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Name: Lab Partners:
Date:
Pre-Lab Assignment:
Introduction to Light
(Due at the beginning of lab)
Directions: Read over the lab handout and then answer the following questions about the procedures.
Question 1 When is ray or geometrical optics a very good approximation?
Question 2 In Activity 1.2, what is the original source of the light measured with the light probe?
Question 3 What are the two miniature light bulbs used for in Investigation 2?
Question 4 Why are various portions of the lens blocked in Activity 2.1?
PHYS-204: Physics II Laboratory i
Name: Lab Partners:
Date:
Introduction to Light
Objectives
- To examine how we are able to see objects–how light from objects reaches our eyes.
- To discover how light spreads out from point and extended sources.
- To discover how light rays can be used to represent light as it spreads out from a point source.
- To discover how light rays from an object interact with a lens to form an image.
Overview
Our ability to see the objects around us depends on light traveling from the objects to our eyes, and we usually think of this light coming to our eyes along straight paths. Newton was a strong believer that light was made up of particles, and that it could be described by straight line rays drawn in the direction of motion. He did not conceive of light as waves. However, during the 19th century a number of observations of light interference firmly established that light propagated through space as waves. It is useful to consider light from both of these points of view–as particles or waves. Although light truly is a wave, when it interacts with objects much larger than its wavelength it acts like particles and can be described by straight line rays to a very good approximation. When it interacts with small objects–near the size of its wavelength–then a wave model is needed to accurately describe the interactions. Since optical elements like lenses are generally much larger than the wavelengths of light (which are of the order of half a millionth of a meter), the ray model–usually called ray optics or geometrical optics– is quite adequate. This laboratory unit will deal with such situations. These labs have been adapted from the Real Time Physics Active Learning Laboratories [1]. The goals, guiding principles and procedures of these labs closely parallel the implementations found in the work of those authors [1, 2, 3].
Investigation 1:
Light From Here To There
Let’s look at the light coming from various objects around you. To do this, you can use a light probe that measures the light intensity incident on the surface of its light detecting element (called a photo-diode). Light intensity is the light energy per unit time (power) incident upon unit area. It is associated with the brightness of the light. To carry out this activity, you will need the following:
- computer-based laboratory system with Logger Pro software.
- Lab Pro interface.
- light probe.
- sheets of black and white paper.
Question 1.3 Which piece of paper appeared brighter? Is this also the one for which you measured the larger intensity? Describe your observations.
Step 5: Use the light probe to determine where the light from the surface of the white paper is coming from by rotating the probe and pointing it in different directions around the room.
Question 1.4 Where did the light originate? What is your evidence?
Question 1.5 How can you see objects which are not sources of light? Do objects like your body or the tabletop emit light? Where does the light come from?
Question 1.6 Explain the differences in light intensity observed with the different objects in Questions 1.1 and 1.2.
You have seen in the previous activity that an object is visible either because it is a source of light or because it reflects light incident on it from a source. The brightness of an object seen by reflected light depends on how effectively it reflects the light incident on it.In order to better understand how we see objects, we need to know how light interacts with an optical component like a lens. And to better understand this, we must see what happens to light after it leaves a source or is reflected from the surface of an object. We want to see what happens to the intensity of light as you move away from an object. First we will look at an extended object–a sheet of paper, then at a small light source, a single light bulb.
Prediction 1.3 If you think of the light reflected from a large area like a whole sheet of paper, you would picture this as many rays coming out parallel to each other, as in Fig. 1a below. On the other hand, if the light is emerging from a small object, like a light bulb, you could picture this as rays spreading out as they get farther from the source, as in Fig. 1b.
(a) (b)
Figure 1: (a) light reflecting from an extended source. (b) light from a small source
If you use the light probe to measure the change in intensity of light as you move away from the sheet or white paper or the light bulb, how will the changes in the light intensity compare? Will they be different for these two objects? To test your predictions you will need:
- computer-based laboratory system with Logger Pro software.
- light probe.
- sheet of white paper.
- 6-volt light bulb and socket.
- 6 Volt battery.
Activity 1.2: Light propagating out in space – Light From Extended
and Small Objects
The room lights will be turned off for you to do this activity. Use the goose-neck lamp to provide enough light to work by.
Step 1: Place the sheet of white paper on the floor next to you table. Set up a goose-neck lamp so that it extends over the edge of the table and shines on the paper.
Step 2: Set the switch on the light probe amplifier box to the highest sensitivity (0-600) setting.
Step 3: Open experiment file L10A1-2 (Light from Objects).
Step 4: Place the Light Sensor 2 cm from the paper and note the value of intensity in the meter window. Be sure that you hold the probe so that it is pointing directly at the paper. Make sure that the intensity changes as you move the sensor. If it does not, the probe is becoming saturated; the intensity is too large for it to read. Move the lamp higher or more off to the side of the paper until the probe no longer saturates. Move the sensor away from the bulb and watch the displayed intensity values. Do the meter readings seem to agree with your prediction for the intensity of light reflected from the paper?
Step 5: Click. Collect to begin data collection. Place the Light Sensor 2 cm from the paper. Be sure the sensor is pointing directly at the paper.
(a) (b)
Figure 2: Light detected by light sensor: (a) close to and (b) twice as far away from an extended source.
through 9 above. Be sure the light sensor is pointing directly at the light bulb when you make you measurements.
Question 1.9 How does this graph differ from the one for the whole sheet of paper? Did your results agree with your prediction?
Question 1.10 For which object does the intensity fall off faster from its initial value? Was it much faster? Can you explain this result?
Question 1.11 Fig. 3 below depicts a small light bulb with light coming out along paths in all directions, and two light probes at different distances.
Which probe will intercept more of the light paths? Based on this, explain the change in intensity of light from the light bulb.
Prediction 1.4 In the Fig. 3 above the light rays are spread over an area that is pro- portional to the square of the distance from the light bulb. Based on this, decide what kind of mathematical relationship you think exists between these two variables. Let I be the intensity of the light, and let d be the distance between the light source and the light sensor:
A
B
Figure 3: Light spreading out from a small source.
- If the relationship is direct, then I = kd where k is a proportionality constant.
- If the relationship is inverse, then I = k/d.
- If the relationship is inverse square, then I = k/d^2
Step 11: To see if you made the right choice, click the Curve Fit button, f (x) =. Select a
fit type from the list of curve fits displayed, then click Try Fit. A best-fit curve will be displayed on the graph. If you made the correct choice, the curve should closely match the data. If the curve does not match well, try a different fit and click Try Fit again. When you are satisfied with the fit, click OK.
Question 1.12 Write down an equation that relates the intensity of light to the distance from a small source.
Investigation 2:
How Are Images Formed By Lenses?
When light is emitted or reflected by an object, each point on the object serves as a source of light. The light from each of these points spreads out in all directions in space. To understand what we see or what image is formed by a lens or mirror, we must first see what happens to the light from each of these point sources. The filament of a light bulb, while not exactly a point source of light, is small enough to be a good approximation.
Place the convex lens model in the position shown in Fig. 4. The lens model should be about 3 cm from the edge of the board. Trace the outline of the lens model on the large piece of paper. Sketch on the diagram above the beam of light that comes out of the lens with only bulb 2 turned on. If light rays coming out of the lens are hard to see, try raising or lowering the far end of the cork board to make them more visible.
Step 3: Place the screen with slits between the light bulbs and the lens standing up against the edge of the board, so that the light from the bulb is divided up into rays.
Question 2.1 Describe what the lens does to the rays from bulb 2. (What direction are they moving when they leave the bulb, and what direction are they moving when they leave the lens?)
Step 4: Place an X at the point where the image of bulb 2 is formed (where the rays from bulb 2 are converged by the lens).
Question 2.2 Are rays from bulb 2 hitting only part of the front surface of the lens or all of the front surface?
Step 5: Repeat steps (4) and (5) for bulb 1. Connect the two x’s with an arrow. Note, the head of the arrow should be at the location of the image of bulb 1
Step 6: Now turn both bulbs on at the same time. Put the colored filter in front of bulb 2 so that you can distinguish the light from the two bulbs.
Question 2.3 Would the image of the arrow be upright or inverted? How do you know?
Question 2.4 Is the image of the arrow enlarged or reduced in size compared to the arrow itself?
Prediction 2.1 Suppose that you moved the lights and arrow closer to the lens. How would the image be changed?
Prediction 2.2 Suppose that you moved the lights and arrow farther from the lens. How would the image be changed?
Test your predictions.
Step 7: Move the lights closer to the lens. Move the lights only about 3 cm.
Question 2.5 Describe what happens to the image. Is it closer or farther from the lens? Is it now larger or smaller?
Step 8: Move the lights farther from the lens than they were initially.
Question 2.6 Describe what happens to the image. Is it closer or farther from the lens? Is it now larger or smaller?
Question 2.7 Is there a distance between the arrow and lens so small that the light rays would no longer converge to form an image?
Step 9: Experiment by moving the light bulbs closer to the lens model.
Question 2.8 Do you find a position at which the light rays no longer converge? Do the light rays then diverge, with the rays from each bulb behaving as if they come from a point.
Step 11: Block the center of the lens with a card.
Question 2.10 Carefully describe what happened to the image? Explain your observa- tions based on what happens to rays from each of the bulbs that hit the unblocked portions of the lens.
Prediction 2.5 Suppose that you cover the top half of the arrow with a piece of paper. How would the image be changed? Would the whole image of the arrow still be formed?
Step 12: Block the top bulb with a piece of paper.
Question 2.11 Carefully describe what happened to the image? Explain your observa- tions based on what happens to rays from each of the bulbs.
Prediction 2.6 Suppose that you removed the lens. How would the image be changed? Would the whole image of the arrow still be formed?
Step 13: Remove the lens.
Question 2.12 Describe what happened to the image.
Question 2.13 Carefully describe the function of a lens in forming an image. Describe what the lens does to the light coming from each point on the object.
Activity 2.2: Concave lens
Step 1: Place a clean sheet of paper on the cork board. Place the concave lens model in the position shown in Fig. 6. The lens model should be about 3 cm from the edge of the board. Trace the outline of the lens model on the large piece of paper. Screen with slits (^) cork board
Figure 6: Concave lens
Step 2: Place the screen with slits between the light bulbs and the lens, standing up against the edge of the board, so that the light from the bulb is divided up into rays.
Step 3: Turn on bulbs 1.
Question 2.14 Do the light rays diverge or converge?
Step 4: Trace some of the rays where they emerge from the lens on the sheet of paper. Label these ”lens 1”.
Step 5: Now turn of bulb 1, and turn on bulb 2. Again trace the rays emerging from the lens. Label these ”lens 2”.
Step 6: Remove the lens and the light bulb. Use two meter sticks lined up with two of the rays you have traced from bulb 1. Locate a point at which the meter sticks cross. This is the point that rays from light bulb 1 appear to come from.
Question 2.15 Do the rays appear to start out from a point on the same side of the lens as the light bulbs?
