Interactive practical

Reflection and refraction

Turn a ray box to check the law of reflection, compare a mirror with white and black card, then measure refraction into a glass block and into water with a protractor.

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GCSE Physics

Investigating reflection and refraction

CCEA 1370 — matched to your course.

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At a glance

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Practical focuses1

Questions available2

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Available question marks4

Practical focus

Measure how light changes direction at reflecting surfaces and transparent boundaries.

Practical method

Aim

Investigate how light is reflected by a plane mirror and refracted by a glass block, by measuring angles of incidence, reflection and refraction.

Variables

Independent variable: angle of incidence
Dependent variable: angle of reflection or angle of refraction
Control variables: the same glass block (or mirror); a narrow ray of white light from the same ray box; angles measured from the normal.

Apparatus

Ray box with a single slit
Power supply
Rectangular glass block
Plane mirror
Protractor
Ruler
Sharp pencil and plain paper

Method

Draw round the glass block on paper. Draw a normal (at 90° to the surface) where the ray will enter.
Shine a narrow ray at the block along a line making 20° with the normal.
Mark the ray where it leaves the block with two crosses, remove the block and join the points to show the ray inside the glass.
Measure the angle of refraction between the refracted ray and the normal.
Repeat for angles of incidence of 30°, 40°, 50° and 60°.
Repeat with a plane mirror, measuring the angle of reflection each time.

Sample results

Rectangular glass block; all angles measured from the normal. The angle of reflection from the mirror equalled the angle of incidence each time.

Angle of incidence / °	Angle of refraction / °	Angle of reflection / °
20	13.2	20
30	19.5	30
40	25.4	40
50	30.7	50
60	35.3	60

Analysis

The angle of reflection always equals the angle of incidence. On entering glass the ray bends towards the normal, so the angle of refraction is smaller than the angle of incidence. The angle of refraction increases with the angle of incidence but not in proportion. Light slows down in glass because glass is optically denser than air.

Errors and improvements

Problem	Effect	Improvement
Wide ray	Hard to judge the centre of the ray	Use a narrow slit and mark the centre of the ray
Normal not drawn at 90°	Every angle wrong	Use a protractor or set square to draw the normal
Thick pencil lines	Angles hard to measure precisely	Use a sharp pencil and long ray lines

Risk assessment

Hazard	Risk	Control measure
Ray box gets hot	Burns	Do not touch the bulb housing; let it cool
Glass block dropped	Cuts from broken glass	Handle carefully; report breakages
Working in a dim room	Tripping	Keep bags clear of walkways

Common mistakes

Measuring angles from the surface instead of the normal.
Saying light bends away from the normal when it enters glass.
Not removing the block before drawing the ray inside it.

Worked analysis

A ray hits a mirror at 35° to the normal. Angle of reflection = 35°. If the ray makes 55° with the mirror surface, the angle of incidence is 90° − 55° = 35°.

Common misconception

Angles of incidence, reflection and refraction are measured from the normal, not from the surface.

Variations required by some exam boards

Using a prism (OCR): trace a triangular glass prism and shine a ray into one face. The ray bends towards the normal as it enters and away from the normal as it leaves, so it is deviated towards the base of the prism. A ray of white light is dispersed into a spectrum because each colour refracts by a different amount.

Using a plane mirror: the image is the same distance behind the mirror as the object is in front, upright, the same size and virtual.

Assessment coverage

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