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Light - How light travels in straight lines

Grade 5Cambridge (IGCSE)

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

🔑Concepts

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Light travels in straight lines. This principle is often referred to as the rectilinear propagation of light.

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A light ray is a line with an arrow showing the direction in which light is traveling. Light rays are represented as →\rightarrow.

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Shadows are formed when an opaque object blocks light. Because light travels in straight lines and cannot 'bend' around the object, the area behind the object remains dark.

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The size of a shadow changes depending on the distance. If the distance between the light source and the object dsod_{so} decreases, the shadow becomes larger.

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We see non-luminous objects because light from a source reflects off the object and enters our eyes. The path of light is: Source→Object→Eye\text{Source} \rightarrow \text{Object} \rightarrow \text{Eye}.

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Transparent materials allow light to pass through completely, translucent materials allow some light to pass, and opaque materials block all light, creating a distinct shadow.

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Light travels at a very high speed, approximately c≈3×108 m/sc \approx 3 \times 10^8 \text{ m/s} in a vacuum.

📐Formulae

Angle of Incidence (i)=Angle of Reflection (r)\text{Angle of Incidence } (i) = \text{Angle of Reflection } (r)

Speed (v)=Distance (d)Time (t)\text{Speed } (v) = \frac{\text{Distance } (d)}{\text{Time } (t)}

c≈300,000 km/sc \approx 300,000 \text{ km/s}

💡Examples

Problem 1:

A student uses a torch to shine light on a wooden block. If the student moves the torch closer to the block, what happens to the shadow on the wall?

Solution:

The shadow will increase in size.

Explanation:

Because light travels in straight lines, moving the source closer to the object causes the object to intercept a wider angle of light rays, casting a larger area of darkness (shadow) on the screen behind it.

Problem 2:

Draw the path of light that allows a person to see a book on a table when a lamp is turned on.

Solution:

Lamp→Book→Eye\text{Lamp} \rightarrow \text{Book} \rightarrow \text{Eye}

Explanation:

Light travels in a straight line from the source (Lamp) to the object (Book), reflects off the surface of the book, and then travels in a straight line to the observer's eye.

Problem 3:

If a light ray hits a flat mirror at an angle of incidence of 45∘45^\circ, what is the angle of reflection?

Solution:

45∘45^\circ

Explanation:

According to the law of reflection, the angle at which light hits a surface is equal to the angle at which it bounces off, expressed as i=ri = r.