Apply the reflection-angle rule

Supplied materials

• All measurements and observations below are supplied classroom examples. They are not data from actual learners or a claim about a particular real site.
• Paper or browser document; an ordinary calculator is optional.
• Rule: Reflection angle from the normal equals supplied incidence angle from the normal. Formula: a. Variable key: a = incidence angle from normal (degrees)
• Distinct worked inputs and result: A distinct ray arrives at 30° from the normal: reflected angle is 30° from the normal.
• Assessment case 1: Supplied case 1: incidence angle from normal (degrees)=15, using the explicitly named quantities. Reflection angle from the normal equals supplied incidence angle from the normal. What result follows?
• Assessment case 2: Supplied case 2: incidence angle from normal (degrees)=25, using the explicitly named quantities. Reflection angle from the normal equals supplied incidence angle from the normal. What result follows?
• Assessment case 3: Supplied case 3: incidence angle from normal (degrees)=35, using the explicitly named quantities. Reflection angle from the normal equals supplied incidence angle from the normal. What result follows?
• Assessment case 4: Supplied case 4: incidence angle from normal (degrees)=45, using the explicitly named quantities. Reflection angle from the normal equals supplied incidence angle from the normal. What result follows?
• Assessment case 5: Supplied case 5: incidence angle from normal (degrees)=55, using the explicitly named quantities. Reflection angle from the normal equals supplied incidence angle from the normal. What result follows?
• Assessment case 6: Supplied case 6: incidence angle from normal (degrees)=65, using the explicitly named quantities. Reflection angle from the normal equals supplied incidence angle from the normal. What result follows?

Learn the idea

In a ray model the angle of reflection equals the angle of incidence, both measured from the normal to the surface. Measuring from the surface instead gives the complementary angle.

Reflection angle from the normal equals supplied incidence angle from the normal. The formula in the supplied model is a. Variable key: a = incidence angle from normal (degrees). Measure both angles from the normal. The normal is perpendicular to the surface.

A distinct ray arrives at 30° from the normal: reflected angle is 30° from the normal. Do not measure one angle from the surface and the other from normal. Track the quantity and unit before calculating. Explain the meaning of a positive, negative or fractional result when relevant. A numerical answer supports the stated model calculation; broader claims require the additional measurements and assumptions described here.

1. Name the measured quantities and their units before substituting values.
2. Write the stated rule and use the variable key to match each symbol to its explicitly named quantity.
3. Calculate each supplied case separately, keeping intermediate arithmetic and the result unit.
4. Explain the result in context and identify the assumption that would invalidate direct use of the rule.

Check your reasoning

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Take it further

Changed-input task: use the same stated model with incidence angle from normal (degrees)=16. Expected result: 16 degrees. Explain what changes and keep this assumption: Do not measure one angle from the surface and the other from normal.

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