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IB DP Physics Study Notes

4.3.2 Refraction

Refraction, a fundamental wave characteristic, plays a pivotal role in numerous optical phenomena and applications. This section aims to explore Snell's law in depth and demystify how refraction operates across different mediums.

Snell's Law: Unravelling the Science

Snell's law provides the mathematical foundation for understanding refraction, establishing a relationship between the angles of incidence and refraction when a wave transitions from one medium to another. This can be expressed as:

n1 * sin(θ1) = n2 * sin(θ2)


  • n1 and n2 denote the refractive indices of the first and second mediums, respectively.
  • θ1 represents the angle of incidence.
  • θ2 corresponds to the angle of refraction.

The Refractive Index

The refractive index (often termed 'index of refraction') quantifies how much a wave, such as light, slows down upon entering a medium from a vacuum. Defined as the ratio of the speed of light in a vacuum to its speed in a given medium, it provides insights into the medium’s optical properties. A higher refractive index typically results in a more pronounced bending of light, affecting its path considerably.

Delving Deeper into Refraction Across Mediums

Refraction isn't uniform across all media. The degree to which waves bend depends on the refractive indices of both the initial and the subsequent medium.

Air to Water Transition

Light, upon moving from air (less dense) to water (denser), decelerates, resulting in a bend towards the normal line. An everyday illustration is observing a pencil partially submerged in a water glass. The pencil seems disjointed or bent due to the light rays refracting as they transition from the water to the air.

From Glass to the Open Air

In contrast, when light progresses from a denser medium, such as glass, to a rarer one like air, it accelerates, deviating away from the normal. This bending of light, fundamental to optical devices, aids in focusing light rays to produce sharp images in cameras or correct vision in eyeglasses.

Diamond's Dazzling Display

Diamonds, renowned for their brilliance, owe their sparkle to refraction. Possessing a high refractive index, diamonds bend and manipulate light extensively. Internal facets of diamonds create a playground for light, where it undergoes multiple reflections and refractions. This phenomenon results in the dispersion of light into various colours, producing the gem's characteristic shimmer.

Factors Steering Refraction

Refraction isn't a random event; specific factors influence its magnitude and direction:

  • Difference in Medium: A pronounced refraction is usually observed when there's a stark contrast in the refractive indices of the two media. Transitioning from air to a diamond, for instance, induces a more noticeable bending compared to moving from air to water.
  • Angle of Approach: The angle at which light hits the boundary (angle of incidence) directly impacts the resultant angle of refraction. As this angle increases, veering away from the normal, the refractive angle elevates. There exists a specific angle, termed the 'critical angle', beyond which light ceases to refract into the secondary medium and instead undergoes total internal reflection.

Refraction in Everyday Life & Technology

The Magic of Eyeglasses

Eyeglasses are a quintessential application of refraction. Depending on the visual ailment, eyeglasses employ lenses to adjust the refraction of incoming light. For myopic individuals, concave lenses disperse light rays. In contrast, convex lenses, aiding hypermetropic persons, converge the rays, ensuring the retina receives a clear image.

Mirages: Nature's Illusion

Hot roads or deserts, with their layers of air at varying temperatures, offer a stage for differential refraction. This phenomenon can render distant objects appearing as if they're shimmering or floating above the ground, often mistaken for water puddles.

Optical Fibres & Modern Communication

Optical fibres harness total internal reflection to transmit data as light pulses over extensive distances. Ensuring minimal data loss and high-speed transmission, they've revolutionised telecommunications.

Aquatic Life Perception

Ever wondered why fish in a pond appear closer to the surface than they truly are? This optical illusion is a result of light rays refracting as they move from water to air, making the fish appear shallower.


Diamonds sparkle with multiple colours because of a combination of refraction, dispersion, and total internal reflection. Diamonds have a high refractive index, meaning they bend light significantly. Additionally, as white light enters a diamond, it disperses into its constituent colours. Each colour (or wavelength) refracts by a slightly different amount. As the light bounces inside the diamond through total internal reflection, this dispersion causes the colours to spread out. When the light finally exits the diamond, the different colours radiate in various directions, giving the diamond its characteristic sparkle.

Refraction is not exclusive to light; it is a phenomenon that affects all types of waves, including sound waves, water waves, and other electromagnetic waves besides visible light. Whenever a wave encounters a change in the medium it's travelling through, and there's a variation in the speed of the wave between the two media, refraction occurs. For instance, sound waves refract when they pass from cold air to warm air due to the speed difference, and water waves refract as they move from deep to shallow water, causing them to change direction.

The critical angle is a specific angle of incidence in a denser medium where the refracted ray emerges exactly along the boundary of the denser and rarer medium. If the angle of incidence surpasses this critical angle, total internal reflection occurs, meaning the light doesn't pass into the rarer medium but instead reflects entirely within the denser medium. This concept has vital applications, especially in optical fibres where light signals are transmitted over long distances without escaping the fibre, thanks to total internal reflection.

When you place a straw in a glass of water, it appears to "break" or "bend" at the surface where the air and water meet. This illusion is due to refraction. As light travels from the water (a denser medium) to the air (a rarer medium), it bends away from the normal line. Our eyes and brain trace the light rays backward as if they travelled in a straight line, but due to the bending (refraction) of light, the submerged part of the straw appears to be at a different location, making the straw seem bent.

Light, as a form of electromagnetic radiation, travels fastest in a vacuum. When it enters a medium, interactions with the particles of the medium slow it down. The denser the medium, the more particles there are for light to interact with, causing greater impediment to its speed. When light moves from a denser medium to a rarer one, there are fewer particles to interact with, leading to fewer disruptions to its speed. As a result, it speeds up. This change in speed is the primary cause for refraction as light travels between different mediums.

Practice Questions

A beam of light travels from air into a glass block with a refractive index of 1.5 at an angle of incidence of 30°. Calculate the angle of refraction using Snell’s Law.

To determine the angle of refraction using Snell's Law, one would use the formula: n1 * sin(θ1) = n2 * sin(θ2). Here, n1 represents the refractive index of air, which is close to 1. The angle of incidence θ1 is given as 30°. For glass, n2 is 1.5. Our objective is to calculate the angle of refraction, denoted as θ2.

Plugging in the values, we have: 1 * sin(30°) = 1.5 * sin(θ2). Given that sin(30°) is 0.5, our equation becomes: 0.5 = 1.5 * sin(θ2).

Solving for θ2, sin(θ2) = 1/3. Using inverse sine, the angle of refraction in the glass block is approximately 19.5°.

How does the angle of refraction change if light travels from a denser medium to a less dense medium, and why? Describe using Snell’s Law.

When light moves from a denser to a less dense medium, the angle of refraction is generally larger than the angle of incidence. This can be understood through Snell's Law: n1 * sin(θ1) = n2 * sin(θ2). If n1 is significantly larger than n2, for the equation to hold true, sin(θ2) (and thereby θ2 itself) must be much greater than sin(θ1). This means that the light ray bends further from the normal, resulting in a greater angle of refraction, θ2. This shift is primarily because of the change in speed the light undergoes as it transitions between mediums. In a denser medium, light moves slower, but it speeds up in a less dense medium, causing it to bend away from the normal.

Dr Shubhi Khandelwal avatar
Written by: Dr Shubhi Khandelwal
Qualified Dentist and Expert Science Educator

Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.

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