Change the Size of a Shadow!

Introduction

Background

The saying “where there is a shadow there must be light” tells us that you need some kind of light source to generate a shadow. For example, your own shadow that constantly follows you around on a sunny day is generated by the sun. But shadows don’t necessarily disappear with the sun—other light sources such as the headlights of a car, the light of a table lamp, or a simple flashlight are able to cast shadows, too. All light sources emit light waves that travel away from their source. In optics, which is a special field of physics, this emitted light is modeled as a straight line called a light ray. A light ray indicates the direction in which the light travels.

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

A shadow play is a good way to demonstrate how a shadow is cast. Shadows are projected on a screen, a light source is located behind the screen and the players, and cardboard figures are used to block the light rays. The light rays travel to the screen, which makes it look very bright. Shadows only appear on the screen once a player introduces a cardboard figure in between the light source and the screen. This is because the cardboard blocks the light traveling from its source to the screen, making the area behind it look dark instead of bright.

But how can you vary the size of a shadow? The closer an object is to the source of the light, the larger the shadow it casts. This is because an object closer to the light 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, a toy car, or a ball
• Flashlight
• Table
• Light-colored wall
• Dimmed work space
• Paper
• Pen

Preparation

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

Procedure

1. 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?
2. Switch the flashlight off, but do not move it. Take the object and place it next to the flashlight, about 25 cm (10 inches) away. Then move it towards the wall until it is located 20 cm (8 inches) away from the wall. What do you think you will see once you switch on the light?
3. 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?
4. Next, move the object directly between the wall and the flashlight, so it sits on top of the measuring tape. Place it at the 20 cm (8 inch) mark of the measuring tape. How will the shadow change once you move the object?
5. Again, 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 height of the shadow from the table surface to its top and write its size on a piece of paper. How big is the shadow?
6. Once you have measured the height of the shadow, move the object closer to the wall along the measuring tape until it reaches the 10 cm (4 inches) 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 further away from the light source? Do you notice any other differences beside the size?
7. Finally, move the object closer to the flashlight and place it at the 40 cm (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?
8. 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?

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 straight lines and as the object was not located in the direct path of light, it could not block the light from its source. However, once you placed the object directly in the path of light, 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 all the light from the light source, creating the dark shadow in the wall.

You should have observed that the size of the shadow decreased when you moved the object farther from the light source and increased when you moved the object closer to the light source. The explanation for this is that you simply block a much larger portion of the light from the light 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!

More to Explore

Light and Shadows, from toppr

Light does appear to travel in straight lines, from University of Leicester, UK

Shadow play, from Wikipedia

Science Activity for All Ages!, from Science Buddies

Credits