SCIENCE BLOG
How are rainbows formed?
Since the beginning of mankind, nature’s display of spectacular natural phenomena in the sky has led early civilisation into many superstition and mystical beliefs. When scientists began to discover more about light and reason out what causes rainbow beginning from the 14th century, people move to the other extreme of believing that there is no supernatural forces since such mysterious displays are a result of simple physics theory. In this blog post, we look at the simple theory of what causes a rainbow to appear and how the God of this universe uses simple things.
Sunlight is made up of 7 colors
Sunlight appears white to our naked eyes but it is actually made up of seven colors –red, orange, yellow, green, blue, indigo and violet.
If we let a ray of sunbeam pass through a prism, we will see that the ray of white light disperse into a display of rainbow colors:
Photo by Kelvinsong, CC0, via Wikimedia Commons
Photo by Daoudi Aissa on Unsplash
The Physics Theory of Rainbow
In the blog post on The Mysteries of Light, we learned that our earth is surrounded by an atmosphere. In the atmosphere, there are water droplets. This is especially so on rainy days. The air is filled with water droplets, though it is invisible to our eyes. When the sunlight shines through the water droplets, the white sunlight gets dispersed into 7 colors because of the following 3 properties of light:
1. Light travels in straight light
As you can see, light travels in straight line. Sunlight comes from the sun. It takes some time for the light from the sun to reach our eyes as we stand on earth. So the question is how fast does light travels?
2. Speed of light
Speed measures how fast an object travels. (Click on the Lesson 1 preview to watch a 2 min video lesson on what is speed here.)
In air
Scientists have discovered that in air or vacuum (space without air) , light travels at a speed of 299 792 458 metres per second (commonly written as 3.8 x 108 m/s). This means the light from the sun travels 299 792 458 metres in one second.
In water or glass
However when light enters a denser medium, e.g water or glass, its speed decreases, i.e. light travels slower in water or glass than in air.
Different speed for each of the 7 colors of white light
In air, each of the 7 colors of white light travels at the same speed, i.e. 3.0 x 10 8 m/s. However, in a denser medium, e.g. water or glass, each of the colors of light travels at a different speed. Red light travels fastest while the violet light travels slowest in water.
3. Refraction
When we shine a beam of light from air into a glass block at an angle as shown in the photo on the right, we will notice that the beam of light which travels in straight line is now bent. The bending of light is a result of light travelling at a slower speed in the glass. This is known as refraction of light.
If we draw a line perpendicular to the boundary surface between air and glass, we will notice that the light ray bends towards the normal when it enters the glass and bends away from the normal when it leaves the glass.
Photo by ajizai, Public domain, via Wikimedia Commons
How Rainbow is formed
Each water droplet in the air acts like a tiny prism dispersing the white light into the 7 colors of light.
Prism Dispersion
When we shine a beam of white light into a prism, as each of the colored light travels at different speeds in glass, the extend the light is bent for each of the color is different. Red light travels the fastest in glass and the violet let travels the slowest. Hence the bending of red light is least while the bending of violet light is the greatest. As a result, white light gets dispersed when shined through the glass prism.
Rainbow formation through Droplet Dispersion
Likewise, when sunlight enters a water droplet, the ray of light is refracted. The refraction of different colors of light is different because each of the coloured light travels at different speed in the water droplet. This leads to dispersion of the sunlight into its 7 colors, forming the display of rainbow.
Why we do not see rainbow all the time?
For a rainbow to be visible to an observer on the ground, several conditions have to take place. Before we go into that, we need to understand another property of light, i.e. Total Internal Reflection.
Total Internal Reflection
As we see earlier that when light travels from a denser medium (e.g. water) into a lighter medium (e.g. air), the light ray will bend away from the normal. As we increase the angle of incidence, the bending will increase too. When we hit an angle of incidence where the light ray bends 90 degrees, we call the angle of incidence the critical angle. When the light ray has an angle of incidence more than the critical angle, total internal reflection takes place whereby the light ray does not go out into the air but is reflected back into the water.
Josell7, CC BY-SA 3.0, via Wikimedia Commons
How rainbow is seen by us
Not all rays of sunlight that passes through the droplets reach our eyes. For us to see the rainbow, the sun has to behind us. Note the following:
1. If a ray of sunlight is incident perpendicularly at the droplet, it will pass through directly and will not reach us and be seen by our eyes.
2. If a ray of sunlight is incident at an angle and refracted away from our eyes, we will also not see the rainbow.
3. Hence, only those light rays that is incident at an angle between 40 to 42 degrees and experience total internal reflection, will be reflected towards us and hence can be seen by us.
4. The critical angle for red light is 42 degrees. Hence, all droplets reflect the red waves of sunlight at an angle of 42 degrees. If you use geometry to compute, you will discover that an observer will only see the red light from droplets suspended at an altitude where the angle of elevation of the observer from the horizon is 42 degrees.
5. Violet light has a critical angle of 40 degrees. Hence it is visible only from droplets suspended 40o above the horizon. The other colors are reflected at angles between those of red and violet.
Swipe on the images below to see the illustrations.
Sunlight is made up of 7 colors
Sunlight appears white to our naked eyes but it is actually made up of seven colors –red, orange, yellow, green, blue, indigo and violet.
If we let a ray of sunbeam pass through a prism, we will see that the ray of white light disperse into a display of rainbow colors:
Photo by Kelvinsong, CC0, via Wikimedia Commons
Photo by Daoudi Aissa on Unsplash
The Physics Theory of Rainbow
In the blog post on The Mysteries of Light, we learned that our earth is surrounded by an atmosphere. In the atmosphere, there are water droplets. This is especially so on rainy days. The air is filled with water droplets, though it is invisible to our eyes. When the sunlight shines through the water droplets, the white sunlight gets dispersed into 7 colors because of the following 3 properties of light:
1. Light travels in straight light
As you can see, light travels in straight line. Sunlight comes from the sun. It takes some time for the light from the sun to reach our eyes as we stand on earth. So the question is how fast does light travels?
2. Light travels at slower speed in water
Speed measures how fast an object travels.
In air
Scientists have discovered that in air or vacuum (space without air) , light travels at a speed of 299 792 458 metres per second (commonly written as 3.8 x 108 m/s). This means the light from the sun travels 299 792 458 metres in one second.
In water or glass
However when light enters a denser medium, e.g water or glass, its speed decreases, i.e. light travels slower in water or glass than in air.
Different speed for each of the 7 colors of white light
In air, each of the 7 colors of white light travels at the same speed, i.e. 3.0 x 10 8 m/s. However, in a denser medium, e.g. water or glass, each of the colors of light travels at a different speed. Red light travels fastest while the violet light travels slowest in water.
3. Refraction
When we shine a beam of light from air into a glass block at an angle as shown in the photo on the right, we will notice that the beam of light which travels in straight line is now bent. The bending of light is a result of light travelling at a slower speed in the glass. This is known as refraction of light.
If we draw a line perpendicular to the boundary surface between air and glass, we will notice that the light ray bends towards the normal when it enters the glass and bends away from the normal when it leaves the glass.
Photo by ajizai, Public domain, via Wikimedia Commons
How Rainbow is formed
Each water droplet in the air acts like a tiny prism dispersing the white light into the 7 colors of light.
Prism Dispersion
When we shine a beam of white light into a prism, as each of the colored light travels at different speeds in glass, the extend the light is bent for each of the color is different. Red light travels the fastest in glass and the violet let travels the slowest. Hence the bending of red light is least while the bending of violet light is the greatest. As a result, white light gets dispersed when shined through the glass prism.
Rainbow formation through Droplet Dispersion
Likewise, when sunlight enters a water droplet, the ray of light is refracted. The refraction of different colors of light is different because each of the coloured light travels at different speed in the water droplet. This leads to dispersion of the sunlight into its 7 colors, forming the display of rainbow.
Why we do not see rainbow all the time?
For a rainbow to be visible to an observer on the ground, several conditions have to take place. Before we go into that, we need to understand another property of light, i.e. Total Internal Reflection.
Total Internal Reflection
As we see earlier that when light travels from a denser medium (e.g. water) into a lighter medium (e.g. air), the light ray will bend away from the normal. As we increase the angle of incidence, the bending will increase too. When we hit an angle of incidence where the light ray bends 90 degrees, we call the angle of incidence the critical angle. When the light ray has an angle of incidence more than the critical angle, total internal reflection takes place whereby the light ray does not go out into the air but is reflected back into the water.
Josell7, CC BY-SA 3.0, via Wikimedia Commons
How rainbow is seen by us
Not all rays of sunlight that passes through the droplets reach our eyes. For us to see the rainbow, the sun has to be behind us.
1. If a ray of sunlight is incident perpendicularly at the droplet, it will pass through directly and will not reach us and be seen by our eyes.
2. If a ray of sunlight is incident at an angle and refracted away from our eyes, we will also not see the rainbow.
3. Hence, only those light rays that is incident at an angle between 40 to 42 degrees and experience total internal reflection, will be reflected towards us and hence can be seen by us.
4. The critical angle for red light is 42 degrees. Hence, all droplets reflect the red waves of sunlight at an angle of 42 degrees. If you use geometry to compute, you will discover that an observer will only see the red light from droplets suspended at an altitude where the angle of elevation of the observer from the horizon is 42 degrees.
5. Violet light has a critical angle of 40 degrees. Hence it is visible only from droplets suspended 40o above the horizon. The other colors are reflected at angles between those of red and violet.
Swipe on the images below to see the illustrations.
What is the truth?
As you can see from the above explanation, a simple physics theory on refraction of light explains how a magnificent rainbow is formed in the sky. Hence when people discover such scientific truths, they completely discard all the unfounded superstition and also discard the existence of God. While science has proven unfounded superstition such as fairies and rabit on the moon to be untrue, has science really proven that there is no God.
When we learn a simple recipe, e.g., how to make an oat pancake, we discover it is simple. However, the fact that the recipe is simple and the pancake appears on our table does not mean that the farmer who plant the oat or the chicken that laid the eggs or the mummy who make the pancakes did not exist. Likewise, understanding how rainbow is formed by the physics theory of refraction does not mean that science is not set by God. Science does not mean there is no Creator of this universe. Knowing the refraction theory does not explain how and when water droplets in the air can be at the right place right time for us to see them. We do not see rainbows on every rainy days when the sun starts to shine. Yet, a double rainbow appeared during Queen Elizabeth’s funeral. Was that God or coincidence?
Did you realise?
When you read the above explanation on how rainbow is formed, did you realise the following:
1. Scientists can only analyse and discover the properties of light, e.g. the refractive nature of light, the speed of light in different medium and that light travels in straight line. However, why does light has such properties?
2. How does a change in speed in a denser medium cause light to bend? We can only discover this nature of light but we have yet to be able to explain the bending.
3. Scientists discovered that there are 7 colors of white light. However, why are there 7 colors and not more nor lesser?
There are still many mysteries that scientists have yet to discover the secrets of all sorts of mysterious displays in the sky.
Did you realise?
When you read the above explanation on how rainbow is formed, did you realise the following:
1. Scientists can only analyse and discover the properties of light. However, no one can explain why light has such properties?
2. How does a change in speed in a denser medium cause light to bend? We can only say it happens as a result of a speed change. However why does a change in speed leads to a bending?
3. Scientists discovered that there are 7 colors of white light. However, why are there 7 colors and not more nor lesser?
There are still many mysteries that scientists have yet to discover the secrets of all sorts of mysterious displays in the sky.
DISCOVER THE TRUTH OF A God who uses simple things
When we read about a simple theory such as refraction, we should recognise that our God uses simple things to create wonders as described in 1 Corinthians 1:27 and as described in this article on 11 bible verses where God uses small things.
Photo taken on 21 Jul 2021, 6:57pm, West Coast , Singapore.
Photo taken on 25 Dec 2020, 6:58pm, Labrador Park, Singapore.
God sets the rainbow
Read full chapter of the promises in this covenant with God in Isaiah 54-55 here. If you would like to believe and accept Jesus as your LORD and Saviour and become a child of the God who created this entire universe, all it takes is to believe in your heart and a prayer as follows:
“Dear Lord Jesus, thank you for dying for my sins. I believe you are the Son of God and that You love me and desire to bless me with the abundant life and eternal life that You came to give me. I believe and receive You as my Lord and Saviour. In Jesus name I pray, amen.”
Science does not replace God. Science reveals the infinite wisdom of a covenant keeping God who loves us deeply.
If you would like to believe and accept Jesus as your LORD and Saviour and become a child of the God who created this entire universe, all it takes is a believe in your heart and a prayer as follows:
“Dear Lord Jesus, thank you for dying for my sins. I believe you are the Son of God and that You love me and desire to bless me with the abundant life and eternal life that You came to give me. I believe and receive You as my Lord and Saviour. In Jesus name I pray, amen.”