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Physics - Light: Sources, reflections, and how shadows are formed

Grade 4IGCSE

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

🔑Concepts

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Light is a form of energy that travels in straight lines called raysrays. We use arrows to represent the direction of light.

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Luminous sources are objects that create their own light, such as the Sun, stars, or a lit torch. Non-luminous objects, like the Moon or a book, do not emit light but are seen because they reflect light from a source.

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Reflection happens when light hits a surface and bounces off. Smooth, shiny surfaces (like mirrors) reflect light rays evenly, while rough surfaces scatter light in many directions.

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The Law of Reflection states that the angle of incidence θi\theta_i is equal to the angle of reflection θr\theta_r relative to the normal line.

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Shadows are dark areas formed when an Opaque object blocks the path of light. Light cannot bend around the object because it travels in straight lines.

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Transparent materials (like clear glass) allow all light to pass through. Translucent materials (like frosted glass) allow some light to pass but scatter it. Opaque materials (like wood) block all light.

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The size of a shadow depends on the distance dd between the light source and the object. If the object is closer to the source, the shadow is larger.

📐Formulae

θi=θr\theta_i = \theta_r

c≈3×108 m/sc \approx 3 \times 10^8 \text{ m/s}

Distance=Speed×Time\text{Distance} = \text{Speed} \times \text{Time}

💡Examples

Problem 1:

A student moves a toy car closer to a lamp. What happens to the size of the shadow cast on the wall?

Solution:

The shadow will become larger.

Explanation:

Because light travels in straight lines, moving the car closer to the lamp causes it to block a larger angle of the light rays, resulting in a bigger shadow on the wall.

Problem 2:

Explain why we can see the Moon even though it is not a luminous source.

Solution:

Reflection.

Explanation:

The Moon is non-luminous. We see it because light from the Sun travels to the Moon, hits its surface, and reflects (bouncesbounces) toward our eyes on Earth.

Problem 3:

If light hits a mirror at an angle of 30∘30^\circ to the normal, what will the angle of reflection be?

Solution:

The angle of reflection will be 30∘30^\circ.

Explanation:

According to the Law of Reflection, θi=θr\theta_i = \theta_r. If the angle of incidence is 30∘30^\circ, the reflected ray must also be at 30∘30^\circ.