Backgrounders

Reflection and Refraction

Large diamond refracting light

Large diamond refracting light (DiamondGalaxy, iStockphoto)

Large diamond refracting light

Large diamond refracting light (DiamondGalaxy, iStockphoto)

Physics,  Waves, Sound, Light
Let's Talk Science
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AB 8 Knowledge and Employability Science 8, 9 (revised 2009) Unit C: Light and Optical Systems
AB 12 Physics 30 (2007, Updated 2014) Unit C: Electromagnetic Radiation
AB 8 Science 7-8-9 (2003, updated 2014) Unit C: Light and Optical Systems
BC 8 Science Grade 8 (June 2016) Big Idea: Energy can be transferred as both a particle and a wave.
MB 8 Science Grade 8 (2000) Cluster 2: Optics
MB 11 Senior 3 Physics (2003) Topic 2: The Nature of Light
NB 6 Science 6: Wayfinding: Making sense of your world (2020) Behaviour and Properties of Light
NB 11 Physics 11 (2003) Waves
NL 8 Grade 8 Science Unit 3: Optics (revised 2012)
NS 12 Physics 12 (2015, 2019) Waves and Modern Physics
NU 8 Knowledge and Employability Science 8 (Alberta, Revised 2009) Unit C: Light and Optical Systems
NU 8 Science 8 (Alberta, 2003, updated 2014) Unit C: Light and Optical Systems
NU 12 Physics 30 (2007, Updated 2014) Unit C: Electromagnetic Radiation
ON 10 Science Grade 10 Academic (SNC2D) Strand E: Light and Geometric Optics
ON 10 Science Grade 10 Applied (SNC2P) Strand E: Light and Applications of Optics
PE 8 Science Grade 8 (revised 2016) Unit 3: Optics
PE 11 Physics 521A (2009) Waves
QC Sec III Science and Technology Material World
QC Sec V Physics Geometric optics
QC Sec III Applied Science and Technology Material World
YT 8 Science Grade 8 (British Columbia, June 2016) Big Idea: Energy can be transferred as both a particle and a wave.
SK 8 Science Grade 8 (2009) Physical Science – Optics and Vision (OP)
SK 11 Physical Sciences 20 (2016) Properties of Waves
NT 8 Knowledge and Employability Science 8 (Alberta, Revised 2009) Unit C: Light and Optical Systems
NT 8 Science 8 (Alberta, 2003, updated 2014) Unit C: Light and Optical Systems
NT 12 Physics 30 (Alberta, 2007, Updated 2014) Unit C: Electromagnetic Radiation

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Learn about reflection and refraction and meet Emily Altiere, a PhD student in physics who studies lasers.

We experience reflection almost every day. When we see ourselves in a mirror, we are seeing a reflection. When we see the sky on the surface of a still lake, we are seeing a reflection.

Shown is a colour photograph of a city at night, and its reflection in water.
Reflection of Toronto Skyline in Lake Ontario (Source: jameswheeler via Pixabay).
Image - Text Version

Shown is a colour photograph of a city at night, and its reflection in water.

High rise office buildings are close together along the horizon. Their windows twinkle with light. The CN Tower is to the left, lit up in red. All the buildings and their lights are reflected, upside down, in the calm water in the foreground. The water, and the sky above are both dark blue.

Reflection occurs when light traveling through one material bounces off a different material. The reflected light continues to travel in a straight line, but in a different direction.

Here are some things to remember about reflection.

  1. Light is reflected at the same angle that it hits the surface.
    The angle between the incoming light and a line perpendicular to the surface we call the angle of incidence. The angle between the reflected light and a line perpendicular to the surface we call the angle of reflection. The perpendicular line we call the normal.
  2. The angle of incidence is equal to the angle of reflection.
    The symbol Ɵ means “angle'' and arrows represent rays of light.
Shown are two colour illustrations of the angles of light reflecting off surfaces.
Light reflecting off two surfaces (©2019 Let’s Talk Science).
Image - Text Version

Shown are two colour illustrations of the angles of light reflecting off surfaces.

Both illustrations have a long, flat, grey rectangle along the bottom edge. These are labelled "Surface." Vertical, dashed lines stretch down the centre of each illustration, ending at the top centre of each rectangle. These lines are labelled, "Normal."

In the left illustration, a blue arrow points diagonally, down and right, to top centre of the surface. This is labelled "Incident Light" in blue letters. A red arrow points diagonally up and right from the top centre of the surface. This is labelled, "Reflected Light" in red letters. 

A curved line leads from
"Normal" out to each of the arrows. The one leading to the "Incident Light" arrow is labelled with a theta symbol and a lower case, italicized i. The one leading to the "Reflected Light" arrow is labelled with a theta symbol and a lowercase, italicized r.

The right illustration is the same as the left, except that the angles of the arrows are further away from "Normal". Each of these angles are also labelled with the theta symbol and a lowercase, italicized i, and the theta symbol and a lowercase, italicized r.

Sometimes light reflects off a smooth surface, like a mirror or a still lake. When this happens, all of the light is reflected in the same direction. We call this specular reflection.

Along a smooth surface, the normal always points the same way. This means that all of the light is reflected in the same direction.

For this type of reflection, the image that is reflected looks the same as the original image.

Shown is a colour diagram of light reflecting off a smooth surface.
Example of specular reflection (©2022 Let’s Talk Science).
Image - Text Version

Shown is a colour diagram of light reflecting off a smooth surface.

A long, flat, grey rectangle runs along the bottom edge of the illustration. This is labelled, "Surface." Three blue, parallel arrows point diagonally down to the top of the rectangle. Where the blue arrows touch the surface, three red arrows point diagonally up at the same angle. These look like three V shapes.

Sometimes light reflects off a rough surface. When this happens, the light is reflected in different directions. We call this diffuse reflection.

Along a rough surface, the normal points in different ways. This means that all of the light is reflected in different directions.

The arrows show in which direction the reflected image will appear when light reflects off a rough surface.

Shown is a colour diagram of light reflecting off a rough surface.
Example of diffuse reflection (©2022 Let’s Talk Science).
Image - Text Version

Shown is a colour diagram of light reflecting off a rough surface. 

Along the bottom of the diagram is a long grey shape, with a flat bottom edge and a jagged, uneven top edge. This is labelled, "Surface." 

Three diagonal, parallel blue arrows point down to the surface. Where each arrow touches the surface, a red arrow points up. These are at three completely different angles.

A good example of specular and diffuse reflection is when the headlights vehicles shine on the road at night.

If the road is dry, the reflection will be diffuse, since the pavement is rough.

Shown is a colour diagram of light reflecting off a rough surface.
Light reflecting off a dry road at night (©2022 Let’s Talk Science).
Image - Text Version

Shown is a colour diagram of light reflecting off a rough surface.

The background of the diagram is dark blue. Along the bottom is a long grey shape, with a flat bottom edge and a jagged, uneven top edge. This is labelled, "Dry road."

Three diagonal, parallel white arrows point down to the surface. Where each arrow touches the surface, a yellow arrow points up. These arrows point in three different directions, at different angles.

If the road is wet, the water makes the road surface smoother. This results in more specular reflection from the car’s headlights. This causes what we call glare. Glare makes it hard for drivers to see.

Shown is a colour diagram of light reflecting off a rough surface.
Light reflecting off a wet road at night (©2022 Let’s Talk Science).
Image - Text Version

Shown is a colour diagram of light reflecting off a rough surface.

The background of the diagram is dark blue. Along the bottom is a long grey shape, with a flat bottom edge and a jagged, uneven top edge. This jagged surface is covered with a smooth, blue, stripe, creating a flat top surface. This is labelled, "Wet road."

Three diagonal, parallel white arrows point down to the surface. Where each arrow touches the surface, a yellow arrow points up at the same angle. The two sets of arrows resemble three V shapes.

Not only can light bounce off materials, it can also sometimes travel through materials. When light travels through a uniform material, like air, it goes in straight lines. When light travels through one material and into a second material, interesting things happen!

When light traveling through one material reaches a second material, some of the light is reflected. The rest of the light enters the second material.

At the point where the light enters the second material, the light will travel in a different direction than the incident light. We call this refraction. Refraction happens because the speed of light is different in different materials!

Shown is a colour photograph of light refracted through a lens marked with angles.
Refraction through a lens (Let’s Talk Science using an image by Zátonyi Sándor (ifj.) Fizped [CC BY-SA] via Wikimedia Commons).
Image - Text Version

Shown is a colour photograph of light refracted through a lens marked with angles.
At the centre of the image is a circle marked around the edge with degrees in each quarter. In the lower half of this circle is a half-circle of clear, shiny material.

A narrow beam of light extends from the end of black cylinder in the top left corner of the photograph. The beam crosses the 60 degree mark of the top left quarter of the circle. This beam is labelled "Incident light."
When the light hits the edge of the lens, it goes in two different directions.
One faint beam turns diagonally down through the lens at about 35 degrees. This beam is labelled "Refracted light."

Another, even fainter beam turns up, diagonally away from the edge of the lens, through the 60 degree mark in the top right quarter of the circle. This beam is labelled "Reflected light."

A good way to think about this is to imagine pushing a shopping cart along cement, and then reaching grass.

It is harder to push the cart in the grass. As each wheel hits the grass, it slows down. Since the wheels on the pavement are still moving faster, the cart changes directions. In this case, the cart turns to the right.

Shown is a colour diagram of a shopping cart moving from pavement onto grass.
Shopping cart example of refraction (©2020 Let’s Talk Science).
Image - Text Version

Item contShown is a colour diagram of a shopping cart moving from pavement onto grass. 

The bottom two thirds of the image is grey. This is labelled "Pavement." The top is green and labelled "Grass." A straight yellow line stretches across the illustration, from near the lower right corner, to near the upper right corner. 

Two shopping carts are shown from above, along the yellow line. This illustrates that one cart is moving in a straight line, across the grey pavement, toward the grass. 

The front right corner of the second cart is on the green background, while the rest is on the grey. The front right wheel is labelled "Right wheel slows down." The front left wheel of the cart is labelled "Left wheel keeps going at the same speed."

A third cart is completely on the green background. It does not follow the yellow line of the other two.There is a red line under this cart, leading from the edge of the pavement, to a point near the top centre of the illustration. This cart is labelled "Cart turns to the right."ent

The way light changes directions has to do with the properties of the material it is travelling through. Every material has a unique index of refraction. This measurement is identified using the letter n.

The index of refraction of a material is equal to the speed of light in a vacuum, divided by the speed of light in the material. The higher the index of refraction, the slower light travels in that medium.

If light is travelling in one material and then refracts in a second material, it will bend towards the normal if the index of refraction of the second material, n2, is greater than the index of refraction of the first material, n1.

n1 < n2

This is because the light travels slower in the second material.

Shown is a colour diagram of light bending as it refracts through material.
Light bends towards normal when n1 < n2. (©2022 Let’s Talk Science).
Image - Text Version

Shown is a colour diagram of light bending as it refracts through material.

The upper half of the diagram has a light grey background. This is labelled, "Material 1."  The lower half has a darker grey background. This is labelled, "Material 2."

A dashed, black, runs vertically through the centre of the diagram. This is labelled "Normal". 

A blue arrow points diagonally from the left side of the top edge of the diagram, through Material 1, to the point where normal crosses from Material 1 to Material 2. This is labelled "Incident Light." It is marked with the theta symbol and the letter i to indicate it depicts the angle of incident light.

A green arrow points from here, diagonally through Material 2, to the right part of the lower edge of the diagram. This arrow is labelled "Refracted Light." The space between the arrow and normal is marked with the theta symbol and the letter r to indicate this is the angle of refracted light.

If the second material has a lower index of refraction, the light will bend away from the normal.

n1 > n2

This is because the light travels faster in the second material.

Shown is a colour diagram of light bending as it refracts through material.
Light bends away from the normal when n1 > n2. (©2022 Let’s Talk Science).
Image - Text Version

Shown is a colour diagram of light bending as it refracts through material.

The upper half of the diagram has a light grey background. This is labelled, "Material 1." The lower half has a darker grey background. This is labelled, "Material 2."

A dashed, black, runs vertically through the centre of the diagram. This is labelled "Normal".

A blue arrow points diagonally from the left side of the top edge of the diagram, through Material 1, to the point where normal crosses from Material 1 to Material 2. This is labelled "Incident Light." It is marked with the theta symbol and the letter i to indicate it depicts the angle of incident light.

A green arrow points from here, diagonally through Material 2, to the lower part of the right edge of the diagram. This arrow is labelled "Refracted Light." The space between the arrow and normal is marked with the theta symbol and the letter r to indicate this is the angle of refracted light.

Unlike reflection, the angle of incidence is not equal to the angle of refraction.

This relationship between the angle of incidence and angle of refraction are known as Snell’s Law.

We can make light go in different directions using lenses. A lens is an optical device made of plastic or glass. When light passes through a lens, it can be refracted in predictable directions. The directions depend on the shape of the surfaces of the lens.

The surface of a lens may be curved outward on one side or on both sides. When lenses curve outward, we call them convex lenses.

The surface of a lens may also be curved inward on one side or on both sides. When lenses curve inward, we call them concave lenses.

Shown is a colour illustration of six different blue, curved shapes.
Top row: Examples of convex lenses. Bottom row: Examples of concave lenses (©2022 Let’s Talk Science).
Image - Text Version

IShown is a colour illustration of six different blue, curved shapes.

In the top row, from the left, the first shape has two sides that are curved outward. In the second shape, the left side is curved outward, while the right side is flat. In the third shape, the left side is curved outward and the right side is curved inward.

In the bottom row, from the left, the right and left sides of the first shape are curved inward. In the second shape, the left side is curved inward, while the right side is flat. In the last shape, the left side is curved outward, while the right side is curved inward.

When parallel beams of light pass through a lens that is convex on both sides (biconvex lens), the light is refracted inwards. These beams of light are said to be converging. The beams of light cross at a point that we call the focal point. With this type of lens, the focal point is behind the lens.

Shown is a colour photograph of three beams of light converging after passing through a lens.
Light as it passes through a biconvex lens (Let’s Talk Science using an image by Fir0002 [CC BY-SA] via Wikimedia Commons).
Image - Text Version

Shown is a colour photograph of three beams of light converging after passing through a lens.

The lens looks like the top left shape in the illustration above. It is clear, and has two sides that curve outward. 

Three thin white beams of light shine from the left edge of the image. When they hit the edge left side of the lens, they turn in toward each other.

The beams continue at the same angles through the lens and out the right side. They meet and cross over each other at a point to the right of the lens. This is labelled "Focal Point" with a yellow arrow.

The light beams can also be seen reflecting, faintly off the left side of the lens. These diverge from each other.

When light passes through a lens that is concave on both sides (biconcave lens), the light is refracted outwards. These beams of light are said to be diverging. With this type of lens, the focal point is in front of the lens.

Shown is a colour photograph of three beams of light diverging as they pass through a lens.
Light as it passes through a biconcave lens (Let’s Talk Science using an image by Fir0002 [CC BY-SA] via Wikimedia commons).
Image - Text Version

Shown is a colour photograph of three beams of light diverging as they pass through a lens.

The lens looks like the bottom left shape in the illustration above. It is clear, and has two sides that curve inward.

Three thin white beams of light shine from the left edge of the image. When they hit the edge left side of the lens, they turn in outward, away from each other.

The beams continue at the same angles through the lens and out the right side. 

The light beams can also be seen reflecting, faintly off the left side of the lens. Here, they turn in toward each other. They meet and cross over at a point to the left of the lens. This point is labelled "Focal Point" with a yellow arrow.

Light entering a plastic or glass prism, refracts as it enters the prism and again as it exits the prism.

When white light passes through a prism, it is refracted into all of its colours. If you project this light onto a white surface, you see what looks like a rainbow. All of the colours that make up white light are separated at different angles. This is because each individual colour refracts by a different amount through the prism.

Light refracted through a prism/Lumière réfractée dans un prisme
Light refracted through a prism (Source: Wikimedia Commons).
Image - Text Version

Shown is a colour illustration of a light beam passing through a prism.

The illustration has a large, pale grey, equilateral triangle on a black background. 

A thin white line extends from the left edge of the image, to the right edge of the triangle. Here the line fans out into a rainbow of colours. Starting from the top, these are: red, orange, yellow, green, blue, indigo and violet. The rainbow grows wider as it moves through the prism and out the other side.

 

Learn More

What's in a Rainbow? (2019)
This article, by Let’s Talk Science, explains how a rainbow can form in the sky.

What is white light?
This hands-on activity, by Let’s Talk Science, shows how it is possible to combine colours to make white light.

References

Ducksters. (n.d.). Lenses and lights.

The Editors of Encyclopaedia Britannica. (n.d.). Refraction.

Encyclopaedia Britannica. (n.d.). Angle of reflection.

Florida State University. (n.d.). Specular and diffuse reflection.

Fore, M. (n.d.). Refraction: Definition, Snell's law & refractive index. Sciencing.

The Physics Classroom. (n.d.). Converging lenses: Ray diagrams.

The Physics Classroom. (n.d.). The law of reflection.

Related Topics

Reflection & Refraction
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