A straw in a glass of water looks bent. A coin at the bottom of a pool appears shallower than it is. Both illusions happen because light changes speed when it crosses into a new medium, and when it speeds up or slows down at an angle, it bends. The number that captures how much a medium slows light is its refractive index, and it is built from one elegant equation.
A beam of light travels from air into a block of glass, hitting the surface at an angle.
Before reading: does the light speed up, slow down, or stay the same speed inside the glass, and does it bend towards or away from the normal? Write your prediction and one sentence of reasoning.
Refraction happens because light changes speed at the boundary between two media
Light bends not because it "wants" to, but because it changes speed when it crosses into a new material, and when one edge of the wavefront slows before the other, the whole wave pivots.
When light passes from one medium to another at an angle, the part of the wavefront that enters the new medium first changes speed first. This makes the wavefront pivot, so the ray bends:
Refraction is the bending of light caused by a change in its speed at a boundary. Light slowing down (entering a higher-index medium) bends towards the normal; light speeding up bends away from the normal.
Pause, copy the highlighted statement into your book before moving on.
Did you get this? True or false: light bends towards the normal when it enters a medium in which it travels more slowly.
A single number that measures how much a medium slows light
We just saw that bending depends on a change of speed. That raises a question: how much does a particular medium slow light down? This card answers it → the refractive index $n = c/v$ puts a number on it.
The refractive index is simply how many times slower light travels in a medium than in a vacuum, a glass with $n = 1.5$ slows light to two-thirds of its vacuum speed.
The refractive index of a medium is defined as the ratio of the speed of light in a vacuum to the speed of light in that medium:
$$n = \frac{c}{v}$$
Because light never travels faster than $c$, we always have $v \le c$, so $n \ge 1$. A vacuum has $n = 1$ exactly; air is very close ($n \approx 1.0003$); water is about $1.33$; common glass is about $1.5$.
Light travels through a transparent plastic with a refractive index of $1.49$. Calculate the speed of light in the plastic. (Take $c = 3.00 \times 10^8\ \text{m s}^{-1}$.)
The refractive index is $n = \dfrac{c}{v}$, where $c$ is the speed of light in a vacuum and $v$ is its speed in the medium. Since $v \le c$, $n \ge 1$. Rearranged, the speed in the medium is $v = \dfrac{c}{n}$, a higher $n$ gives a slower $v$.
Add the highlighted definition and rearrangement to your notes before the check below.
Quick check: Water has $n = 1.33$ and diamond has $n = 2.42$. In which does light travel faster?
Pattern A, Calculate then reason
The speed of light in a sample of crown glass is $1.97 \times 10^8\ \text{m s}^{-1}$. In your book: (1) calculate the refractive index $n$; (2) state whether light is faster in this glass or in water ($n = 1.33$); (3) explain your answer in one sentence using $n = c/v$.
Two truths, one lie: Two truths and a lie about refractive index and the speed of light. Which statement is the lie?
Dot point: Relate the velocity of light to the refractive index of a medium
A simple procedure to show light bending between two media
We just defined the refractive index from the speed of light. That raises a question: how can you actually see refraction happen and link it to that change of speed? This card answers it → a ray-box-and-glass-block investigation.
You can demonstrate refraction by shining a narrow beam of light into a glass block and tracing how the ray bends towards the normal on entry and back away on exit.
A reliable procedure to demonstrate refraction between two media:
To improve reliability, repeat at several angles of incidence and keep the ray narrow so the entry point is well defined. This links the observed bending directly to the change in the speed of light, the larger the slow-down, the larger the bend.
To demonstrate refraction: trace a glass block, shine a single ray in at an angle, mark and join the rays, and draw the normal. The ray bends towards the normal entering the slower medium and away from it on leaving, repeat at several angles for reliability.
Pause, copy the highlighted procedure into your book before the check below.
Last check: Decide if the statement is true or false.
As a ray of light leaves a glass block and re-enters the air, it bends away from the normal because it speeds up.
Fill the gaps: Complete the account of a refraction demonstration, then press Check.
A ray box directs a narrow beam onto a rectangular glass block. On entering the glass the light slows down and bends the normal. On leaving the glass it speeds up and bends from the normal. Because the two surfaces are parallel, the emerging ray is to the original ray.
Dot point: Conduct a scientific investigation to demonstrate refraction between 2 media
A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
UnderstandBand 4(4 marks) 1. Light travels from air into a glass block of refractive index $1.50$. Calculate the speed of light in the glass, and explain why the ray bends towards the normal as it enters the block. (Take $c = 3.00 \times 10^8\ \text{m s}^{-1}$.)
1 mark: uses $v = c/n$ · 1 mark: $v = 2.00 \times 10^8\ \text{m s}^{-1}$ · 1 mark: light slows down entering the glass · 1 mark: slowing causes bending towards the normal
ApplyBand 4(3 marks) 2. The speed of light in a certain type of glass is $2.0 \times 10^8\ \text{m s}^{-1}$. Calculate the refractive index of the glass, and state what the value tells you about how light travels in it compared with a vacuum.
1 mark: correct use of $n = c/v$ · 1 mark: $n = 1.5$ · 1 mark: interprets this as light travelling 1.5 times more slowly than in a vacuum
Dot point: Solve refractive index problems using n = c/v, where n is the refractive index of the medium and v is the speed of light in the medium
AnalyseBand 5(3 marks) 3. Describe a scientific investigation to demonstrate refraction between two media, and explain what observation confirms that refraction has occurred.
1 mark: describes a workable method (ray box and glass or perspex block on paper, tracing the incident and emerging rays) · 1 mark: marks the normal and the ray path at the boundary · 1 mark: the observation is a change of direction at the boundary, bending towards the normal on entering the denser medium
Dot point: Conduct a scientific investigation to demonstrate refraction between 2 media
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Q1 (4 marks): Using $n = c/v$, rearranged to $v = c/n = \dfrac{3.00 \times 10^8}{1.50} = 2.00 \times 10^8\ \text{m s}^{-1}$, so light travels at $2.00 \times 10^8\ \text{m s}^{-1}$ in the glass. Because the glass has a higher refractive index than air, light travels more slowly in it. As the wavefront crosses the boundary at an angle, the edge that enters the glass first slows first, causing the wavefront to pivot, so the ray bends towards the normal as it enters the denser, slower medium.
Q2 (3 marks): The refractive index is defined as $n = c/v$, where $c$ is the speed of light in a vacuum and $v$ is its speed in the medium. Substituting, $n = (3.0 \times 10^8)/(2.0 \times 10^8) = 1.5$. Refractive index is a ratio of two speeds, so it has no units. A value of $1.5$ means that light travels $1.5$ times more slowly in this glass than it does in a vacuum. A larger refractive index therefore always corresponds to a slower speed in that medium, which is the reason the light bends towards the normal on entering it.
Q3 (3 marks): Place a rectangular glass or perspex block on a sheet of paper in a darkened room and draw around it. Direct a narrow beam from a ray box at one face at an angle, and mark the path of the incoming ray with two pencil dots and the path of the emerging ray with two more. Remove the block and join the dots, then draw the ray inside the block by joining the entry and exit points, and construct the normal at right angles to the surface at the point of entry. The observation that confirms refraction is that the ray changes direction at the boundary: on entering the glass, where the light slows down, the ray bends towards the normal, and on leaving it bends away again. Measuring the angle of incidence and the angle of refraction from the normal shows they are unequal, which is what distinguishes refraction from a ray simply passing straight through. Repeating at several angles of incidence shows the effect is systematic rather than a single mismeasurement.
You can now explain refraction as a change of speed, calculate with $n = c/v$, and describe how to demonstrate refraction. Next lesson turns the bending into a precise equation, Snell's law, $n_1\sin\theta_1 = n_2\sin\theta_2$.
↩ Back to focus-area hubAt the start you predicted whether light speeds up or slows down in glass, and which way it bends. Light slows down in glass (higher $n$, so smaller $v$) and bends towards the normal. If you thought a higher refractive index meant faster light, revisit Card 2, $n = c/v$ means higher $n$ gives lower $v$.
A short spaced-retrieval set, mixing this lesson with earlier ones. It gets harder to guess and easier to remember.
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