3.2.2Waves

Investigating Reflection — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 10 parts Lesson 10 of 35 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision

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Lesson summary

3.2.2 Investigating Reflection — IGCSE PHYSICS notes: Describe and evaluate an experiment that tests the law of reflection with a plane mirror.Aim — investigate reflection by a plane mirrorIndependent variable — angle of incidence iDependent variable — angle of reflection rContro…

Control variables
Expected result
Small differences
Systematic
Marking points
Protractor

Part 1 of 8Aim and variables

Aim

Aim investigate reflection by a plane mirror

Independent variable

Independent variable angle of incidence i

Dependent variable

Dependent variable angle of reflection r

Control variables

Control variables distance of the ray box from the mirror, width of the beam and wavelength of the light

Part 2 of 8Equipment

Equipment	Purpose
Ray box	Provide a narrow beam of light
Protractor	Measure the angles of incidence and reflection
Sheet of paper	Mark the lines of the rays
Pencil	Draw the ray lines
Ruler	Draw straight ray lines
Plane mirror	Reflect the beam

Part 3 of 8Method

Mark a straight line about 10 cm long in the middle of the paper.

Use a protractor to draw a 90° line that bisects it, which is the normal.

Place the mirror along the first line.

Aim a beam from the ray box at the point where the two lines cross.

Mark a point on the beam just after the ray box and a point on the reflected beam about 10 cm from the mirror.

Remove the ray box and mirror.

Join the marked points to the crossing point.

Measure the angle of incidence and the angle of reflection from the 90° line.

Repeat at different angles, for example 15°, 30°, 45°, 60° and 75°.

Part 4 of 8Analysis

Table

Table columns for angle of incidence i and angle of reflection r

Expected result

Expected result the two angles are equal within the protractor reading error of about ±1°

Graph

Graph r against i is a straight line through the origin with gradient 1

Small differences

Small differences are random errors, not a failure of the law

Part 5 of 8Pin method for the image

Place a pin in front of a plane mirror, for example a thin glass sheet held upright.

Look from one direction and line up two pins with the image of the first pin so there is no parallax.

Draw the line through the two pins.

Repeat from a second direction.

Extend the lines behind the mirror, where they cross is the image position.

Check
- the object and image distances from the mirror are equal

Part 6 of 8Errors and improvements

Errors and improvements

Systematic

90° lines drawn incorrectly, so use a set square

Mirror

Mirror a distorted or marked mirror changes the angle, so check it has no blemishes

Marking points

Marking points use a sharpened pencil and mark the middle of the beam, with dots far apart

Protractor

Protractor use a higher-resolution protractor and read it with the eye directly above the scale

Beam

Beam use a narrow beam because a wide beam is hard to judge

Repeats

Repeats repeat at several angles

Part 7 of 8Safety

Ray box

Ray box the light can burn if touched, so run burns under cold water for at least 5 minutes

Eyes

Eyes avoid looking directly into the light and stand behind the ray box

Liquids

Liquids keep them away from electrical equipment and paper

Mirror

Mirror take care using it, because damage affects the results

Part 8 of 8More from the specification

1Total internal reflection in optical fibres

Total internal reflection in optical fibres light rays strike the core cladding boundary above the critical angle, guiding signals along optical fibres.

2Refraction and Snell's Law

When light waves cross an optical boundary between two media of differing optical density, the wave speed changes,
causing refraction governed by Snell's law: n₁ sin θ₁ = n₂ sin θ₂.

3Refractive Index of a Medium

The absolute refractive index n is defined as the ratio of light speed in vacuum to the phase speed of light in the material (n = c / v), determining how much light bends towards the normal.

4Converging and Diverging Lenses

A converging (convex) lens brings parallel incident rays to a real focal point,
forming real or virtual images depending on object distance,
whereas a diverging (concave) lens always produces an upright, diminished virtual image.

5Real vs Virtual Images

A real image is formed where light rays physically converge and can be captured on a screen,
whereas a virtual image is located where rays only appear to diverge from behind the optical element.

6Optically dense

Having a high refractive index

7Refraction

Change in direction of light when it changes speed between media.

8Boundary

Surface between two media at which waves are partially reflected

9Refractive index

Ratio of the speed of light in a vacuum to its speed in a material

10Critical angle

Incline angle at which sliding just begins.

11Virtual image

Image formed where rays only appear to meet, not projectable on a screen

12Optical fibre

Thin transparent strand that guides light by TIR

13Principal focus

Point where parallel rays meet or appear to come from

14Focal length

Distance from the lens centre to the principal focus

Key terms

Term	Meaning
Independent variable	The variable deliberately changed, here angle of incidence
Dependent variable	The variable measured, here angle of reflection
Normal	Line at 90° to the mirror at the point of incidence
Parallax	Apparent shift between objects caused by viewing from different positions
Ray box	Light source that gives a narrow beam

Avoid these mistakes

Measuring angles from the mirror — measure from the normal.

Drawing the normal by eye — use a protractor or set square to draw it at 90°.

Moving the mirror while marking points — draw round its back to keep it fixed.

Treating small differences between i and r as disproving the law — they are random errors.

Last updated: 15 October 2026