Refraction

    OCR
    GCSE

    Refraction defines the change in direction of a wave passing from one medium to another, caused by a change in wave speed due to differing optical densities. Candidates must articulate that while wave speed and wavelength change, frequency remains constant across the boundary. Analysis requires constructing accurate ray diagrams relative to the normal and, at higher levels, applying Snell's Law to calculate refractive indices and critical angles. Mastery includes understanding the conditions for Total Internal Reflection and its application in optical fibres.

    0
    Objectives
    3
    Exam Tips
    4
    Pitfalls
    4
    Key Terms
    4
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating that light slows down when entering a strictly optically denser medium
    • Credit ray diagrams showing the refracted ray bending towards the normal when entering a denser medium
    • Award 1 mark for explicitly stating that frequency remains constant while wavelength and speed change
    • Credit responses that explain refraction via wavefronts: one part of the wavefront hits the boundary and slows down before the rest, causing a pivot

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the change in direction; now explain *why* this happens using wave speed"
    • "Ensure your normal line is dashed and exactly 90 degrees to the surface—precision marks are lost here"
    • "Remember: Frequency is a property of the source and does not change. Only speed and wavelength change in the new medium"
    • "Your ray diagram is accurate, but you measured the angle from the surface. Always measure from the normal"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that light slows down when entering a strictly optically denser medium
    • Credit ray diagrams showing the refracted ray bending towards the normal when entering a denser medium
    • Award 1 mark for explicitly stating that frequency remains constant while wavelength and speed change
    • Credit responses that explain refraction via wavefronts: one part of the wavefront hits the boundary and slows down before the rest, causing a pivot

    Examiner Tips

    Expert advice for maximising your marks

    • 💡Always draw the normal line as a dashed line to distinguish it from the light rays
    • 💡For Higher Tier wavefront questions, use the analogy of a vehicle's wheels entering mud to explain the change in direction
    • 💡When asked to 'Describe' the path, specify both the change in speed and the direction relative to the normal

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Measuring the angle of incidence or refraction between the ray and the boundary surface instead of the normal
    • Drawing freehand rays that curve or lack precision; examiners require straight lines drawn with a ruler
    • Incorrectly stating that frequency changes when the wave enters a new medium (only speed and wavelength change)
    • Failing to draw the normal line perpendicular (90°) to the surface at the exact point of incidence

    Study Guide Available

    Comprehensive revision notes & examples

    Read Study Guide

    Key Terminology

    Essential terms to know

    Wave speed, density, and direction relationships
    Ray diagrams: Normal, angle of incidence, angle of refraction
    Refractive Index and Snell's Law
    Total Internal Reflection (TIR) and Critical Angle

    Likely Command Words

    How questions on this topic are typically asked

    Draw
    Explain
    Describe
    Calculate
    Suggest

    Practical Links

    Related required practicals

    • •{"code":"PAG P4","title":"Investigation of reflection and refraction","relevance":"Tracing rays through glass blocks to measure angles of incidence and refraction"}

    Ready to test yourself?

    Practice questions tailored to this topic