Name __ __ Date __ __
##### What’s the Size of What You See? ##### Sample Answers
In this activity, you’ll learn how to use scale bars on images and movies, and find out how to calculate the size of specimens you examine with your compound microscope.
**Note: **Students will be estimating sizes. In the case of *Volvox,* several different sizes are shown in the image. The sample answers are “ballpark” guidelines.
**Part One: Using scale bars**
**1. Determining the size of a red blood cell**
Observe the __image of red blood cells__. Notice that the scale bar says “50µm,” which is 50 micrometers or 0.05 mm. Estimate how many blood cells could fit end to end along the scale bar. Now, use the equation below to calculate the size of an individual blood cell:
Your calculations:
Diameter of one red blood cell: **0.008 mm** (Don’t forget the size unit!)
**2. Determining the size of a ***Volvox* colony
Observe the __image of __*Volvox globator*. Notice that the scale bar says “150 µm,” which is 150 micrometers or 0.15 mm. Estimate how many of the *Volvox* can fit along the scale bar. Does one entire *Volvox* fit, or does only part of one fit on the scale bar? Show your work below.
Your calculations (include the correct equation for this problem, as modeled in the first problem):
Diameter of one *Volvox*: **0.3 mm**
**3. Determining the diameter of a sea urchin sperm**
Observe the __image of sea urchin sperm__. Notice that the scale bar is __50__ µm. Determine the diameter of the head (round portion) of a sea urchin sperm.
Your calculations:
Diameter of the sea urchin sperm: **0.003 mm**
continued
**4. Finding the size of a sea urchin embryo**
Observe the image of the __sea urchin embryo__. Notice that the size of the scale bar is __50__ µm. Keeping this image available, watch the sea urchin embryo cell division movie on the same page. Calculate the diameter of a sea urchin embryo at the one-, two-, and four-cell stages. Show your calculations below.
**a.** One cell—your calculations:
Diameter of single-celled sea urchin embryo: **1.67 mm**
**b.** Two cells—your calculations:
**Same as “a.“**
Diameter of two-celled sea urchin embryo: __________
**c.** Four cells—your calculations:
**Same as “a.”**
Diameter of four-celled sea urchin embryo: __________
**d.** As the embryo divides, does it become larger, stay the same size, or become smaller?
**Stays the same size.**
**e.** What do your calculations tell you about the size of the individual cells of the early sea urchin embryo? Do they increase in size as the embryo divides, decrease in size, or stay the same?
**The individual cells must be getting smaller, since there are four cells in the same size embryo as at the one-cell stage.**
**Note: **For *Part Two: Finding the size of microscope specimens,* sample answers aren’t provided because answers will vary depending on the particular microscope.
Exploratorium Microscope Imaging Station What's the Size of What You See?
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