How do we see? 1.2 Observe and explain - light rays Observe the laser pointer and the spot that it projects on the wall.

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1 How do we see? 1.1 Observe and explain Go to a room that is isolated from all external light sources natural and artificial. Turn off the internal lights and wait in the dark room for several minutes. Devise several reasons why you cannot see anything. 1.2 Observe and explain - light rays Observe the laser pointer and the spot that it projects on the wall. 1. What is the path of light rays from the laser to the screen? How can you tell? 2. Why can you see the spot on the wall? Respond Illustrate with a sketch 3. Where can you stand to see the spot on the wall? What does this mean about the rays reflected from the wall? 4. Why can't you see the beam of light before it hits the screen? 5. What do you observe when chalk dust is sprinkled in the beam? How can you explain this? 6. Summarize the conditions necessary for you to see something

2 1.3 Observe and explain - luminous and non-luminous objects 1. In a completely dark room, why can't you see your hand? Respond Illustrate with a sketch of light rays and objects 2. If you light a candle, why are you able to see the candle flame? 3. If your hand is near the lighted candle, why are you able to see your hand? 4. Which is a better picture of the light rays emitted by the candle flame? How do you know? Alice Bob 5. Draw the light rays emitted from the spot on the candle. Draw the light rays reflected from the spot on the hand. Explain how you knew what to draw

3 1.4 Predict and test Place a powered, frosted light bulb on a table in the center of a dark room and observe that the walls are almost uniformly lit. A friend draws two ray diagrams to try to explain this observation: (I) (II) (a) Describe the difference between the two diagrams. (b) Design an experiment to test which of the diagrams represents the way a light bulb emits light does each point emit one ray or does each point emit rays in all directions? Describe the experiment(s) in words and write a prediction based on each diagram. (c) Perform the experiment and decide which diagram did NOT predict the outcome. Hint: Predict what you will see on the wall if you block the light with a small obstacle placed close to the bulb.

4 1.5 Predict and test - shadow and semishadow You have a light bulb, a ball, and a nearby wall. For each question, draw light rays to predict the image on the wall. Use the idea "all points on the light bulb emit light rays in all directions". 1. The ball is close to the wall. Draw a ray diagram to predict the shadow of the ball. 2. The ball is part way between the wall and the lamp. Draw a ray diagram to predict the shadow of the ball. 3. The ball is close to the lamp. Draw a ray diagram to predict the shadow of the ball. 4. Will the shadows be different? If so, in what way(s)? How do you know? 5. Perform the experiment. Do the results match your predictions?

5 1.6 Observe and explain Shadows and semishadows. You have a lamp, a sharpened pencil, and a wall. Draw ray diagrams to predict what you will see on the screen in each case. Remember the lamp emits light rays from all points on the filament, in all directions. 1. Pencil is close to wall Sketch and describe what you see on the wall Explain what you see with a ray diagram 2. Pencil is part way between lamp and wall Sketch and describe what you see on the wall Explain what you see with a ray diagram 3. Pencil is close to lamp Sketch and describe what you see on the wall Explain what you see with a ray diagram

6 1.7 Predict and test - triangular shadows You have a candle, a piece of cardboard with a shape cut out of it, and a wall. Draw ray diagrams to predict what you will see on the screen in each case. Remember, the bulb emits light rays from all points on the bulb, in all directions. Hint: use a ruler or straight edge to draw rays 1. Cardboard is close to wall Describe what you should see on the wall 2. Cardboard is part way between bulb and wall Describe what you should see on the wall 3. Cardboard is close to bulb Describe what you should see on the wall

7 1.8 Predict and test - pinhole experiments A candle illuminates a piece of cardboard with a small hole in it. Draw a ray diagram to predict what you will see on the wall. Remember, all points of the candle flame emit light rays in all directions. Perform the experiment. Do the results match your prediction? 7. What would you expect to see if you repeat the experiment with a slightly larger hole? Perform the experiment. Do the results match your prediction?

8 1.9 Predict and test 1. A frosted bulb is behind a screen with a very small hole in it. Draw a ray diagram to predict what you will see on the screen. 2. A group of students are inside a completely dark room. A small hole is made in one wall. Outside, a tree and a deformed squirrel are brightly illuminated by the sun. Make a ray diagram to predict what the class will see when they look at a white board placed in front of the small hole.