Core Practical: Investigating Refraction and Snell's Law — Reflection & Refraction | IGCSE Physics

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 9% of your exam marks.

stable

Medium

Stable9%

Ray diagrams, Snell's Law and critical angle calculations appear regularly.

Measure pairs of angles of incidence and refraction as light passes from air into a rectangular block, plot a graph of sin i against sin r, and use the gradient to find the refractive index of the block

Independent variable: angle of incidence, i (degrees), set by rotating the incoming ray
Dependent variable: angle of refraction, r (degrees), read off the paper after tracing the ray
Control variables: the same block (same material, same shape, same orientation); the same narrow beam from the ray box; the same room lighting

Equipment	Purpose	Resolution
Ray box with single-slit attachment	Provides a thin, well-defined beam of light	—
Rectangular perspex (or glass) block	The material whose refractive index is being measured	—
Plain white A3 paper	Provides a flat working surface on which to trace rays	—
Sharp pencil and 30 cm ruler	Mark and join the points along each ray	1 mm
Protractor (preferably 360°)	Measures i and r	1°
Set square	Helps draw the normal exactly at 90° to the block's edge	—

Lay the paper flat and draw round the outline of the rectangular block with a sharp pencil
At a chosen point on the long edge of the outline, use the set square to draw a dashed normal line perpendicular to that edge, extending well into the air outside the outline
Replace the block exactly within its outline, turn on the ray box, and angle the beam so that it meets the chosen entry point at a sensible angle (begin near i ≈ 20°)
With the block still in place, mark four pencil dots:
- one along the incident beam, about 5 cm before it reaches the block
- one where the beam enters the block
- one where the beam exits the far side of the block
- one along the emergent beam, about 5 cm beyond the exit point
Turn the ray box off, remove the block, and use a ruler to join the four dots with two straight lines: the incident ray and the emergent ray (and where they intersect the outline)
With the protractor, measure the angle of incidence i and the angle of refraction r (the angle the ray makes with the normal inside the block)
Repeat steps 3–6 for at least six more angles of incidence (sensible values are 25°, 35°, 45°, 55°, 65° and 75°), using a fresh sheet of paper or a clean part of the same sheet each time

For every pair, calculate sin i and sin r on the calculator
Plot sin i (y-axis) against sin r (x-axis); draw the best straight line through the points and through the origin
The gradient of the line equals the refractive index of the block:

gradient = sin i / sin r = n

Systematic errors:
- the normal not drawn exactly at 90° to the surface (use the set square, not the protractor)
- the ray box positioned slightly above or below the paper, so its beam is at an angle to the surface; keep the ray box level with the paper
Random errors:
- the beam fades and broadens with distance, so mark each ray near to its endpoints so the dot is in the middle of the visible line
- the protractor only reads to the nearest degree, so repeat each angle three times and average
Safety:
- the ray box lamp gets hot, so do not touch the front bulb or aperture
- do not stare into the beam, even via a reflection from the block