TUTORIAL 4 LIGHT AND SHADOW. Instructor: Kazumi Tolich

Transcription

1 TUTORIAL 4 LIGHT AND SHADOW Instructor: Kazumi Tolich

2 Light and shadow 2! The activities in this tutorial involve a series of experiments that must be conducted in a darkened room.! In each experiment, you will be asked to make a series of predictions. You should then compare your predictions with those of your classmates.! You will then observe each experiment.

3 3 I. Light: point sources! A small bulb is placed in front of a cardboard mask with a circular hole and a screen. 1. Predict what you would see on the screen, and sketch your prediction in your notebook. 2. Predict how each of the following changes would affect what you see on the screen: 1. Moving the bulb upward 2. Replacing the circular hole with a triangular hole 3. Moving the bulb farther away from the mask! A second small bulb is placed above the first as shown. Perspective view Perspective view 3. Predict the size and location of the second spot. How would moving the top bulb upward change what you see? Screen Mask Screen Mask

4 Quiz: T4-1 answer 4! The second spot of light (due to the upper bulb) is below the first spot, but the same size.! Moving a bulb farther upward means that only light shining farther downward can make it through the mask.! Both bulbs are the same distance from the hole, so both spots will be the same size.

5 5 I. Light: extended sources 4. Imagine that you held a string of closely-spaced bulbs one above the other. What would you expect to see on the screen? 5. Predict what you would see on the screen if you used a bulb with a long filament as shown. (Hint: How is this similar to the previous question?) 6. Predict what you would see on the screen if a triangular hole was used with the long-filament bulb. Longfilament bulb

6 Quiz: T4-2 answer 6! Each spot on the filament is sending out light, so we can view this as a string of small bulbs.! The spot at the top of the filament sends light down through the hole, and the spot at the bottom sends light up through the hole.! The overall shape of the spot of light is due to the overlap of all of these individual triangular spots of light.

7 7 I. Light: extended sources! A second long-filament bulb is placed perpendicular to the first as shown. 7. Predict what you would see on the screen. 8. What would you see on the screen if the mask was removed? Perspective view Triangular hole! A frosted bulb is placed in front of the mask as shown. 9. Predict what you would see on the screen. Perspective view Triangular hole

8 II. Light: quantitative analysis 8! A light source is 60 cm from a mask, which is 40 cm from the screen as shown. Side view (not to scale) Circular hole 1 cm in diameter 10. How would the diameter of the lit region change if the diameter of the hole were halved? (In particular, would it be half as big?) 60 cm Mask 40 cm Screen 11. Determine the diameter of the bright spot with the original 1 cm hole.! Suppose the bulb was replaced by a long-filament bulb, whose filament was 7.5 cm long. Side view (not to scale) 7.5 cm Circular hole 1 cm in diameter 12. How would the height of the lit region change if the diameter of the hole were halved? (In particular, would it be half as tall?) 60 cm Mask 40 cm Screen

9 Quiz: T4-3 answer 9! The height of the lit region is smaller than before, but more than half as high.! Every spot of the filament will create a circle of light centered at the same location, but the diameter of these circles are smaller than before. 1 cm hole 0.5 cm hole

10 10 III. Shadows! A small bead is hung in front of two small bulbs. 13. Predict what you would see on the screen at the back of the box. 14. Must a region be completely without light for a shadow to be formed?! Suppose the light from the top bulb were red and the light from the bottom bulb were green. 15. Predict what you would see on the screen. Perspective view

11 Quiz: T4-4 answer 11! There can be a shadow even when a region is not completely without light.! If the light from one source is blocked but the light from another source is not, there will still be an observable shadow.! Think about your shadow during the day the direct light of the sun is blocked, but reflected light from other sources still illuminates the ground (you can still see the ground where your shadow is, it s just darker).

12 12 III. Shadows! A small bead is hung in front of two small bulbs. 13. Predict what you would see on the screen at the back of the box. 14. Must a region be completely without light for a shadow to be formed?! Suppose the light from the top bulb were red and the light from the bottom bulb were green. 15. Predict what you would see on the screen. Perspective view