Shadows & Time Lab

 

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Vocabulary:

 

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alignment: to bring into line

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shadow: partial darkness in a space from which light rays are blocked or cut off

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angle: the figure formed by the meeting of two (2) lines in a point

 

During this lab, we will become familiar with the orientation of shadows; their size in relation to the object casting them; and what the alignment of the Sun, object, and the shadow tells us about how shadows work.

 

The height of a tilted light source (in other words, the angle between the light source and the ground) and the size of the object it is illuminating, determine the length of the shadow that the object casts.  The object blocks the light coming from the source so that nothing behind the object gets any direct light.  The length of the shadow is a result of how high above or below the top of the object the light source is.  Imagine if the light source were directly above the top of the object.  Would there be a shadow? 

 

If you said no, you would be correct.  No shadow would be visible around the object.

 

Move the light source a little down from the top, and a shadow appears behind the object, but is very short.  This is because as the light source moves down, the shadow is being created by the small area of the object blocking the light.  Imagine straight lines coming down from the light and hitting the object.  The higher the light, the fewer light rays are blocked by the object and hence the less shadow.  Thus, the lower the light source is aimed at the object, the more the object blocks the rays of light.

The key to understanding shadows is to realize that the light source and object must be lined up in order to make a shadow appear.  In fact, if the object is placed anywhere along that line, it will produce a shadow of the same length behind the object.  It is only when you change the orientation of the light source that the shadow changes.  That makes sense - light hits an object and casts a shadow.  The connection comes when realizing that the path of the sun across the sky will cast a moving shadow on a stationary object effectually enabling one to tell time.

During one of the Mars missions, a sundial was carried and placed on Mars.  The sundial was included to allow a prominent public display of time.  The sundial idea was the brainchild of Bill Nye the Science Guy, who noticed that a post originally used for camera calibration could be redesigned which would allow it to function as a sundial.  Millennia ago, sundials were state-of-the-art timekeepers for humans on Earth.  Since the Sun casts similar shadows on Mars and Earth, an accurate calibration of the shadow placement on the Martian Sundial will tell a curious inspector of returned images both the time of day and the season.  An amazing use of ancient technology to solve a modern day problem!

(click for larger image)

Laboratory Activity

Objective:

This lab will show how a shadow's length changes as sunlight strikes the ground at different angles.

 

Background:

The sun's position in the sky changes during the day.  It appears to rise in the eastern sky.  In the Northern Hemisphere, the sun travels due south to its highest point in the sky at noon.  Later, the sun appears to set in the western sky.  As a result, an objects shadow changes size and direction during the day.

 

Materials:

pencil; newsprint paper; 40 cm of string; flashlight; metric ruler and protractor.

 

Procedure:

1. Find center of newsprint paper by folding paper in half length-wise and then in half again width-wise.

2. Mark the directions North, South, East and West on the newsprint.

3. Tie one end of the string to the top of the pencil; set the pencil upright in the center of the newsprint by holding it at its base.

4. While standing at the south, your partner should place the flashlight in the "sunrise" position.

5. Measure the length of the shadow.  Extend the string to the pencil's shadow.  Record the direction of the shadow.  Finally, using your protractor, measure the angle of the string to the table.

6. Repeat steps 5 and 6, placing the flashlight in the "noon" and "sunset" positions.  Note: remember the sun is due south at noon.

 

Date Table:

 

Time of Day Length of Shadow Direction of Shadow Angle of String
Sunrise      
Noon      
Sunset      

 

Questions:

 

1. Assume the string represents a ray of sunlight.  When does sunlight hit the ground at its greatest angle?  When does sunlight

    hit the ground at its smallest angle?

 

2. When is the pencil's shadow longest?  When is the pencil's shadow shortest?

 

3. As the sun rises higher in the sky, what happens to the length of an object's shadow?  What happens as the sun moves lower

    in the sky?

 

4. Suppose you are outdoors one clear day.  The sun is in the southwestern part of the sky.  In which direction will your shadow

    point?  What is the approximate time?

 

5. How would you change the procedure to show the sun through a day in the Southern Hemisphere?

Remember that when writing up your lab report, you must follow the proper format.

Lab Notes  >