# Basic Physics Lesson-22: Reflection and Refraction of Light

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## Introduction

Light is a form of energy. It travels from one place to another as an electromagnetic wave. It is that small band of the large electromagnetic spectrum, that is visible to our eyes. Human eyes are capable of seeing the visible light that is coming from sources like Sun or a candle or an electric bulb or the burning wood or lightening in the sky etc.

There are so many electromagnetic radiations which are outside the visibility of human eyes and naturally we cannot see them and have to detect them using certain equipments which are capable of detecting their presence. For example X-Rays used in medical tests is not visible to eyes but is recorded on a photographic plate.

## Dual nature of light

To explain the various characteristics of light scientists have put two theories about it's nature. The first says that it is comprised of infinitesimal small energy particles (known as photons) and a light ray is nothing but a shower of these photons having a particular energy level. Many properties of light like photo electric effect can be nicely explained by this theory. At the same time there are many things which can be explained only by assuming it to be having a wave form. So the second theory says that it is in form of a wave. Hence, light is having a dual character sometimes behaving like a wave and sometimes like small energy particles called photons.

## Light travels in a straight path

Light travels in a straight path and is coming straight to our eyes from the light source. If we keep an opaque material in between the light source and our eyes then we don't see that light source.

Do you remember the science experiment shown to us during our school days where we used two pinholes separated by a small distance to view a light source through those holes and were able to see the source only when all the three were in a straight line. That was a basic experiment to depict that light travels in a straight line.

One thing that we must remember about light is that it travels with a very high speed beyond our imagination and at the same time another equally interesting thing about it is that it does not require a medium for travelling from one place to another unlike the sound waves which require a medium to travel. That is the reason why light from the sun is able to reach the earth in spite of the big vacuum space in between in the universe.

The speed of light is mind boggling and is about 300000000 km/s. It will be interesting to note here that the distances in this universe are so huge that even with this enormous velocity light takes time to travel from one place to another in the universe. For example if we divide the distance between Earth and Sun by this velocity of light then we can find out how much time light takes to reach Earth from Sun. If we calculate it then it comes to about 8 minutes. If we do same calculation for the stars which are situated very far from us then the time that light takes to reach Earth from them is very high and is if the order of millions of years!

The visible light is actually composed of its constituents ranging from violet light to red light. Using a prism we can see these constituents easily. In fact we also see them when a rainbow is formed during the rainy times in our surroundings. The little water drops in the atmosphere act like small prisms only.

A wave is characterized by its wavelength or frequency and they are actually related to its speed in the following way -

Speed = frequency x wavelength

Symbolically it is represented as -

v = fλ where f denotes frequency and λ is the wavelength.

Visible light is the electromagnetic wave having wavelength around 400 nm to 700 nm (nm means nanometer and 1 nanometer is one-billionth of a meter) and in terms of frequency this range is around 400 THz to 800 THz.

One end of visible light is violet colour light while the other end is red colour light beyond which the region of infra red exists which is of course not visible to us. Similarly beyond violet light also a region of ultra violet exists which is also not visible to us.

## Effect of sunlight on Earth

The most obvious source of light in our solar system is Sun. It is a continuous source of light for us on earth and whatever life form is today available on Earth is there due to the heat and warmth that the Sun continuously provides us. Students generally ask a question as how this heat is coming from the sun to us. The answer is quite straight forward and the light waves or light particles whatever we call them are coming from Sun to Earth through the void space in the uni verse and only when the light enters the atmosphere of the Earth then it starts giving its energy to the atmosphere and also to the Earth surface which starts increasing the surface temperature of Earth till evening when Sun sets in and night envelopes us bringing down the temperatures.

## Reflection, absorption, and refraction of light

When the light falls on any material then there are many things that could happen first thing is that a part of the light might get reflected from the surface of the matter while another part might get absorbed in the medium itself and there can also be a possibility that light passes through the medium and emerges from the other side and if it so happens then it is known as the refraction of light. For example light passing through a glass pane experiences all these phenomenons. Another example is light falling on the surface of the lake is partly reflected and partly enters the water body and travels down and attenuates as it is absorbed in the water. Only remaining part will reach the bottom of the lake. An observer at the bottom of the lake may see some little light depending on the depth of the lake.

## Reflection of light

The most easy example of reflection of light is a mirror. When the light falls on a mirror it is reflected back and we can tilt the mirror to focus that reflected light to any target and that target will be illuminated with the reflected light. To understand the mechanism of reflection of light for the sake of simplicity the ray of light is assumed as a straight line with an arrow. Basically the reflected light is also having the same character as the incident light so it is also shown with a line and the arrow. The arrow shows the direction of travel of the light.

Every object does not reflect light like a mirror. Some of the materials reflect light dimly while others do not reflect at all and absorb the light falling on them. The most interesting example of reflection of light is the moonlight. The moon has not got its own light like sun and is a simple heavenly body circling around the Earth and is actually a natural satellite of Earth. When the sunlight falls on moon then a part of it is reflected and what we see is the reflected light reaching our eyes coming from the moon. Depending on the plane of rotation and positions of these heavenly bodies that is Sun, Earth, and Moon and the relative positions we see different shapes of moon as perceived by us due to the reflected light.

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So, for all the materials which do not have their own light we are seeing them because of the reflected light only. In a dark room we don't see anything. As soon as we switch on the light everything starts reflecting a part of it and we start seeing them. It is as simple as that.

When the light falls on a mirror then it can fall on it head on or making an angle with the mirror. The angle between the direction of light and perpendicular to the mirror at the meeting point is known as angle of incidence. The reflected light also makes an angle with the perpendicular line and that is known as angle of reflection. The laws of reflection state that -

1. Incident light, reflected light, and perpendicular line from the meeting point all these three lines lie in the same plane.

2. Angle of reflection is equal to angle of incidence.

Exercise:

Q. Using a plane mirror I want to divert a beam of light by 60 degrees from its original direction. What should I do?

A. As we know that the angle of reflection is equal to angle of incidence, so, if we keep an angle of incidence as 30 degree then the reflected light would be at 60 degree with respect to the incident light and that is what we want. So, we keep the mirror tilted accordingly.

## Refraction of light

There are some materials which allow a part of the light to pass through them to other side for example glass, a fine sheet of paper, a fine thin cloth etc. When light falls on them then a part of light is reflected and a part enters the material. Now some amount of this light is then absorbed by the material and rest comes out of it. If the material is thick enough then it is quite possible that good amount of light is absorbed and nothing or very little light comes out of it.

Whenever light enters from one medium to another medium then its speed also changes and it slightly changes its direction which is also called bending of light rays and that actually depends on the difference of refractive index of the mediums.

So, the angle of incidence and angle of refraction will be different depending upon the refractive index of the mediums.

For example when light from the air enters a glass slab then its speed decreases and it bends towards the normal (normal means a line perpendicular at the point of incidence) and obviously the angle of refraction is less than angle of incidence. Now if light leaves the glass slab and emerges out of it in the air on the other side then its speed would increase back to what it had in air and it will bend away from the normal.

Please also note that when light falls in the perpendicular direction then though it's speed would change in the 2nd medium but its direction will remain same.

Just like the laws of reflection there are laws of refraction also -

1. The incident ray, the refracted ray, and the normal (perpendicular at the point of incidence), all lie in the same plane.

2. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for the pair of given mediums. This constant is known as the refractive index of the 2nd medium with respect to the 1st medium.

The formula for refractive index (μ) is -

μ = Sin i / Sin r

Where i is the angle of incidence and r is the angle of refraction.

## Total internal reflection

Sometimes the light falls on a boundry in such an angle that it does not propagate ahead of the boundary and reflects back then this phenomenon is known as total internal reflection.

This happens when light is moving from a denser medium to a rarer medium and the refracted light bends away from the normal. If the angle of incidence increases much then a stage comes when the light reflects back to the denser medium itself and this situation is called total internal reflection.

## Conclusion

Understanding the reflection and refraction of light is essential for knowing the behaviour of light while it moves through different mediums.

Designing of lenses, mirrors, and prisms are based on the above understanding and is useful in many research and educational work.

## Other lessons in this series on basic Physics

Lesson-1: Distance and Displacement.

Lesson-2: Speed and Velocity.

Lesson-3: Acceleration.

Lesson-4: Mass and Weight.

Lesson-5: Gravity.

Lesson-6: Volume and Density.

Lesson-7: Momentum.

Lesson-8: Force Work Done and Energy.

Lesson-9: Heat and Temperature.

Lesson-10: Circular Motion.

Lesson-11: Friction.

Lesson-12: Rotational Motion.

Lesson-13: Simple Harmonic Motion.

Lesson-14: Voltage and Current.

Lesson-15: Magnetism.

Lesson-16: Light.

Lesson-17: Sound.

Lesson-18: Electrical Resistance.

Lesson-19: Capacitance.

Lesson-20: Atomic Structure.

Lesson-21: Kinetic Energy.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.