How Do We See Colors

2ndSem.-Physics – LO Lab-03What Color of Light Do You See?

LO - Lab #03 - What Color of Light Do You See?

Task #1 - Revisiting Previous Results

Here is a summary of some results that were obtained by students last week.

1.Write a brief description (no longer than one short paragraph!) of the experiment conducted last week. What was measured and under what conditions was it measured?

2.In words, describe the physical significance of the optical density. (That is, don't describe how it is calculated, instead, describe what the values mean in relation to the three different colors of filters.) What does it mean for a material to be optically denser than another material?

3.Suppose you repeated the experiment from last week, except instead of using "white" light, you used red light (or blue light or green light or …). Do you think this would change your results for the optical density of the different filter colors? Why or why not? If it would change, how do you think it would change? Explain your reasoning fully.

Research Questions

For today's lesson, you are going to investigate different consequences of the color of visible light. These investigations will examine the your own visual response to color as well as how color filters affect different colors of light. That is, you are going to complete investigations to answer the following questions.

• How do your own eyes respond to different colors of stimuli?

• Does the amount of light transmitted through color filters depend on the wavelength (color) of the incident light? If it does, then how does it depend?

• Does the optical density of color filters depend on the wavelength of incident light? If it does, then how does it depend?

Task #2 - Examining the Color Response of Your Own Eyes

• Due to limited equipment, only one group may complete this activity at a time.
• If the equipment is currently being used, go on to the next activity (Task #3) until the equipment is available.
• When it is your turn, each member of your group should complete Task #2. This only needs to be done before you leave lab today.

The human eye perceives light and color by the stimulation of the photoreceptors (detectors of photons) of the retina. The human eye has two kinds of photoreceptors – rods and cones. Rods work in very low levels of light, but do not help with color vision. Cones are responsible for color vision, but are only sensitive to relatively bright light. The rods provide information on the presence or absence of photons – irrespective of the wavelength of the visible light. Therefore, while the rods are not able to distinguish among different colors of light, they are responsible for human night vision. This is why in conditions of very low light, people only see in shades of gray.

People with normal color vision are able to perceive all the visible colors from mixtures of just these three colors. As the cones are stimulated they send appropriate signals to the brain, which are then perceived as different colors. For example, if yellow light enters the eye, then the green and red cones will be stimulated and your brain will interpret these signals as yellow.

However, about 8 % of men and about 0.4 %of women have some degree of color blindness. Typically, color blindness does not refer to the inability to perceive colors. Instead, most cases of color blindness result in a difficulty in being able to distinguish among certain hues, primarily pastel reds and greens.

• Do not write on the test images!!

• When completing the test, do not say the numbers out loud. If someone in your group has mild/moderate color blindness, they may be able to recognize the number after they hear it, but they may not have been able to distinguish the correct number on their own.

• Keep in mind that these images are not of the same quality as the ones you would find in a doctor’s office. However, they should give you a good sense of how your color vision is in general.

• If you find that you have some degree of color blindness, don’t panic! You are in good company since about 5% of the total population share this same condition!

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2ndSem.-Physics – LO Lab-03What Color of Light Do You See?

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2ndSem.-Physics – LO Lab-03What Color of Light Do You See?

Task #3 - Examining the Effect of Color Filters on Visible Light with Your Eyes

You should find the following equipment at your station:

• Spectrophotometer (with glass prism and MBL sensors)
• Incandescent light source mounted on optics bench
• Power supply for light source
• Black cloths
• Packet of color filters (with two of the following colors: red, magenta, blue, or cyan)

Before you begin your study, prepare the equipment by completing the following checklist.

a)Turn on the light source and cover the extraneous light with a black cloth.

b)Check that the light source is aligned and focused into the spectrophotometer. The light from the bulb should pass through slit #1 and the focusing lens, pass through the prism, and result in a crisp image of the white light going through the slit at 0 and a rainbow spectrum like you observed previously in lab #06.

c)Ask your instructor for assistance before you begin your experiment if the alignment seems off.

White Light:

Move the arm of the spectrometer so that you are able to clearly view the resulting spectrum on the white screen. Using the provided crayons, carefully draw the band of colors as they appear to your eyes. Try to accurately depict the relative width of each color band in your drawing.

Color Light:

Select a color filter from the packet. Have one person hold this filter over the front of the focusing lens (this is between the light source and the prism). Record which filter you used and carefully draw the band of colors as they now appear to your eyes. Keep the same scale as you depicted in the first drawing. Which colors were absorbed by this filter and which were transmitted?

Repeat this for the other filter color. Identify the color of the filter, draw what you see, and list which colors were absorbed and transmitted.

5.If you had an orange filter, what colors do you think it would transmit? What colors do you think it would absorb? Explain your reasoning for your prediction.

• You might complete Task #2 before you continue if the equipment is available.

Task #4 - Examining the Effect of Color Filters on Visible Light with an MBL Sensor

You will now use the MBL sensor to quantitatively study the effect of color filters.

Before you begin your study, prepare the equipment and software by completing the following checklist.

a)Start up the interface box and Lab#08 MBL software. Check that the light sensor is set to a gain of 100.

b)Turn on the light source and cover the extraneous light with a black cloth.

c)Check that the light source is aligned and focused. The light from the bulb should pass through slit #1, the focusing lens and enter the prism. It should then result in a crisp image of the white light going through the slit at 0 and a rainbow spectrum like you observed in lab #06 at about 110.

d)Ask your instructor for assistance before you begin your experiment if the alignment seems off.

Next you will want to check the brightness of the light source for collecting data with the Pasco light sensor. To do this, place the light sensor against the stop position on the rotating platform. Start recording data and move the sensor clockwise from the stop position past the area where you are able to see the light's visible spectrum (a total of about 17).

Examine your data. Did it appear that the MBL sensor was saturated ("maxed out")? If so, for what color(s) of light was it maxed out?

If the light seems to be too bright for the sensor, you can change the gain setting to 10. Try this out and see how it changes the data. Is the sensor still saturated? How did the values change? That is, did they change by a factor of 10? Or…?

You will want to adjust the bulb's brightness as measured by the sensor so that it is as bright as possible, but not so bright that the visible data saturates the sensor. Therefore, you may need to collect some of your data with a gain of 100 (like blues and greens) and some of your data with a gain of 10 (like red and infrared). When possible, use a gain of 100.

6.a)Describe what adjustments, if any, you made to the equipment setup.

b)Explain how you can tell if the light sensor is being saturated. Why is this a problem when you are collecting intensity data?

You might ask your instructor to check the setup before you continue.

Now that you have tested the software, aligned the light source, and adjusted the brightness of the light source, you are ready to begin your experiment! You will measure the intensity of visible light with and without the use of filters. You will then use this data to characterize the effect of each of the filter colors.

Data Collection – Nonfiltered and Filtered Visible Light

Start with the light sensor set to a gain of 100. Set the light sensor so that it is at an angle just beyond where your eyes can perceive any violet light, say around 114. Press the monitor button so that the computer will continually measure the light intensity.

Record the angle (according to the rotating platform) and the light intensity, along with the current gain setting. Since the light intensity will fluctuate due to fluctuations in the light source, you should adopt a consistent strategy for determining the value of the light intensity like always recording the maximum value that appears for a certain condition.

You will want to create a large data table such as the following for this experiment.

After you have measured the intensity of the light with no filters, then remeasure it one by one with each of the filter colors. Be sure to clearly identify which color of filter is being used in which data column.

Once you have collected your data for that angle, then move the sensor clockwise by 0.5 degrees. Repeat all of the measurements. Continue to collect data until you have gone past the region of visible red light, say to about 122. Be sure to adjust the gain setting should your sensor become saturated.

• Turn off the light source once you have finished collecting your data.!

Data Analysis

The transmission and absorption of light is extremely important in many applications of optics and spectrophotometry. These two concepts are often represented by the variables Percent Transmission and Optical Density. These two variables are defined below.

and

7.a)If all the incident light on the filter were transmitted, what value would you estimate for its percent transmission? For its optical density? Do these values make sense? Explain.

b)If there was no incident light before the filter was added, then what does this tell you about the resulting optical density and percent transmission? Can you identify values for the filters in such cases? Why or why not?

You will now examine the properties of your filter colors in terms of their percent transmission and their optical density as functions of wavelength.

Open the Excel file "Lab #08 Excel– O.D.". You can use this file to assist you in calculating percent transmission and optical density values for light bent through to different angles by the spectrophotometer.

Enter your data into the Excel spreadsheet. We will again use Excel because it is very efficient at making lots of calculations and at making graphical models.

Once you have entered your intensity data into Excel, save this file and record its name in your logbook. You should save this file often as you complete your work to keep from losing any information.

Use Excel to calculate the percent transmitted and the optical density for the light incident at each stated angle for each filter color. Recall that you can enter a formula such as:

Once you have made your calculations, then create two graphs for each filter color: a XY Scatter linear graph of Percent Transmitted vs. Angle and a XY Scatter linear graph of Optical Density vs. Angle.

Be sure to make all necessary adjustments to these graphs. For example, include meaningful titles and labels. You should also look to see that the values make sense. For example, you should not have any data point on your graph that is meaningless (i.e., a point for which you had no valid data). If you decide to delete a data point from your data table, be sure to record an explanation in your logbook as to why you deleted that point.

Once a graph is finished, then select it by clicking on the graph once with the mouse. Print a copy of the graph for each person in your group. Repeat this for the remaining graphs.

To facilitate in analyzing these graphs, use the colored crayons to identify what color of visible light is represented by which part of your graph. Also, be sure you identified the filter color represented by each graph.

Data Analysis– Filtered Light #1

Examine the graphs you made for the first filter color. Use these graphs to answer the following questions.

8.How does the percent of light transmitted by the filter depend on the color of the incident light? What color(s) are transmitted most efficiently? Which color(s) are not transmitted efficiently? Explain your answers and refer to specific angles on your graphs.

9.How does the optical density of this color filter depend on the color of the incident light? To which color(s) of light is the filter most dense? To which color(s) of light is the filter least dense? How does this compare to your prediction in question #3? Explain your answers and refer to specific angles on your graphs.

Data Analysis– Filtered Light #2

Now answer these same two questions for the other filter color.

Concluding Questions

10.If possible, find another group (or groups) that studied the same color of filter as you studied. How similar are your graphs and conclusions for each filter? Can you account for any differences?

11.Compare the percent transmitted graphs for the two different colors of filters that you investigated. How does the transmitted light differ for the two? Did they treat the different colors of light in the same ways? Be specific, referring to actual numbers for angles and/or percentage values. Does this behavior agree with what you saw with your own eyes?

12.Compare the optical density graphs for two different colors of filters. How does the optical density differ for the two? Did they react to different colors of light in the same ways? Be specific, referring to actual numbers for angles and/or percentage values. Does this behavior agree with what you saw with your own eyes?

13.Make a sketch of the optical density and percent transmission graphs that you think you would obtain if you had an orange filter to investigate with this equipment. Be sure to identify which colors of visible light are represented by which part of the graphs.

14.In order for a human to perceive colors, the light must be absorbed by the cones of the person's eye. In addition, humans have been shown to be most sensitive to yellow/yellow green light (as pictured in last week's lab). Looking at the spectral response curves pictured earlier in this lab, does it make sense that this is the color we see the best? Explain your thoughts.

• Make sure you have completed Task #2 before you leave!

Lab Cleanup

• Be sure the light source has been turned off before you leave.
• Place the two filters back into the storage envelope.

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