The Colour Story!

rose apple

How are different objects spot different colours? and how can we see even see colours? What are colours anyway?

Interaction of light with matter contributes to the colour appearances around us. So, let’s start with light first. We cannot see anything without light anyway.

What is light?

Light is energy, simply. The visible light or the visible spectrum is that tiny portion of the electromagnetic spectrum with wavelengths of 400-700nm visible to human eyes.

Visible Spectrum

The light spectrum contains 7 colours; violet, indigo, blue, green, yellow, orange and red and are called spectral colours. These spectral colours have specific wavelengths. Other colours are made from a mix of different wavelengths.

Human Colour Vision

A typical human eye will respond to wavelengths from about 390 to 700nm. Human eye and brain work together to translate light into colour when the specific colour (wavelength) reaches the retina in the eye. Retina has numerous light-sensitive cells which process the light into nerve impulses and pass them to the brain via the optic nerve.

Colours of Objects

Colour is not inherent in objects. We see an object coloured because the surface of the object reflects some wavelengths and absorbs all the others. We perceive only the reflected colors.

Animal Colour Vision

Many animals can see light outside the ‘human visible spectrum’. Honey bees are able to detect wavelengths of 300 to 700nm through their eyes. Bee eyes have photo-receptors capable of detecting ultraviolet light.

Rattlesnakes are able to detect wavelengths of 480 to 850nm. They detect these through special pit organs on their heads which pick of infrared heat signals from warm objects in the environment; the reason they navigate at night when there is no visible light.

Colour of Sky

Colour of sky depends on the amount of atmosphere the light has to pass through before it reaches our eyes and also its angle of entry.

A clear, cloudless sky appears blue. This is because the tiny molecules in the air scatter blue wavelengths from the visible spectrum, a phenomenon called Rayleigh scattering, which reaches our eyes. Blue is scattered more than other colours because it travels as shorter, smaller waves while larger wavelengths (yellows and reds) are large enough to not be scattered by these tiny particles. The sky appears blue most of the time.

blue sky

Blue sky

At sunrise and sunset, the sky spots a reddish colour. This is when the sunlight has to travel longer and the angle of entry is greater. By the time the light reaches us, the shorter wavelengths (blue) are scattered out and only the longer wavelengths (red) are left which reach our eyes.


Colour of the sky during sunset

Sky also appears white sometimes (or lighter blue); especially in Summer. During Summer, air is warm and can hold more moisture. The water molecules in the air are large enough to scatter all wavelengths of light equally and hence we see the sky as almost white.

The sky appears white at noon as well. This is when the sun is right above our head and the light doesn’t have to travel much through the atmosphere. So, all wavelengths are scattered less and equally and hence we see a white sky.

Rayleigh scattering is the elastic scattering or light (or the EM radiation) by particles in the medium whose size is much smaller than the wavelength of light (radiation). The particles may be individual atoms or molecules. Rayleign scattering is most prominent when light passes through gases; though it occurs in transparent solid and liquid medium as well. The amount of scattering is inversely proportional to the fourth power of wavelength. Particles responsible for Rayleigh scattering neither loses nor gains energy. So, the scattered light has the same frequency as the radiation from which it is produced.

Colour of Sun

Technically, the colour of Sun is white. However, Sun spots many colours (yellow to red) depending on the amount of atmosphere sunlight has to pass through. At noon, when the scattering of light is less, the Sun appears almost white in the sky.

Mid day sun

Mid-Day Sun

evening sun

Evening Sun

In short, the colour we see around us is culturally determined. What we see depend on the response of our eyes to the sunlight.


Blue moon, blood moon, supermoon…


Earlier this week, we had a rare lunar spectacle in the form of the super blue blood moon. Many shutterbugs have flooded their social media accounts with a reddish full moon. It was a combination of blue moon, super moon and blood moon.

What are these terms mean anyway?

Blue Moon

Blue moon refers to the additional full moon in a year; either the third full moon of a four full moon season or the second full moon of a month with two full moons. There is nothing blue about the moon. Then why it is called a ‘blue’ moon. Because it is rare… like once in a blue moon.

Blue moon occurs once in two to three years!

Blood Moon or Red Moon

The Moon does not have any light of its own. It shines because its surface reflects the sunlight.

A total lunar eclipse occurs when Sun, Earth and Moon are aligned and the moon falls in the shadow of Earth.


Lunar Eclipse

When sunlight enters the Earth’s atmosphere, it strikes the atmospheric particles and gets scattered.  Colors in the light spectrum with shorter wavelengths, violet and blue colours, are scattered away before the sunlight hits the surface of the Moon during a lunar eclipse. The red and orange colours in the light spectrum, because of their longer wavelengths, pass through the atmosphere, gets refracted around Earth and hits the surface of the Moon thus giving it the reddish-orange glow.


Blood moon


A supermoon is a full moon or new moon and appears particularly large in the sky when it is at its closest approach to the earth, the perigee.

The Moon orbits Earth in an elliptical path, which means one side of the path is closer to the Earth than the other. An opposite phenomenon called micromoon occurs when the moon is at its farthest approach to Earth, the apogee.


Supermoon and micromoon


Few bits about ‘HEAT’


As you know the Universe is made of matter and energy. The matter is made up of atoms and molecules. And it is the energy that causes (triggers) these atoms and molecules in motion. They bump into each other or vibrate back and forth and increased the atomic/molecular kinetic energy and thus creates a form of energy called ‘thermal energy’.

Any form of energy can be converted into thermal energy via various processes. Heat is the amount of thermal energy transferred during these processes. Let’s look into few examples:

  1. Mechanical energy to thermal energy: Consider bouncing of a ball. Everytime the ball bounces back from the ground, some of the energy of ball’s motion  (KE) is converted into heat (making the ball warm) and thus slowing it down.
  2. Electrical energy to thermal energy: Most electrical appliances heat up when we switch it on. We know that electrical energy is the flow of charges, carried by moving electrons, through a wire.  As they move, these electrons encounter hinderance to its flow due to many collisions with atoms in the wire (resistance). So, the electrical energy needs to push the current through the wire against the resistance which results in current converting electrical energy to heat.
  3. Chemical energy to thermal energy: An example is a fuel releasing heat. And a good example of fuel is the food we eat which is stored chemical energy. During digestion, chemical reactions cause bonds to break releasing kinetic and thermal energy. This heat keeps our body warm.
  4. Light energy to thermal energy: Light is emitted by discrete packets of energy called photons. Light, for example, Sun’s rays, falling on an object changes the energy levels of electrons in the atoms and molecules of the object. The atoms and molecules thereby lose energy in the form of re-radiating photons or collision with other atoms or molecules releasing heat.

The hotness of a body depends on how fast the molecules in a body are vibrating. This depends on the amount of energy put into it and the thermal conductivity of the body. The more the thermal conductivity of a body, more the hotness; examples – hotness of an iron road and wood piece with the same amount of thermal energy supplied. The degree or intensity of heat present in a substance or object is ‘temperature’.

The intensity of atomic/molecular vibrations/collisions is proportional to the increase in atomic/molecular kinetic energy of a body which is directly proportional to the temperature. So, fast the vibrations/collisions, more the temperature.

Transfer of heat: thermal energy can be transferred from one point to another where there is a temperature difference between the two points.

Bubbly and Boiling…


What are bubbles?
Bubbles, in general, are globules of one substance in another. Usually, it is a gas in a liquid. It is globular in shape since it has the most stable and lowest energy state. Bubbles collapse when the pressure of the substance inside matches that of outside.

Let us understand how the bubbles are formed in liquids. Let us consider the case of water.

Bubbles in boiling water
We know that boiling water produces bubbles; few at the beginning… then we see those rising… and more and more as the temperature is increased and the water reaches the boiling point… The boiling phenomena is accompanied with a sound.. What causes  all these?

It is a known fact that water has a lot of air mixed in it. When the temperature of the water is raised, this solubility of air in water is decreased and air gets separated out in water. That is what we see as the bubbles at the bottom and sides of a pan of water over fire!

As the temperature is increased, water at the bottom of the pan (close to the source of heat) starts vaporizing into the bubbles. As these water vapours have density less than that of water around them, they rise to the surface of water and try to escape from the water.

It should be noted that the atmospheric pressure (weight of air on the surface of liquid) acts against the vapour escaping water when the vapour pressure is low. At temperatures less than the boiling point, the vapour pressure is less than the atmospheric pressure. So, at a temperature lower than the boiling point of water, the bubbles may rise, but they won’t reach the surface of water. It cools down on its way up and collapses. The sound we hear prior to the boiling water is nothing but the sound of collapsing of bubbles.

At the boiling point, when all the liquid molecules have the willingness to vaporize, they form bubbles rapidly. The vapour pressure at boiling point will be higher and equals the atmospheric pressure. So, the bubbles rise to the surface of water and burst releasing the vapour to the surrounding.

Now, a question: What causes water to boil faster at higher altitudes?
Answer: At higher altitudes, the atmospheric pressure is less than normal. So, the vapour pressure of the boiling water will be equalling the atmospheric pressure at a lower temperature and vapour escapes from water. So, boiling point will be lower at higher altitudes.

Vaporization vs Evaporation vs Boiling


While working at my previous organization where I was a development editor for school books, I had quite a lot of interaction with school teachers. The topics of these interactions vary from new teaching methods to the content which goes into the text books to various other subject related matter. Mostly, the discussions were on their views regarding the explanations of certain “confusing” topics.

Teachers can be intransigent sometimes. Teachers, especially the experienced ones, usually follow only those books they have started their teaching career with. So, they get upset if the new content in our books deviate slightly from the wordings they are familiar with, or the artwork or diagrams they are used to. There were certain topics where the rewording confused the teachers and they ended up confusing us with their corrections. A set of topic I came across while working on the Physics books was vaporization/evaporation/boiling. These terms drove my series editor to a confused state. She sent different sets of corrections. I managed to “negotiate” with this middle school teacher and a general explanation was given for the book. But I realized, looking up in internet, many people are confused as well: the difference between vaporization, evaporation and boiling. So, in this post, I briefly discuss what these are and how they differ from each other.

Vaporization: It is nothing but the change of state of matter from a liquid state to a gaseous state. For example, change of water to steam. This change of state from liquid to gas can happen by two methods: evaporation and boiling.

Evaporation: It is the change of state of liquid to gaseous at a temperature below the boiling point of the liquid. There is no boiling of liquid involved in evaporation. Evaporation takes place at the surface of liquid which is exposed to the surroundings.

Boiling: When the liquid is heated and attains the boiling point, the vaporization process is called boiling. Boiling happens throughout the liquid. This is because, the heating process (convection) heats up the entire liquid.

Let us understand these better:

What causes a liquid to change its state to gas?
We know that the various states of matter are characterized by the arrangement of molecules in them and also the intermolecular force of attraction.

In solids, the intermolecular force of attraction is strong. That is why the molecules are held together tightly and the matter is rigid.
In liquids, the intermolecular force of attraction is less, which gives the matter the ‘fluid’ characteristics.
In gases, the intermolecular force of attraction is least. That is the reason, they don’t usually hold together as a substance and easily spread everywhere.

For a solid molecule to change its state to liquid, it should have enough kinetic energy to overcome the intermolecular force of attraction which binds it to other molecules. You know that heating can provide that kinetic energy. Similarly, heating provides enough kinetic energy to a liquid molecule to overcome the intermolecular force of attraction and change to gas.

During evaporation, the liquid absorbs heat from the surroundings. The molecules at the surface of the liquid acquire enough kinetic energy and get converted to vapor and escape. The more heat the liquid can absorb, the more will be the rate of evaporation: On hot summer days, we see the water in the pond getting evaporated faster than other seasons.


Evaporation of lake water

When we apply heat externally, for example, heating up a pan of water, the kinetic energy of the molecules increases rapidly and they start to vaporize. There will come a point at which all the molecules in the liquid have enough kinetic energy to change their state to gas. This is called the boiling point and the process is called boiling. Vaporization during boiling will be quite rapid.


Boiling of water

Vapor Pressure
Another factor which differentiate these processes is vapor pressure. It is the pressure exerted by the liquid on the walls of a container. Liquids which can vaporize easily will have a higher vapor pressure. During evaporation, the vaporization is a slow process and the vapor pressure will be less as compared to the atmospheric pressure.

During boiling, at the boiling point, the vaporization is fast and the vapor pressure is equal to the atmospheric pressure. That is the reason, the bubbles formed (with vapor inside) during boiling rise up and burst at the surface of the liquid.

Summing up, both evaporation and boiling are same kind of phenomenon where the liquid change its state to gas. The difference depends on the supply of heat: Evaporation can take place at any temperature while boiling happens only at the boiling point. Another difference is, evaporation takes place only at the surface of the liquid while the boiling is a bulk process where vaporization takes place throughout the liquid. Also, during evaporation no bubbles are formed, but, bubbles are formed during boiling.


Evaporation and boiling

So, the concluding question: What causes the formation of bubbles during boiling? That calls for another blog post.

The levers in our body


Archimedes once said, “Give me a place to stand and I will move the Earth.” It was Archimedes who came up with the idea of levers which will reduce our effort in moving heavy load. Levers are simple machines which makes our lifting and moving of things easier. We use a variety of levers in our daily life to make many tasks done with less effort.

A lever works around a fixed point called pivot or fulcrum (F). It is a point about which the lever moves. Load is the weight to be lifted or moved using the lever and is represented by L. Effort is the input force applied by us and is represented by E. Levers are classified into three based on the positions of the Fulcrum, Load and Effort.

Class 1: Fulcrum between Load and Effort. Examples include scissors and wire cutters.

Class 1 lever

Class 1 lever

Class 2: The load is between fulcrum and effort. Examples include bottle opener and nut cracker.

Class 2 lever

Class 2 lever

Class 3: The effort is between the fulcrum and the load. Everyday example include thongs, tweezers and staplers.

Class 3 lever

Class 3 lever

Working of levers is based on the components fulcrum and effort to balance the load. We consider our body as a complex machine with a number of simple machines working together. The bones, muscles and the many joints in our body form different types of levers for us to lift and balance weight or load. Imagine lifting a weight without the use of your joints and standing on your toes without an effort by your muscles. Human body has all the classes of levers.

Class 1 lever: Move your head up and down feeling the muscles at the back of your neck. The movement of the head is supported by these muscles and the head moves around a point where the top of the spine meets the skull. This forms the fulcrum of a class 1 lever. Here head is the load and the muscle at the back of the neck which supports the skull up is the effort.

The movement of head is based on Class 1 lever

The movement of head as a Class 1 lever

Class 2 lever: Stand on your toes, and we can feel the calf muscles stretching up to support your weight. When you stand on your toes, your toes act as a fulcrum. Your calf muscles provides the effort to balance the weight (load).

Standing on toes is based on Class 2 lever

Standing on toes as a Class 2 lever

Class 3 lever: Lifting things requires bending your arm. Your arm works as a class 3 lever. The fulcrum is at the elbow. The biceps muscles provide the effort to bend the forearm against the weight of the forearm (load). If you are bending your arm with some weight in hand, that weight adds to the load.

Lifting weights or bending our arm is based on Class 3 lever

Lifting weights or bending our arm as a Class 3 lever

We maintain the tools and machines we use every day with great care: oiling them and use them regularly so that they work smooth. In the same way, it is necessary to maintain the simple machines in our body. Give the joints and muscles the exercise they need every day. Work out, walk, run,… stay active… never be idle because the body machinery will become less efficient without being charged up!!!

Static Electricity

static el 10

Have you ever wondered what causes a bad hair day or the sparks from your woollen blanket during winter or the random shocks you get from random objects now and then or the standing fur of your pet cat or the most obvious, disastrous lightning dashing across the evening sky!? There can be one answer to all these – Static electricity!!!

The general definition of static electricity is that it is an imbalance of electric charges in a material. The material remains charged until it can discharge these charge (electric current) trough any medium.

Static electricity effects are evidently annoying. However, it should be said that life is impossible without the forces associated with static electricity. Static forces, both positive and negative, hold the world of atoms and molecules together in the right balance. But, what is static electricity? Is it really harmful? Why it is more during winter? To understand the nature of static electricity, one should know what things around are made of.

Atomic Structure
We know that all material objects are composed of atoms (elements). These elements combine to form compounds (molecules). Protons, neutrons and electrons constitute the sub-atomic particles. The massive (comparatively) protons (positively charged) and neutrons (neutral in charge) are located at the centre (nucleus) of the atoms which forms a dense positive core, while ‘electron shells’ which are concentric spherical regions with distinct energy levels are the home of the negatively charged, mass-less electrons.

Electrons are loosely bound in the atoms while protons and neutrons are tightly bound inside the nucleus. A charged atom: A neutral atom has equal number of protons and electrons: i.e., equal number of positive and negative charges. Provided with sufficient energy, electrons can be freed from its attraction to the nucleus, thus removing them from the atom. Electrons from different atoms can enter into the electron shells of an atom. This entering or leaving of electrons makes an atom charged due to the imbalance of positive and negative charges in it. This charged atom is referred to as ion. Ions can be positive (cation) or negative (anion) depending on electrons leaving or entering an atom.

static el 5

Formation of cation and anion from a neutral atom by loosing and gaining electron(s) respectively

Transfer of charge
What happens when we have a charged object? Every charged object tends to redistribute the excess charge across its surface by electron movement. This movement of electrons can take place freely only in the case of a conducting body. When a conducting body is charged from a point, the body will distribute the charges across its entire surface.

Additional charged transferred to a conductor getting evenly distributed across its surface of the conductor

Additional charges transferred to a conductor getting evenly distributed across the surface of the conductor

This redistribution of charges can be explained by the basics of charge interaction: unlike charges attract and like charges repel, thus making the excess charges spread across the entire surface away from each other. It should be stressed that additional charges, positive or negative, is redistributed only through the movement of electrons. While additional negative charges (excess of electrons) get distributed by the movement of electrons itself, additional positive charges (deficiency of electrons) attract electrons from elsewhere creating positive charges wherever the electrons have come from.

Like charges attract and unlike charges repel

Like charges repel and unlike charges attract

Conducting human body
We know that human body is a good conductor of electricity. When charges are transferred human body, they spread across the entire body even through the strands of hair. When the hair gets charged (like charges), they repel each other making them rise upward and outward separated from each other as possible. This is the common ‘hair-rising’ phenomenon of static electricity.

Why static electricity effects more during winter?
Shocks from door knobs, bad hair days, etc. are frequent during winter compared to summer. Why? This has to do with the humidity which affects static charges. Water molecules can remove the extra charges in a material. We often fix our hair with water! Winter is the driest of all seasons with little moisture in air and the static charges remain more on the surfaces of the body often getting discharged via static electricity.

Polarization of charges
We know that like charges attract each over and unlike charges repel each other. But what about the interaction between a charged and neutral particle? There is always an attractive force between a charged (positive or negative) and neutral particles. This is due to the polarization of charges in a material. Polarization is the process of separation of charges within a material; i.e., the positive and negative charges get separated forming two poles at either ends as shown in the following image:

Polarisation of charges

Polarisation of charges

Consider the classic example of a charged balloon held against the flowing water from a tap;  the stream of water gets attracted towards the charged balloon. In the vicinity of the charged object (negatively charged balloon), the neutral water molecules will get polarized: Hydrogen as the positive pole and oxygen as the negative pole. Molecules in a liquid are free to move. Hence, the water molecules get realigned and the positively charged oxygen molecules move towards the negatively charged balloon, pulling the entire steam of water with it thereby deflecting its flow.

Polarisation of charges in water causing a deflection of water stream

Polarisation of charges in water causing a deflection of water stream

Charging of objects
So far we have discussed the effects of static charges and charge interactions. Now, let us look into how these charges are created in objects. Charging can be done via various methods such as conduction, induction, friction, etc.

The word conduction  means two objects in contact. Charging by conduction takes place when a charged object is brought in contact with a neutral object, transferring the charges from the charged to the neutral object.

Charging by conduction

Charging by conduction

Charging by induction takes place when the charged object is brought in the vicinity of the neutral object (no contact). This causes a polarization of charges in the neutral object and with proper earthing, one can remove one type of charge from it making it charged.

Charging by induction

Charging by induction

Charging by friction  is the simplest way of charging objects. This is done by rubbing the objects together. As objects of different materials are rubbed together, electrons with weaker bonds (in least electron loving material) are removed from one material and migrate to the other material making both materials charged. Friction is usually the reason behind the static charges getting accumulated on our woollen clothes (when rubbed against us), cat’s fur (when they move about the carpets and us), the balloons (when rubbed against other substance), and so on.

When a glass rod is rubbed on a silk cloth, the glass rod loses electrons to sink, making the silk cloth negatively charges and the glass rod positively charged

When a glass rod is rubbed on a silk cloth, the glass rod loses electrons to sink, making the silk cloth negatively charges and the glass rod positively charged

Lightning is one of the dangerous effects of static electricity. It is the discharge of the static charge built up in the clouds. There is a polarization of charges in the clouds; the top parts acquire positive charges while the bottom parts acquire negative charges. This static charging of the clouds is believed to be due to frictional charging.

Frictional charging of clouds
Clouds contain millions of ice particles and water particles suspended in them. When additional water from the ground gets evaporated and rises up forming droplets on its way up towards the cloud, the upward motion of moisture collides (frictional) with the water droplets already suspended in the cloud. During these collisions, electrons are ripped off from the rising droplets causing the bottom of clouds to be negatively charged  and the positively charged droplets rises up to the top of the clouds making the region positively charged..

Lightning strike
The polarization of charges in the cloud, with the electrons gravitating towards the Earth’s surface causes the Earth’s surface to be charged positive. As the build up static charge in clouds increases, there is an increased electric field in the atmosphere surrounding the cloud making it highly conductive. This makes way for an easy path for the transfer of charges from the clouds to the surface of Earth. Since Earth’s atmosphere has numerous dust particles and other impurities, there wont be a straight line transfer of charges from the cloud to the Earth: The reason behind the zig-zag motion and branching in a lightning. Also, the purplish glow of the lightning is a characteristic of the ionized air molecules.



Lightning is not a discharge from the cloud to the Earth. It can be between cloud to cloud or even within a cloud because of the polarization of charges in the clouds.

Various kinds of discharge from clouds

Various kinds of discharge from clouds

The sudden discharge from the clouds heats up the surrounding air which expands violently. This creates a shock wave which we hear as thunder.