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Light and Sound - How Shadows are Formed

Grade 3IB

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Light travels in straight lines. This is known as the rectilinear propagation of light, represented by rays r⃗\vec{r}.

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A shadow is formed when an opaque object blocks light rays. Since light cannot bend around the object, the area behind it remains dark.

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Materials are classified by how much light they let through: Opaque (blocks all light), Translucent (blocks some light), and Transparent (blocks no light).

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The position of the light source affects the shadow. If the light source is directly above an object, the shadow is short (Lshadow≈0L_{shadow} \approx 0). If the light source is at an angle, the shadow becomes longer.

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The size of a shadow depends on the distance between the light source (dsd_s), the object, and the screen (dscreend_{screen}).

📐Formulae

Shadow Size∝1Distance between Light and Object\text{Shadow Size} \propto \frac{1}{\text{Distance between Light and Object}}

Total Light=Absorbed Light+Reflected Light+Transmitted Light\text{Total Light} = \text{Absorbed Light} + \text{Reflected Light} + \text{Transmitted Light}

Distancetotal=dsource−to−object+dobject−to−screen\text{Distance}_{total} = d_{source-to-object} + d_{object-to-screen}

💡Examples

Problem 1:

Sara is holding a puppet 50 cm50\text{ cm} away from a flashlight. If she moves the puppet to 20 cm20\text{ cm} away from the flashlight, what happens to the size of the shadow on the wall?

Solution:

The shadow will become larger.

Explanation:

As the distance dd between the light source and the object decreases, the object blocks a wider angle of light rays, causing the shadow area to increase in size.

Problem 2:

At 12:00 PM (noon), the Sun is directly overhead. What will the length of the shadow (LL) of a vertical stick look like compared to 5:00 PM?

Solution:

The shadow at 12:00 PM will be very short (L≈0L \approx 0), while at 5:00 PM it will be very long.

Explanation:

When the light source is at a high angle θ\theta (overhead), the shadow is minimized. When the light source is at a low angle (near the horizon), the rays hit the object at a slant, creating a longer shadow.

Problem 3:

Which material will create the darkest shadow: a glass window, a piece of tissue paper, or a wooden block?

Solution:

The wooden block.

Explanation:

The wooden block is opaque, meaning it blocks 100%100\% of the light rays. The glass is transparent and the tissue is translucent, allowing most or some light to pass through, resulting in no shadow or a very faint shadow.