Question 7
Observe what happens to the size of the shadow as you vary the distance between your torch and your object.
- A shadow is cast on a screen when an opaque object blocks the path of light emitted by a torch.
- Varying the distance between the torch (light source) and the object changes the angle at which the light rays diverge around the object's edges, which alters the size of the shadow formed on the screen.
Step 1 · Observe the Effect of Distance on Shadow Size
Place an object between a torch and a screen.
- Moving the torch closer to the object: The light rays diverge at a wider angle around the edges of the object, causing the shadow on the screen to become larger.
- Moving the torch farther from the object: The light rays diverge at a narrower angle, causing the shadow on the screen to become smaller.
The size of the shadow increases as the torch moves closer to the object, and decreases as the torch moves farther from the object.
- Brightness vs. Size Confusion: Assuming that bringing the torch closer makes the shadow smaller because the light is brighter; shadow size depends on the geometry of diverging rays, not brightness.
- Torch vs. Screen Movement: Confusing the effect of moving the light source with moving the screen (moving the screen farther away also increases shadow size).
More questions in A
Build it in Your Imagination
We will start this section by practising visualisation. For each prompt, feel free to talk to your partner, gesture, draw it in the air — but do not actually draw on paper!
- Picture your name, then read off the letters backwards. Make sure to do this by sight, not by sound — really see your name! Now try with your friend's name.
Cut off the four corners of an imaginary square, with each cut going between midpoints of adjacent edges. What shape is left over? How can you reassemble the four corners to make another square?
Mark the sides of an equilateral triangle into thirds. Cut off each corner of the triangle, as far as the marks. What shape do you get?
Mark the sides of a square into thirds and cut off each of its corners as far as the marks. What shape is left?
Net of a sphere? Experiment and see if you can make a paper cutout that can perfectly wrap around a ball without leaving any wrinkles, gaps or overlaps.
Place an object in front of a plane, such as a wall of your room. Shine a torch light on the object in a direction perpendicular to the wall.
What do you see?
Observe what happens to the size of the shadow as you vary the distance between your torch and your object.
Context: Observe what happens to the size of the shadow as you vary the distance between your torch and your object.
Q. Why does this happen?
Construct a model of a cube and use your hands to keep it balanced on one corner vertex. Can you try to understand why all the projected edges have equal length?