Waves and space · GCSE Physics
Refraction
Teacher-written GCSE Physics revision on refraction: waves changing speed and direction in a new medium, towards or away from the normal, light in glass, and why wavelength changes but frequency does not.
Refraction is a change of speed when a wave enters a new medium. If it slows, it bends towards the normal. Frequency stays the same; wavelength changes.
The important bits
What you need to know
- 1
Reflection: angle of incidence equals angle of reflection, both measured from the normal, not from the surface.
- 2
Refraction: waves change speed when they enter a different medium. If they meet the boundary at an angle, they also change direction.
- 3
Light slows down as it goes from air into glass or water, and bends towards the normal. It speeds up as it leaves, and bends away from the normal.
- 4
If the wave hits the boundary along the normal (angle of incidence 0°), it changes speed but not direction.
- 5
Frequency is set by the source and does not change at the boundary. Because v = fλ, a drop in speed means a drop in wavelength.
- 6
A ripple tank shows refraction at a step: ripples slow in shallower water, λ falls, and the wavefronts bend. That is the same physics as light in glass.
- 7
On GCSE Physics (Triple), total internal reflection and critical angle appear for light in fibres and prisms. Combined focuses on the basic towards/away rule.
- 8
Draw the normal as a dashed line at 90° to the surface at the point of incidence. Label i and r. Without the normal, “towards” has no meaning.
Quotations worth analysing
Short evidence. Real method.
“When a wave enters a denser medium it slows down and, if it meets the boundary at an angle, bends towards the normal.”
Two facts: speed change, then direction change. A ray along the normal still slows; it just does not bend.
“Angles of incidence and refraction are measured from the normal.”
Measuring from the surface is a classic way to lose both the diagram mark and the calculation mark.
Go deeper
Speed is the cause; bending is the symptom
A wavefront is a line of points doing the same thing at the same time. When one end of the wavefront hits glass first, that end slows while the other end is still racing in air, so the wavefront swings. That is refraction as a picture, not as a memorised slogan. Frequency cannot change: crests still arrive at the boundary at the same rate they left the lamp. Wavelength must fall if speed falls, because v = fλ. In water waves, shallower means slower. In light, optically denser usually means slower. Students write “light bends towards the normal because glass is denser” without saying slower. Add the speed. For a rectangular block, the ray bends towards the normal going in and away going out, so the emergent ray is parallel to the original but shifted. Draw that displacement; do not invent a rainbow unless the question is dispersion.
Go deeper
Total internal reflection is Physics-route light, built on the same rule
When light in glass meets glass-to-air at a large angle of incidence, it cannot refract away from the normal enough to escape, so it reflects inside. The smallest such angle is the critical angle. Optical fibres use this to carry visible or infrared signals with low loss, which is Triple-style content on AQA. Combined students should still measure from the normal and state towards or away. Dispersion in a prism is refraction that depends on wavelength: violet slows more than red in glass, so it bends more. If your course includes colour and prisms, name speed and wavelength, not “violet is stronger”. Check the specification before you spend revision time on critical angle, but never skip the normal and the speed change — those are universal.
See the idea in action
Water waves travel at 0.24 m/s in deep water with λ = 0.060 m, so f = v/λ = 0.24 / 0.060 = 4.0 Hz. They cross into shallow water where v = 0.16 m/s. Frequency stays 4.0 Hz (the dipper has not changed). New wavelength λ = v/f = 0.16 / 4.0 = 0.040 m. The waves slow, pack tighter, and if they meet the step at an angle they bend towards the normal. A light ray in air hitting glass at 40° to the normal might refract at 25° to the normal — smaller r than i because the light slowed. Measure both angles from the dashed normal, not from the block’s face.
Exam technique
Turn knowledge into marks
Draw the normal first. State whether the wave slowed or sped up, then whether it bent towards or away from the normal. Keep frequency constant at the boundary and change λ with v = fλ.
Common mistakes
Do not give these marks away
- 01
Measuring angles from the surface instead of from the normal.
- 02
Saying frequency changes in glass, or that light bends towards the normal when it speeds up.
- 03
Drawing a ray through a block that does not emerge parallel (for a rectangular block) without a reason.
A water wave slows as it enters shallower water at an angle. What happens?
AFrequency increases and it bends away from the normal
BFrequency stays the same, wavelength decreases, and it bends towards the normal
CSpeed increases and wavelength increases
DThe wave must reflect with i = r and cannot refract
Show the answer
Frequency stays the same, wavelength decreases, and it bends towards the normal. The source sets frequency. Slower speed means smaller λ from v = fλ. Slowing at an angle bends the wave towards the normal.
Quick questions
If this is the bit you searched
Why does light bend towards the normal in glass?
It slows down in glass. If it meets the boundary at an angle, the change of speed turns the wavefront towards the normal.
What stays the same when a wave refracts?
Frequency (and the period). Speed and wavelength change. Direction changes unless the wave is along the normal.
What is the normal?
A line drawn at 90° to the boundary at the point where the wave hits. Angles of incidence, reflection and refraction are measured from it.
Is total internal reflection on Combined Science?
On AQA-style routes it is more typical of GCSE Physics (Triple), along with optical fibres. Combined still needs basic refraction towards and away from the normal.