Waves and space · GCSE Physics
Waves required practical
Plan the GCSE Physics waves investigation: reflection and refraction with a ray box or ripple tank, angle i equals angle r, Snell’s law at GCSE level, measuring angles from the normal and fair testing.
Measure angles from the normal. Reflection: i = r. Refraction: n = sin i / sin r when light slows down. Ripple tank: frequency sets λ; v = fλ. Same medium, same speed.
The important bits
What you need to know
- 1
Aim: investigate reflection and refraction of light (ray box on paper or glass block) or investigate wave speed, frequency and wavelength in a ripple tank.
- 2
Ray box method: shine a narrow ray onto a plane mirror or into a rectangular glass block; mark incident and refracted rays with crosses; join to the surface and draw the normal at 90°.
- 3
Law of reflection: angle of incidence i equals angle of reflection r, both measured from the normal to the surface, not from the surface itself.
- 4
Refraction into glass: light slows down and bends towards the normal (i > r when entering denser medium). Snell’s law at GCSE: n = sin i / sin r, where n is the refractive index.
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Ripple tank: bar dipper frequency f sets wavelength λ; measure λ with a metre rule or strobe; count ripples over a fixed distance. Wave speed v = fλ. Depth of water affects v — keep depth constant.
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Independent variable examples: angle of incidence for refraction; frequency of the dipper in the ripple tank. Dependent variable: angle of refraction; or wavelength / measured wave speed.
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Control variables: same ray box slit, same glass block, same ripple tank depth, same temperature, and the same method of marking rays or counting ripples.
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Safety: ray box lamp gets hot — do not touch the bulb. Keep liquids away from electrical leads in the ripple tank. Protractor precision limits accuracy — repeat and mean angles.
Quotations worth analysing
Short evidence. Real method.
“angle of incidence = angle of reflection (both measured from the normal)”
Draw the normal first. Students lose marks by measuring from the surface — those angles do not equal i and r.
“n = sin i / sin r”
Use degrees on the calculator in degree mode. i is in air, r in glass for a block. Larger n means more bending.
“wave speed v = frequency f × wavelength λ”
If f doubles and v is fixed by the water, λ halves. Measure λ in metres for unit consistency.
Go deeper
Ray box practical: from crosses to Snell’s law
Place a glass block on paper, outline it, and shine a ray at an angle into the block. Mark two points on the incoming ray and two on the ray leaving the block; join them to the surface to draw incident and refracted rays. At the point of entry, draw the normal with a set square. Measure i and r with a protractor from the normal. Repeat for i = 20°, 30°, 40°, 50°. Plot sin r (y) against sin i (x) — gradient = 1/n for air into glass, or calculate n = sin i / sin r for each pair and mean. Reflection on a mirror uses the same normal rule: mark the reflected ray, measure i and r, show they match within protractor uncertainty. Switch off other lights so the ray is visible. A sharp pencil line beats a wide smear.
Go deeper
Ripple tank: prove v = fλ without assuming it
Set the dipper to a known frequency (or count vibrations in ten seconds and divide). Measure the distance across ten wavelengths with a metre rule above the tank, divide by ten for λ in metres. Calculate v = fλ. Change the frequency and repeat at the same water depth — v should stay about constant if depth is unchanged, while λ changes. Plot λ against 1/f and expect a straight line through the origin if v is constant (since λ = v/f). Strobes freeze the ripples for measurement; without a strobe, use slow motion video if allowed. Depth must stay constant because wave speed in water depends on depth. This is the fair-test story examiners want: one variable at a time.
Go deeper
What goes wrong, and how do you evaluate?
Wide ray boxes give fuzzy boundaries — use a single slit. Eyes not level with the paper make parallax error on marks. For refraction, drawing the normal at the wrong place makes both angles wrong together. Protractor resolution is about ±1°; small angles have larger percentage error — use a range of i values. In ripple tanks, reflections from the tank walls create interference patterns; a shallow wedge of water or a barrier can help. Evaluation: repeat angles and take means; use a narrower ray; ensure the block is not nudged between marks. Validity: reflection tests the law i = r in a plane mirror; refraction tests the change of speed at a boundary. Snell’s law is the quantitative refraction link on Higher / Triple routes; Combined may stop at “bends towards the normal when slower”.
See the idea in action
A ray enters a glass block with i = 40° and r = 25°. n = sin 40° / sin 25° = 0.643 / 0.423 ≈ 1.52 (typical glass). For reflection on a mirror at i = 30°, r = 30° ± 1° from the protractor. Ripple tank: f = 10 Hz, ten wavelengths span 0.20 m → λ = 0.020 m. v = fλ = 10 × 0.020 = 0.20 m/s. When f is raised to 20 Hz at the same depth, λ ≈ 0.010 m and v stays ≈ 0.20 m/s. IV: frequency. DV: wavelength (then calculated v). Controls: water depth, same tank.
Exam technique
Turn knowledge into marks
Always draw the normal and measure i and r from it. State i = r for reflection. For refraction, say whether the ray speeds up or slows down and which way it bends. In ripple tanks, show v = fλ with units and keep depth constant.
Common mistakes
Do not give these marks away
- 01
Measuring angles from the surface instead of the normal, so i = r appears false even when the law holds.
- 02
Saying refraction bends the ray away from the normal when light enters glass — it bends towards the normal because light slows down.
- 03
Changing ripple tank depth while changing frequency, or leaving wavelength in centimetres inside v = fλ.
A light ray reflects from a plane mirror. The angle of incidence is 35°. What is the angle of reflection?
A35°
B55°
C70°
D90°
Show the answer
35°. The law of reflection states angle of incidence equals angle of reflection, both measured from the normal. 55° is the angle to the surface, not the reflection angle from the normal.
Quick questions
If this is the bit you searched
What is the law of reflection?
The angle of incidence equals the angle of reflection, when both angles are measured from the normal to the surface.
What is Snell’s law at GCSE?
n = sin i / sin r, where n is refractive index, i is the angle of incidence and r is the angle of refraction, both from the normal.
Why does light refract towards the normal in glass?
Light slows down in a denser medium such as glass. The change in speed at the boundary bends the ray towards the normal on entry.
How do you investigate wave speed in a ripple tank?
Measure frequency f and wavelength λ at constant water depth, then calculate v = fλ. Repeat at different f and show v stays constant if depth is unchanged.