The glass block with optics pins is the most frequent optics question in 0625 Papers 5 and 6, and the examiner reports say the same things every year: pins too close together, the ray lined up with the 90° line of the protractor instead of the zero, and vague precautions. Here you do the whole ray trace by hand: drag the block onto the sheet and draw round it, draw the normal and an incident ray against the protractor, pin the ray, look through the block with the eye until the images of the pins are one behind the other, pin the emergent ray, draw the rays and read i and r through a magnifier. Three angles, a table of sin i / sin r, and the within-10% judgement. Every attempt uses a block with a different refractive index. Allow about 35 minutes.
Checked: Cambridge IGCSE Physics 0625 Papers 5 and 6, 2014 to 2025. Only the papers in our collection are listed, so the real number may be higher. Question papers are available from Cambridge.
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Pins P1P2 and P3P4 more than 5 cm apart in every trace, incident rays within 1° of 20°, 35° and 50° (so the protractor must be centred on N with its edge along the normal), and every refracted ray within 2° of the true path.
Headings with units for the angles and none for the sines and the ratio, i and r to the nearest degree as measured on your own drawing, sin i and sin r to 2 or 3 significant figures, and sin i / sin r to a consistent number of significant figures.
The spread of your ratios as a percentage of the smallest, then a yes or no that matches your own numbers, justified by the limits of experimental accuracy (within 10%).
The mean ratio is the refractive index: one mark for a mean within 0.06 of the true value of the block.
A technique the examiners credit (view the bases of the pins, thin lines), why far-apart pins give an accurate line, and the inaccuracy that remains even in a careful experiment.
The marking follows the conventions in published Cambridge mark schemes. The experiments, readings and wording on this page are our own.