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Refraction of Light; the Prism; Dispersion and Spectra

Physics Grade 9 Difficulty 2/5

Beginning of the lesson

Scientific educational illustration for the physics lesson: Refraction of Light; the Prism; Dispersion and Spectra.
This illustration highlights the main physical model and observable relationships explored in the lesson.

Refraction of Light; the Prism; Dispersion and Spectra

Why a straw looks broken in a glass of water, and how a raindrop paints a rainbow over Issyk-Kul

Big Idea: When light crosses from one transparent substance into another, its speed changes, and this makes the ray bend. This bending is called refraction. Because different colours bend by slightly different amounts, a glass prism spreads white light into a band of colours - the spectrum. This one idea explains lenses, rainbows, the twinkling of stars and how astronomers read the chemistry of the Sun.
By the end of this topic you will be able to:
  • explain why a ray of light changes direction at a boundary,
  • state and use the law of refraction and the refractive index n = c/v,
  • predict which way a ray bends when it enters a denser or a rarer medium,
  • trace a ray through a triangular prism,
  • explain dispersion and why violet bends more than red,
  • name the three kinds of spectra and say what each one tells us.

1. What is refraction?

In the previous topic you saw that light travels in straight lines inside one uniform medium, and that it reflects off a mirror. But watch what happens at the surface of water: a pencil standing in a glass looks bent or even broken at the waterline, and the bottom of a clear mountain river looks closer than it really is. The light has not stopped and it has not simply reflected - it has changed direction as it crossed the boundary between air and water.

Refraction is the change in the direction of a light ray when it passes from one transparent medium into another in which its speed is different. The ray bends at the boundary (the interface) and then continues in a straight line inside the new medium.

Key fact: light travels fastest in a vacuum, c = 3 × 108 m/s (300 000 km/s). In any transparent material - water, glass, a diamond - it travels slower. That change of speed is the real cause of the bending.

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In the full lesson

  • The complete explanation
  • Worked examples
  • Common mistakes and how to avoid them
  • Test questions: 15
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