Key concepts
Physics
Optics
Light

Introduction
Halloween is almost here, and behind every corner there seems to lurk a spooky shadow! Luckily this is all they really are—just shadows that are part of decorations set up to give you a thrill while trick-or-treating. But where do these shadows come from, and how can some of them be so huge? Do this activity and find out how to generate the largest and scariest shadow of all!

Background
You need some kind of light source to generate a shadow. This can be the sun, a lamp or a flashlight. All light sources emit light waves that travel away from them. In optics, which is a special field of physics, this emitted light is modeled as a straight line called a light ray, which indicates the direction in which the light travels.

To cast a shadow you need an object that can block light rays. Not every object is able to do that. Some materials will let the light pass through whereas others will only block some of it. Objects able to block all the light are called opaque and will form a shadow. You also need a large surface such as a wall or screen on which you can make the shadow visible.

How can you vary the size of a shadow? The closer an object is to the light source, the larger the shadow it casts. This is because an object closer to the source will block a larger area of the light, increasing its shadow size. Try it out for yourself in this activity!

Materials

• Two measuring tapes
• Transparent tape
• Opaque objects that block light, such as a cup, toy car or ball
• Flashlight
• Table
• Light-colored wall
• Dimmed work space
• Paper
• Pen

Preparation

• Place a table right next to a light-colored wall.
• Tape the first measuring tape onto the table with the zero mark right at the wall.
• Select an opaque object that is able to fully block light rays from a light source. It shouldn’t be too big or small.
• Put the flashlight on the table so it aligns with the measuring tape facing the wall. It should be located 50 centimeters (20 inches) away from the wall.
• Dim the lights in the room so it is dark enough to see clear shadows on the wall.

Procedure

• Switch on the flashlight and point it to the wall. What do you see on the wall once you switch on the light? How does the appearance of the wall change when you switch the light on?
• Switch the flashlight off but do not move it. Take the object and place it next to the flashlight, 25 centimeters (10 inches) away. Then move it toward the wall until it is located 20 centimeters (eight inches) away from the wall. What do you think you will see once you switch on the light?
• Switch on the light and look at the wall. Does the object placed on the table form a shadow? Can you explain why or why not?
• Move the object directly between the wall and the flashlight so it sits on top of the measuring tape. Place it at the 20-centimeter (eight-inch) mark of the measuring tape. How will the shadow change once you move the object?
• Switch on the light. What do you see on the wall this time? What shape does the shadow have? If there is a shadow, take the second measuring tape and measure the shadow’s height from the table surface to its top and write its size on a piece of paper. How big is the shadow?
• Once you have measured the shadow’s height, move the object closer to the wall along the measuring tape until it reaches the 10-centimeter (four-inch) mark. Then measure the height of the shadow again and record its size. How does the size of the shadow change when you move it farther away from the light source? Do you notice any other differences beside the size?
• Move the object closer to the flashlight and place it at the 40-centimeter (16-inch) mark on the measuring tape. Again, measure and record the height of the shadow. Does the shadow get smaller or bigger when you place the object closer to the light source?
• Look at your results on the piece of paper. If you like, you can make a graph that shows the distance from the light source on the x axis and the size of the shadow on the y axis. How is the size of the shadow dependent on the distance of the object from the light source? Can you explain your results?
• Extra: Play around with different objects. Choose different materials or different sizes. Or make your own shapes! Do they all form a shadow? Why or why not?
• Extra: Introduce a second light source by placing two flashlights in front of the wall. How do the shadows of objects that you put in between both flashlights and the wall look? How do they differ from shadows using just one light source?
• Extra: Try to point your flashlight onto the object from different angles. How does the angle change the appearance of the shadow?

Observations and results
Were you able to change the size of the object’s shadow? Before you could change the size of the shadow you needed to generate it first. You probably didn’t see a shadow when you placed the object next to the flashlight and not directly into its path of light. This is because light travels in a straight line and, because the object was not located in the direct light path, it could not block the light from its source. Once you placed the object directly in the path, however, you should have seen a shadow on the wall that had the same shape as the object. This time the opaque object was able to block the light from the light source, creating the dark shadow on the wall.

You should have observed the shadow’s size decrease when you moved the object farther from the light source and increase when you moved it closer. You block a much larger portion of the light from the source when your object is closer, which increases the size of the shadow. Now you know how to make spooky Halloween shadows grow and shrink!

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