Refraction of light KS3 physics questions ask you to explain why a ray bends as it crosses the boundary between two transparent materials, such as air and water. It happens because light changes speed when it enters a different medium — bending towards the normal when it slows down, and away from the normal when it speeds up again.
What is refraction of light?
Refraction is the change in direction of a light ray as it passes from one transparent medium into another, such as from air into glass or from air into water. The ray bends because light travels at a different speed in each material. This is exactly what refraction of light KS3 physics questions test — can you explain why the bend happens, not just describe that it does?
Refraction only happens when a ray meets the boundary at an angle other than 0° (straight on). A ray that hits the boundary at exactly 90° to the surface passes straight through without bending — it slows down or speeds up, but its direction stays the same.
Why does light bend when it enters water or glass?
Light travels fastest in a vacuum (empty space) and only slightly slower in air. When light enters a denser transparent material — such as glass or water — it slows down. This change in speed is what causes the ray to bend.
- When light travels from a less dense medium (like air) into a more dense medium (like glass or water), it slows down and bends towards the normal.
- When light travels from a more dense medium into a less dense medium, it speeds up and bends away from the normal.
The normal is the imaginary line drawn at 90° to the boundary, at the point where the ray meets it — the same reference line used when studying reflection.
How do you draw a refraction ray diagram?
A refraction ray diagram, in words, for a ray passing from air into a glass block:
- Draw the boundary (the surface of the glass block) as a straight line.
- Draw the normal as a dashed line at 90° to the boundary, at the point where the ray meets the glass.
- Draw the incident ray travelling through the air towards that point, measuring its angle from the normal — this is the angle of incidence.
- Draw the refracted ray continuing into the glass, bent towards the normal, at a smaller angle than the incident ray — this is the angle of refraction.
- If some light also reflects off the surface, add a faint reflected ray at an equal angle on the same side as the incident ray.
Because glass is denser than air, the angle of refraction is always smaller than the angle of incidence when light enters the glass from air.
What is refractive index?
The refractive index of a material is a number that describes how much it slows down light compared with a vacuum, and how strongly it bends a ray of light. A higher refractive index means light slows down more and bends more sharply.
| Medium | Approximate refractive index |
|---|---|
| Air | 1.00 |
| Water | 1.33 |
| Glass | 1.50 |
| Diamond | 2.42 |
Worked example: A ray of light hits a glass block with an angle of incidence of 40° and refracts to an angle of refraction of 25°. Using refractive index = sin(angle of incidence) ÷ sin(angle of refraction):
- sin(40°) ≈ 0.643
- sin(25°) ≈ 0.423
- refractive index = 0.643 ÷ 0.423 ≈ 1.52
This value is close to the accepted refractive index of glass (about 1.50), which confirms the ray diagram is consistent with a glass block. At KS3 you mainly need to understand why the bending happens; the full calculation using sine ratios is developed further at GCSE.
Why does a straw look bent in a glass of water?
When you place a straw in a glass of water, light reflecting off the part of the straw under the water refracts as it leaves the water and enters the air, bending away from the normal. Your eye assumes light has travelled in a straight line, so the underwater part of the straw appears to be in a different position from where it actually is — creating the illusion of a bend at the water's surface.
The same effect makes swimming pools look shallower than they really are, and makes fish appear to be in a slightly different position from where they actually are when viewed from above the water.
Frequently asked questions
What causes refraction of light?
Refraction is caused by a change in the speed of light as it crosses the boundary between two transparent materials of different density. Light slows down when it enters a denser medium and speeds up when it leaves a denser medium for a less dense one, and this change in speed makes the ray change direction.
Does light always bend when it enters a new medium?
No. Light only bends if it meets the boundary at an angle other than 0° to the normal — that is, not straight on. A ray travelling straight through the surface, perpendicular to it, changes speed but continues in the same direction without bending.
What is the difference between reflection and refraction?
Reflection happens when light bounces off a surface and stays in the same medium — for example, air to mirror to air. Refraction happens when light passes through the boundary into a new medium, such as air into water, and its speed and direction both change. Many surfaces, like a glass window, produce both a reflected ray and a refracted ray at the same time.
What is refractive index in simple terms?
Refractive index is a number showing how much a material slows down light compared with a vacuum, and how strongly it bends a ray of light. Materials with a higher refractive index, such as diamond, slow light down more and bend it more sharply than materials with a lower refractive index, such as water.
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