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Chapter 1 – Reflection Of Light

The following Topics and Sub-Topics are covered in this chapter and are available on MSVgo:

Introduction

Have you ever wondered why a sunflower looks yellow while orchids look blue? Or why you cannot see anything in a dark room? You can answer these questions with the help of the concept of reflection of light. 

When a ray of light falls on a smooth surface and bounces back, the phenomenon is called reflection of light. The ray of light that falls on the surface is called the incident ray. The ray of light that bounces back is called the reflected ray.

To study reflection mathematically, we consider a normal, which is perpendicular (90°) to the surface, and lies between the incident and the reflected ray.

Here, ‘∠i’ is the angle of incidence, and ‘∠r’ is the angle of reflection.

  1. The Angle of Incidence: The angle formed between the incident ray and the normal to the surface.
  2. The angle of Reflection: The angle formed between the reflected ray and the normal to the surface.

Laws of Reflection determine how an incident ray will be reflected when it falls on a reflecting surface, including plane mirrors, spherical mirrors, water, metal, etc.

There are three laws of reflection:

  • The angle of incidence = Angle of reflection, i.e. ∠i =∠r.
  • The incident ray, the reflected ray, and the normal all lie in the same plane.
  • The ratio of the sine of the angle of incidence to the sine of the angle of refraction or bending is constant, for a given pair of media and a light of given colour. This is called Snell’s Law.

sin i/sinr = constant, where ‘i’ is the angle of incidence, and ‘r’ is the angle of refraction.

There are two types of reflections:

  • Specular or Regular Reflection: When light rays fall on a smooth surface, like a mirror, they get reflected in one direction at a definitive angle. Thus,,the angle of incidence is equal to the angle of reflection (∠i =∠r). This is called regular reflection. In this case, the image looks exactly like the object.
  • Diffused or Irregular Reflection: When light rays are incident on an irregular or rough surface, they are scattered during reflection, and the image formed is not clearly defined. This is called irregular or diffused reflection. This happens when you check your reflection in the water on a windy day.

Light has a dual nature, i.e. it behaves both as a particle and as a wave.

When light exhibits the properties of a particle, its intensity depends on the number of particles, known as photons. When there are more particles, light is brighter, and when there are fewer particles, light is dimmer.

On the other hand, when light exhibits the properties of a wave, then only those waves are visible to our eyes whose wavelength (distance travelled by light in one oscillation) is between 400 to 700 nanometers. This range is called visible light.

Human eyes are sensitive to visible light, which consists of various wavelengths, both short and long. The colour of visible light depends on its wavelength. Each colour has a different wavelength.

Have you ever seen a rainbow in the sky? The colours of the rainbow are the colours of visible light. There are 7 colours in a rainbow:

V = Violet

I= Indigo

B = Blue

G = Green

Y = Yellow

O = Orange 

R = Red

Red has the longest wavelength, and violet has the shortest wavelength. When all the waves are seen together, they form white light.

The colour of the object that we see depends on the way it interacts with light and how it reflects it. The colour is not within the object itself, but in the light that falls on it and is reflected towards our eyes. When visible light falls on a sunflower, the pigments in it absorb all the wavelengths of white light, except yellow, which is reflected towards our eyes.

Characteristics of Light

  1. Light is an electromagnetic wave. Sunlight, a small portion of visible light, radio waves, X-rays, microwaves, etc., are all EM or electromagnetic waves.
  2. Light travels in a straight line or a linear fashion.
  3. Light is a transverse wave and does not need a medium to travel. It can travel through a vacuum at the speed of 3 × 108 m/s.

The manner in which light travels from one medium to another is called propagation of light. 

When light travels in a straight line, it is called rectilinear propagation of light. Have you ever seen sunlight entering through a small opening in a dark room? You can see that it falls in a straight line, just like the light from headlights, torches, etc.

Fermat’s principle states that a ray of light travels from one point to the other following the path that requires the least amount of time, compared to other paths. It is also called the ‘Principle of Least Time’.

The principle of reversibility of light states that if the direction of a beam of light is reversed, then despite the number of times the beam is reflected or refracted, it will retrace its path.

To understand the concept better, stand in front of a plane mirror and direct a flashlight on its surface. You will see the reflected light tracing its path back to the flashlight in a straight line.

Light interacts differently with various objects. When light travels from one medium to the other, its speed changes and bending of light happens. This is called refraction. This is the reason why a straw appears to be bent in a glass of water or a coin at the bottom of the glass appears to be closer than it is.

We can see everything around us due to the reflection of light, including ourselves in the mirror. The different colours that are visible to us are due to reflection. We hope that you now have a better understanding of reflection of light.

  1. What is the reflection of light?
    When a ray of light falls on a smooth surface and bounces back, the phenomenon is called reflection of light. The ray falling on the surface is called incident ray, and the ray bouncing back is called reflected ray.
  1. What are the types of reflection of light?
    There are two types of reflection which are: 
  1. Regular Reflection: This happens when light rays fall on a smooth and shiny surface like a mirror and are reflected in one direction at a definite angle. 
  2. Irregular Reflection: This happens when light rays fall on a rough surface and are reflected in many directions or are scattered.
  3. What are the three laws of reflection?
    The three laws are:
  1. The incident ray, the reflected ray, and the normal all lie in the same plane.
  2. The angle of incidence = angle of reflection, i.e. ∠i =∠r.
  3. The ratio of the sine of the angle of incidence to the sine of the angle of refraction or bending is constant, for a given pair of media and a light of given colour. This is called Snell’s Law.
  4. Where do we find a reflection of light in our daily life?
    In our daily life, we can see all objects due to the reflection of light. When light rays from the sun fall on an object, like a sunflower, the pigments in it absorb lights of all wavelengths, except yellow, which is reflected towards our eyes. Thus, a sunflower appears yellow.
  1. What is the 1st law of reflection?
    The first law of reflection is:
    The angle of incidence = Angle of reflection, i.e. ∠i =∠r

To learn more about the reflection of light, check out MSVgo. It is an interactive and engaging video library, which explains concepts with examples and explanatory visualisations and animations, and helps you get to the core of the concept. 

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