Refraction

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Lesson notes

Speed of Light and Refractive Index

Light travels at **300 000 000 m/s** in a vacuum – nothing faster.
In a medium (e.g., glass) light slows down; in glass ≈ 200 000 000 m/s.
**Refractive index** measures how much a material slows light: higher index → slower speed.
When light enters a medium with higher refractive index, its **wavelength decreases** but **frequency stays the same**.

Refraction at a Boundary

**Refraction** is the change of direction of light as it passes from one medium to another.
The **normal** is an imaginary line perpendicular to the surface at the point of incidence.
When light goes from a lower to a higher refractive index (e.g., air → glass), it bends **towards the normal**.
The **angle of refraction (r)** is smaller than the **angle of incidence (i)**.
If the angle of incidence is **0°** (along the normal), light continues straight but slows down.

Emergent Ray

When light exits a denser medium (e.g., glass → air), it speeds up and bends **away from the normal**.
The ray that leaves the block is called the **emergent ray**.
The emergent ray is parallel to the incident ray (same direction) but displaced sideways.

The 'Bent Pencil' Illusion

A pencil in water appears bent because light from the underwater part refracts at the water surface.
Light travels slower in water than in air, so it bends away from the normal when leaving water.
Our brain assumes light travels in straight lines, so it traces the rays back to a virtual image that appears higher, making the pencil look bent.

Refraction of light from air into glass: the ray bends towards the normal.

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Practice questions

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1.What is the speed of light in a vacuum?
Easy
- A300 000 000 m/s
- B200 000 000 m/s
- C150 000 000 m/s
- D100 000 000 m/s
2.True or false: Light travels at the same speed in a vacuum as it does in glass.
Easy
- AFalse
- BTrue
- CCannot say
- DOnly if the glass is clear
3.When light passes from air into glass, what happens to its wavelength?
Easy
- AIt decreases
- BIt increases
- CIt stays the same
- DIt becomes zero
4.When light passes from air into glass, what happens to its frequency?
Easy
- AIt remains unchanged
- BIt decreases
- CIt increases
- DIt becomes zero
5.When light travels from air into a glass block at an angle (not 0°), which way does it bend?
Medium
- ATowards the normal
- BAway from the normal
- CIn the same direction as the incident ray
- DIt does not bend
6.In the diagram of refraction from air into glass, which angle is the angle of refraction?
Medium
- AThe angle between the refracted ray and the normal
- BThe angle between the incident ray and the normal
- CThe angle between the incident ray and the surface
- DThe angle between the refracted ray and the surface
7.What is the name given to the ray that leaves a glass block?
Medium
- AEmergent ray
- BIncident ray
- CRefracted ray
- DNormal ray
8.If the angle of incidence of a ray of light entering a glass block is 0°, what happens to the ray?
Hard
- AIt continues in the same direction (but slower)
- BIt bends towards the normal
- CIt bends away from the normal
- DIt reflects back into air

More questions

9.The 'bent pencil' illusion when a pencil is partially submerged in water is caused by:
Easy
- ARefraction of light
- BReflection of light
- CAbsorption of light
- DDispersion of light
10.Which of the following increases when light enters a medium with a higher refractive index?
Medium
- AThe amount of bending towards the normal
- BThe speed of light
- CThe frequency of light
- DThe wavelength of light
11.When light travels from glass into air, what happens to the angle of refraction compared to the angle of incidence?
Hard
- AThe angle of refraction is greater than the angle of incidence
- BThe angle of refraction is less than the angle of incidence
- CThe angle of refraction equals the angle of incidence
- DThere is no refraction
12.A ray of light passes from air into a glass block. The angle of incidence is 40°. What is the angle of refraction?
Medium
- ALess than 40°
- BGreater than 40°
- CEqual to 40°
- D0°
13.A ray of light passes from air into a glass block. The angle of incidence is 45°. Which of the following is the most likely angle of refraction?
Medium
- A27°
- B45°
- C60°
- D90°
14.A student measures the angle of incidence and angle of refraction for light entering a glass block. The results are: i = 0°, r = 0°; i = 20°, r = 13°; i = 40°, r = 25°; i = 60°, r = 35°. What happens to the angle of refraction as the angle of incidence increases?
Hard
- AIt increases, but not as much as the angle of incidence.
- BIt increases by the same amount as the angle of incidence.
- CIt decreases.
- DIt stays the same.
15.A ray of light passes from glass into air. The angle of incidence in the glass is 30°. What can you say about the angle of refraction in air?
Medium
- AIt is less than 30°.
- BIt is equal to 30°.
- CIt is greater than 30°.
- DIt is 0°.
16.What happens to the speed of light when it enters a medium with a higher refractive index?
Easy
- AIt increases.
- BIt decreases.
- CIt stays the same.
- DIt becomes zero.
17.The speed of light in a vacuum is 300,000,000 m/s. In a certain type of glass, the speed is 200,000,000 m/s. What is the refractive index of this glass? (Refractive index = speed in vacuum / speed in medium)
Hard
- A0.67
- B1.5
- C2.0
- D3.0
18.Which of the following properties of light does NOT change when it refracts from air into glass?
Medium
- ASpeed
- BWavelength
- CFrequency
- DDirection
19.A ray of light enters a glass block at an angle of incidence of 0°. What happens to the ray?
Medium
- AIt reflects back.
- BIt refracts towards the normal.
- CIt continues straight through without bending.
- DIt refracts away from the normal.
20.The diagram shows a ray of light entering a glass block. Which labelled angle is the angle of refraction?
Medium
- AAngle A (between incident ray and normal)
- BAngle B (between refracted ray and normal)
- CAngle C (between emergent ray and normal)
- DAngle D (between incident ray and surface)
21.In the diagram, what is the angle between the incident ray and the normal?
Hard
- A40°
- B25°
- C65°
- D90°
22.What is the name of the ray that leaves a glass block after refraction?
Easy
- AIncident ray
- BRefracted ray
- CEmergent ray
- DNormal ray
23.A student shines a laser into a rectangular glass block at an angle. The laser exits the block parallel to the incident ray but offset. Which of the following best explains this?
Hard
- AThe light refracts towards the normal on entry and away from the normal on exit, so the emergent ray is parallel to the incident ray.
- BThe light reflects inside the block.
- CThe light travels in a straight line through the block.
- DThe light is absorbed and re-emitted.
24.A student places a rectangular glass block on a piece of paper and shines a ray of light into it at an angle. Which of the following is the correct method to measure the angle of refraction?
Medium
- AMeasure the angle between the refracted ray and the normal inside the glass block.
- BMeasure the angle between the incident ray and the normal inside the glass block.
- CMeasure the angle between the refracted ray and the surface of the glass block.
- DMeasure the angle between the emergent ray and the normal outside the block.
25.A student investigates refraction using a glass block. She measures the angle of incidence and the angle of refraction for several angles. Her results are: i = 10°, r = 6°; i = 20°, r = 13°; i = 30°, r = 19°; i = 40°, r = 25°. Which of the following is the most likely value for the angle of refraction when the angle of incidence is 50°?
Hard
- A31°
- B35°
- C40°
- D50°
26.A student uses a ray box to shine light into a glass block. She notices that the emergent ray is parallel to the incident ray but shifted sideways. Which of the following explains this?
Medium
- AThe light refracts towards the normal when entering and away from the normal when leaving, causing a lateral shift.
- BThe light reflects inside the block causing the shift.
- CThe light speeds up in the glass and then slows down when leaving.
- DThe light changes frequency inside the block.
27.In a refraction experiment, a student places a pencil in a glass of water. The pencil appears bent at the water surface. Which of the following correctly explains this illusion?
Easy
- ALight from the pencil refracts as it passes from water to air, making the pencil appear bent.
- BLight reflects off the water surface, creating a mirror image.
- CThe water magnifies the pencil.
- DThe pencil absorbs light differently in water.
28.A student sets up a refraction experiment with a glass block. She measures the angle of incidence as 30° and the angle of refraction as 19°. She then increases the angle of incidence to 45°. What will happen to the angle of refraction?
Hard
- AIt will increase to about 28°.
- BIt will decrease to about 10°.
- CIt will remain at 19°.
- DIt will increase to 45°.
29.In a refraction experiment, a student uses a ray box to shine light into a semicircular glass block along the normal (0° incidence). What does the student observe?
Medium
- AThe light passes straight through without bending.
- BThe light bends towards the normal.
- CThe light bends away from the normal.
- DThe light is totally internally reflected.
30.Which of the following properties of light changes when it refracts from air into glass?
Easy
- AWavelength
- BFrequency
- CColour
- DEnergy
31.A student performs an experiment to find the refractive index of a transparent liquid. She measures the angle of incidence in air and the angle of refraction in the liquid. Which of the following calculations gives the refractive index of the liquid?
Hard
- Asin(angle of incidence) / sin(angle of refraction)
- Bsin(angle of refraction) / sin(angle of incidence)
- Cangle of incidence / angle of refraction
- Dangle of refraction / angle of incidence
32.The diagram shows a ray of light entering a glass block. Which labelled angle is the angle of incidence?
Medium
- AAngle between incident ray and normal
- BAngle between refracted ray and normal
- CAngle between incident ray and surface
- DAngle between refracted ray and surface
33.A student shines a laser into a rectangular glass block at an angle. The laser exits the block parallel to the incident ray but offset. Which of the following best explains why the emergent ray is parallel to the incident ray?
Hard
- AThe light undergoes two refractions that cancel the directional change.
- BThe light reflects inside the block and emerges at the same angle.
- CThe glass block is rectangular and the two surfaces are parallel.
- DThe light slows down and then speeds up, returning to its original path.
34.A student measures the speed of light in three different materials. The results are: Material X: 250,000,000 m/s, Material Y: 200,000,000 m/s, Material Z: 300,000,000 m/s. Which material has the highest refractive index?
Medium
- AMaterial X
- BMaterial Y
- CMaterial Z
- DAll have the same refractive index
35.Light travels from air into a glass block. The speed of light in air is 300,000,000 m/s and in glass is 200,000,000 m/s. The wavelength of the light in air is 600 nm. What is the wavelength of the light in glass?
Medium
- A400 nm
- B600 nm
- C900 nm
- D200 nm
36.A ray of light passes from water into air. The angle of incidence in water is 30°. The refractive index of water is 1.33 and of air is 1.00. Using Snell's law (n₁ sin θ₁ = n₂ sin θ₂), what is the angle of refraction in air? (sin 30° = 0.5)
Hard
- A22°
- B41.8°
- C30°
- D19.5°
37.In an experiment, a student measures the angle of incidence and angle of refraction for light entering a glass block from air. The results are shown in the table. Which pair of values is most likely to be correct?
Medium
- Ai = 20°, r = 30°
- Bi = 20°, r = 13°
- Ci = 20°, r = 20°
- Di = 20°, r = 10°
38.A student uses a ray box to shine light into a glass block at an angle. The emergent ray is parallel to the incident ray but shifted sideways. Which of the following is the best explanation for this?
Medium
- AThe light bends towards the normal on entering and away on leaving, causing a lateral shift.
- BThe light reflects inside the block and emerges at a different angle.
- CThe light speeds up in the glass and bends away from the normal.
- DThe glass block absorbs some light, causing the shift.
39.A ray of light enters a glass block at an angle of incidence of 0° (along the normal). The speed of light in glass is 200,000,000 m/s. What is the speed of the light in the glass block?
Hard
- A300,000,000 m/s
- B200,000,000 m/s
- C0 m/s
- D500,000,000 m/s
40.In a refraction experiment, a student places a rectangular glass block on a piece of paper and shines a ray of light into it. She measures the angle of incidence and angle of refraction for several angles. Her results are: i = 0°, r = 0°; i = 20°, r = 13°; i = 40°, r = 25°; i = 60°, r = 35°. What is the refractive index of the glass? (Use sin values: sin 0°=0, sin 20°=0.342, sin 13°=0.225, sin 40°=0.643, sin 25°=0.423, sin 60°=0.866, sin 35°=0.574)
Hard
- A1.33
- B1.52
- C1.00
- D1.20

Revision notes

## Refraction Summary - **Refraction** is the change in direction of light when it passes from one medium to another, caused by a change in speed. - Light travels at **300,000,000 m/s** in a vacuum, the fastest possible speed. In other materials, it slows down. - **Refractive index** measures how much a material slows light: higher refractive index = slower speed. - When light enters a denser medium (higher refractive index), it **slows down**, **wavelength decreases**, and it bends **towards the normal** (unless angle of incidence = 0°). Frequency stays the same. - **Angle of incidence (i)** is the angle between the incident ray and the normal. **Angle of refraction (r)** is the angle between the refracted ray and the normal. When going from air to glass, r < i. - When light exits a denser medium into a less dense one, it **speeds up** and bends **away from the normal**. The emergent ray is parallel to the incident ray. - If the angle of incidence is 0° (light along the normal), light continues straight but changes speed. - **Example**: The "bent pencil" illusion occurs because light from the pencil travels from water (denser) to air (less dense), refracting away from the normal, making the pencil appear bent at the surface.

Refraction: The change in direction of light when it passes from one medium to another, caused by a change in speed.
Refractive index: A measure of how much a material slows down light; higher refractive index means slower speed.
Normal: An imaginary line drawn at 90° to the surface at the point where light hits.
Angle of incidence: The angle between the incident ray and the normal.
Angle of refraction: The angle between the refracted ray and the normal.
Emergent ray: The ray that exits a medium, traveling in the same direction as the incident ray.

Flashcards

What is the speed of light in a vacuum?

300,000,000 m/s.

Can anything travel faster than the speed of light in a vacuum?

No, nothing can travel faster than this.

What happens to the speed of light when it enters a different medium?

It decreases.

What is the approximate speed of light in glass?

200,000,000 m/s.

Why does light travel slower in glass than in a vacuum?

Because glass has a higher refractive index than a vacuum.

How does refractive index affect the speed of light?

The higher the refractive index, the slower light travels through the material.

What two things happen when light slows down entering a medium?

1. Its wavelength decreases. 2. Unless the angle of incidence is 0°, it changes direction.

What property of light remains unchanged during refraction?

The frequency of light is unchanged.

Define the normal in refraction.

An imaginary line at 90° to the surface (e.g., the glass block).

When light passes from air into glass, does it bend towards or away from the normal?

It bends towards the normal.

How does the angle of refraction compare to the angle of incidence when light enters glass from air?

The angle of refraction is less than the angle of incidence.

What happens to light when it exits a glass block into air?

It speeds up and refracts away from the normal.

What is the emergent ray?

The ray that exits the glass block.

How does the direction of the emergent ray compare to the incident ray?

The emergent ray travels in the same direction as the initial incident ray.

What happens if the angle of incidence is 0° when light enters a medium?

Light continues in the same direction (but slows down).

True or false: Light travels at the same speed in a vacuum as in glass.

False.

True or false: When light goes from air into glass, its wavelength decreases.

True.

True or false: When light goes from air into glass, the angle of refraction is less than the angle of incidence.

True.

True or false: When light goes from air into glass, the frequency of light remains unchanged.

True.

Explain the 'bent pencil' illusion.

Light from the pencil travels from water (higher refractive index) to air (lower refractive index), speeding up and refracting away from the normal, making the pencil appear bent.

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