Refraction of light — Eduqas A-Level Physics
Test yourself on Refraction of light with EDUQAS A-Level practice questions.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Refraction of light explained
This topic covers the fundamental concepts of force, free body diagrams, and Newton's laws of motion.
Read the full explanation
It also explores linear momentum, the principle of conservation of momentum, and the application of these concepts to solve problems involving elastic and inelastic collisions.
What to demonstrate
- Newton's 3rd law of motion
- Use of free body diagrams to represent forces
- Application of the relationship ΣF = ma for constant mass
Show all 7 objectives
- Definition of linear momentum as the product of mass and velocity
- Force as the rate of change of momentum
- Principle of conservation of momentum in one dimension
- Distinction between elastic (no kinetic energy loss) and inelastic (kinetic energy loss) collisions
Refraction of light exam tips
Topic Overview
Refraction is the bending of light as it passes from one transparent medium to another, caused by a change in its speed. This topic is central to understanding how lenses, prisms, and optical fibres work, and it explains everyday phenomena like why a straw appears bent in water or why pools look shallower than they are. In the WJEC A-Level Physics specification, refraction is studied alongside reflection and total internal reflection, forming the foundation of wave optics.
The key principle governing refraction is Snell's Law, which relates the angles of incidence and refraction to the refractive indices of the two media. You will learn to calculate refractive indices, critical angles, and apply these to problems involving prisms and optical fibres. Understanding refraction is essential for topics like fibre optics in communications, lens design in cameras and glasses, and even atmospheric effects like mirages.
Mastering refraction requires a solid grasp of wave behaviour and trigonometry. It connects directly to the wave model of light and prepares you for more advanced concepts such as dispersion, interference, and diffraction. In exams, you will be expected to draw ray diagrams, perform calculations using Snell's Law, and explain real-world applications.
Key Concepts
- →Refractive index (n): A measure of how much a medium slows down light relative to a vacuum. It is defined as n = c/v, where c is the speed of light in vacuum and v is the speed in the medium. Higher n means greater bending.
- →Snell's Law: n₁ sin θ₁ = n₂ sin θ₂, where θ₁ and θ₂ are the angles of incidence and refraction measured from the normal. This law allows calculation of unknown angles or refractive indices.
- →Critical angle (C): The angle of incidence in the denser medium for which the angle of refraction is 90°. It is given by sin C = n₂/n₁ (where n₁ > n₂). Beyond this angle, total internal reflection occurs.
- →Total internal reflection (TIR): When light travelling from a denser to a rarer medium hits the boundary at an angle greater than the critical angle, it is completely reflected back. This principle is used in optical fibres and prisms.
- →Dispersion: The splitting of white light into its constituent colours when passing through a prism, because different wavelengths have slightly different refractive indices in the same medium.
Marking Points
- Newton's 3rd law of motion
- Use of free body diagrams to represent forces
- Application of the relationship ΣF = ma for constant mass
- Definition of linear momentum as the product of mass and velocity
- Force as the rate of change of momentum
- Principle of conservation of momentum in one dimension
- Distinction between elastic (no kinetic energy loss) and inelastic (kinetic energy loss) collisions
Examiner Tips
- 💡Always draw a clear free body diagram before attempting to solve force problems
- 💡Ensure units are consistent throughout calculations, particularly when dealing with momentum
- 💡State the principle of conservation of momentum clearly before applying it to a collision problem
- 💡Check if the collision is elastic or inelastic to determine if kinetic energy is conserved
- 💡Always draw a clear ray diagram with the normal line at the point of incidence. Label angles correctly: the angle of incidence is between the incident ray and the normal, not the boundary. Marks are often lost for mislabelling.
- 💡When using Snell's Law, ensure your calculator is in degree mode. Check whether the angle given is measured from the normal or from the boundary. If from the boundary, subtract from 90° first.
- 💡For total internal reflection questions, remember that the critical angle formula sin C = n₂/n₁ assumes n₁ > n₂. If you are asked for the critical angle for a glass-air boundary, n₁ = refractive index of glass, n₂ = 1 (air).
Common Mistakes
- Confusing the conditions for elastic and inelastic collisions regarding kinetic energy
- Incorrectly applying Newton's 3rd law to forces acting on the same body
- Failing to account for the vector nature of momentum in calculations
- Misinterpreting the relationship between force and rate of change of momentum when mass is not constant
- Misconception: Light always bends towards the normal when entering a denser medium. Correction: This is true only if the light is not already travelling along the normal. If the incident angle is 0°, there is no bending. Also, when entering a rarer medium, light bends away from the normal.
- Misconception: The refractive index of a medium is constant for all colours of light. Correction: Refractive index varies slightly with wavelength (dispersion). For example, blue light bends more than red light in glass, which is why a prism separates colours.
- Misconception: Total internal reflection only occurs when light goes from a rarer to a denser medium. Correction: TIR occurs only when light travels from a denser to a rarer medium (e.g., from glass to air) and the angle of incidence exceeds the critical angle.