5 The light ray diagram on the right shows how observer A (the boy in the picture) sees the mirror image of the light source. Another observer, standing to the right of the mirror (but not shown in the picture), would not see the mirror image because light from the flashlight, reflecting off the mirror, cannot get into observer B s eye. Summarizing Questions:. Design an experiment that will help prove the following statement: The angle at which the light strikes the mirror (angle of incidence) is equal to the angle at which the light reflects from the mirror (angle of reflection). 2. Below is a side-view diagram showing a light source, a small mirror, and four observers looking towards the mirror. Which of the four observers would see the reflection of the light source in the mirror? 5