Question 8
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?
The shadow forms because the object blocks light rays from the torch.
Step 1 — Observing Shadow Size
Let us set up a torch, an object, and a screen. We observe the shadow of the object on the screen. Let us move the torch closer to the object. We see the shadow becomes larger. Let us move the torch farther from the object. We see the shadow becomes smaller. The shadow's size changes with the torch's distance.
Step 2 — Explaining with Light Rays
A torch sends out light rays in all directions. These rays spread out as they travel. Imagine the light rays forming a cone shape. The object blocks some of these spreading rays. When the torch is close, light rays spread out more sharply. The object blocks a wider section of these rays. This makes a larger shadow on the screen. When the torch is far, light rays spread out less sharply. The object blocks a narrower section of these rays. This makes a smaller shadow on the screen. The light rays spread out more over a longer distance. So, the shadow's size changes with distance.
Answer
(i) The shadow's size changes because light rays spread out. When the torch is closer, the object blocks more widely spread light. This creates a larger shadow. When the torch is farther, the object blocks less widely spread light. This creates a smaller shadow.
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?