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When a wave reaches a boundary — the edge of one material and the start of another — it does not simply stop. It may bounce back off the boundary (reflection), or it may pass through and change direction (refraction). These two behaviours explain a great deal of everyday physics: why you can see yourself in a mirror, why a straw looks bent where it enters a glass of water, why a swimming pool looks shallower than it really is, and how lenses and prisms work. This lesson, part of Topic P4 (Waves and radioactivity) of OCR Gateway Combined Science A, sets out the law of reflection, distinguishes specular from diffuse reflection, explains refraction as a change of speed at a boundary, introduces wavefronts, and shows how to draw the ray diagrams examiners ask for.
By the end of this lesson you should be able to state and use the law of reflection, distinguish specular and diffuse reflection, explain refraction in terms of a change of wave speed at a boundary, predict which way a ray bends, use wavefronts to describe waves at a boundary, and describe the required practical of investigating reflection and refraction.
This lesson builds AO1 understanding of reflection and refraction, AO2 application when you use the law of reflection and carry out the required practical, and AO3 reasoning when you predict which way a ray bends from the change in wave speed at a boundary.
Reflection is when a wave hits a boundary and bounces back into the material it came from. The most familiar example is light reflecting off a mirror, but all waves reflect — sound echoes off walls, and water waves bounce off the side of a tank.
To describe reflection we draw a normal: an imaginary line at right angles (90°) to the surface at the point where the wave hits. All angles are measured from the normal, never from the surface itself. The ray arriving is the incident ray, and the ray leaving is the reflected ray. We then define:
The law of reflection states:
angle of incidence=angle of reflection(i=r)
The incident ray, the reflected ray and the normal all lie in the same plane. The diagram below shows a ray reflecting off a plane (flat) mirror.
Exam Tip: Always measure angles from the normal, not from the mirror surface. A favourite exam trap gives you the angle between the ray and the mirror; you must subtract it from 90° to get the angle from the normal before applying i=r.
A ray of light strikes a plane mirror. The angle between the incident ray and the mirror surface is 30°. State the angle of incidence and calculate the angle of reflection.
Step 1 — spot the trap. The angle given (30°) is measured from the mirror surface, but angles of incidence and reflection are always measured from the normal. The normal is at 90° to the surface, so the angle of incidence is the complement of the angle given.
Step 2 — convert to the angle from the normal (the angle of incidence):
i=90°−30°=60°
Step 3 — apply the law of reflection, i=r:
r=i=60°
Answer: the angle of incidence is 60° and the angle of reflection is 60°. Notice that if you had wrongly used 30° as the angle of incidence, you would have quoted r=30° and lost the marks — the whole point of the question was to test whether you measure from the normal. A quick check: the two "from-the-surface" angles (30° each side) plus the two "from-the-normal" angles (60° each side) must add up to 180° across the flat surface, and 30+30+60+60=180, which confirms the answer is consistent.
Whether a surface gives a clear reflection or just scatters light depends on how smooth it is.
Crucially, the law of reflection still holds at every single point in diffuse reflection — each tiny part of the rough surface obeys i=r using its own local normal. The rays scatter only because those local normals point in different directions, not because the law is broken.
Exam Tip: A common misconception is that diffuse reflection "breaks" the law of reflection. It does not: the rays scatter because the surface is rough, so the normals point in different directions, but i=r still holds at every point.
A wavefront is a line joining points on a wave that are all in step — for example, a line running along the crest of a wave. Wavefronts are drawn at right angles to the direction the wave travels, and neighbouring wavefronts are one wavelength apart. Drawing waves as wavefronts, rather than as single rays, makes it easier to see what happens at a boundary.
When a wave is reflected, the wavefronts bounce off the boundary and the spacing (wavelength) stays the same. When a wave is refracted into a slower material, the wavefronts become closer together, because the wavelength gets shorter — and it is this bunching that swings the wave round into a new direction, as the next section explains. The frequency, meanwhile, stays exactly the same.
Exam Tip: Remember that neighbouring wavefronts are one wavelength apart. When a wave slows down entering a new material, the wavefronts bunch closer together (shorter wavelength), but the frequency does not change.
Refraction is the change in direction of a wave when it crosses a boundary between two materials and changes speed. The change of speed is the cause; the change of direction is the result.
Why does the speed change? Different materials let waves travel at different speeds. Light, for instance, travels fastest in a vacuum (and air), more slowly in water, and slower still in glass. A material in which light travels more slowly is described as optically denser.
When a wave crosses a boundary at an angle to the normal, one side of the wave reaches the new material — and changes speed — before the other side. This makes the wave swing round, like a marching band wheeling when the people on one side take shorter steps. The rule for which way it bends is:
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