Rectilinear Propagation Of Light


When someone projects something onto a screen, we love to make shadow animals with our hands. We can contort our hands to make a giraffe shadow or a cat shadow or whatever animal we know how to make. This experience actually tells us something: it tells us that light travels in straight lines, which is rectilinear propagation. It is actually a very unique phenomenon.

Motion of projectiles

Think of a football being thrown through the air or an archer launching an arrow at a target 50 meters away. The football and the arrow are called projectiles. And, to arrive accurately at their target, they do not travel in straight lines.

If the archer aimed straight at the bull's eye, the arrow would actually strike below the bull’s eye because gravity pulls it downwards the entire length of its path. The same happens with the thrown football or any mass launched through the air. Light is completely different. If a laser (light beams of the same wavelength that are all aligned) points straight at the bull's eye from the same distance as the archer, it will hit the bull's eye without having to aim above it like the archer has to do with his arrow.


Reflection is when light bounces off an object. If the surface is smooth and shiny; like glass, water or polished metal, the light will reflect at the same angle as it hit the surface. This is called specular reflection.

Types of reflection

Light reflects from a smooth surface at the same angle as it hits the surface. For a smooth surface, reflected light rays travel in the same direction. This is called specular reflection. For a rough surface, reflected light rays scatter in all directions. This is called diffuse reflection.

Diffuse reflection is when light hits an object and reflects in different directions. This happens when the surface is rough. Most of the things we see are because light from a source has reflected off of it.

For example, if you look at a bird, light has reflected off that bird and travelled in nearly all directions. If some of that light enters your eyes, it hits the retina at the back of your eyes. An electrical signal is then passed to your brain and your brain interprets the signals as an image.

The angle at which light hits a reflecting surface is called the angle of incidence, and the angle at which light bounces off a reflecting surface is called the angle of reflection.

If you want to measure these angles, imagine a perfectly straight line at a right angle to the reflective surface (this imaginary line is called ‘normal’). If you measure the angle of incidence and the angle of reflection against the normal, the angle of incidence is exactly the same as the angle of reflection. With a flat mirror, it is easy to show that the angle of reflection is the same as the angle of incidence.

Water is also a reflective surface. When the water in a lake or sea is very still, the reflection of the landscape is perfect, because the reflecting surface is very flat. However, if there are ripples or waves in the water, the reflection becomes distorted. This is because the reflecting surface is no longer flat and may have humps and troughs caused by the wind.

It is possible to make mirrors that behave like humps or troughs, and because of the different way they reflect light. They can be very useful.

Spherical mirror

When the mirror is a part of a sphere, it is called spherical mirror. Spherical mirrors are of two types. In concave mirror, the reflective surface is inside the sphere, i.e. it is depressed. In a convex mirror, the reflective surface is outside the sphere, i.e. it is bulged or protruded.



Image formation in concave mirror: In the case of a concave mirror, the image is usually inverted, real and smaller than the object. When the object is kept too close to the concave mirror; the image is erect, virtual and larger than the object.

Uses of concave mirror:

  • Concave mirror is used by dentists and ENT specialists to focus a beam of light to see inside a patient’s mouth or ears.
  • Concave mirror is used in solar furnaces, because rays of sunlight converge at a point once they reflect from the concave mirror.
  • Concave mirror is used as a barber’s mirror, because it shows a larger image when the object is too close.

Image formation in Convex Mirror:

 In case of convex mirror, the image is erect, virtual and smaller than the object.

Uses of Convex Mirror:

  • Convex mirror is used in rear view mirrors, as it shows smaller images from a bigger field of view.
  • Convex mirror is used on hairpin bends, to see the vehicles coming from other side of the bend.


An image may be defined as that point, where the light rays coming from an object meet or appears to meet after reflection or refraction.

In this definition the word ‘object’ may be defined as anything which gives out light rays. The objects can be of two types: (1) very small objects and (2) large objects or extended objects. The small objects are represented by a dot in a ray diagram, while the large objects can be represented by an arrow pointing in upward direction.

Types of Images

The images are of two types:

  1. Real images
  2. Virtual images

Real Images

A real image is that image which is formed when the light rays coming from an object actually meet each other after reflection or refraction. A real image can be obtained on the screen. The real image is always inverted. The common example of real image is the image formed on the cinema screen.

  Virtual Image

A virtual image is that image which is formed when the light rays coming from an object do not actually meet, but appear to meet when produced backwards. These images cannot be obtained on the screen. The virtual image is always erect. The common example of virtual image is the image formed in the mirror when we stand in front of that mirror.

Components of white light

The white light, or visible spectrum, is composed of seven colors. These colors are VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange and Red). When white light passes through a prism, it breaks down into its component colors. This is the reason rainbows show all the colors of the visible spectrum.


Newton’s Disc: Since Newton was the first to make this; it is called a Newton’s Disc. It is a circular disc on which seven colors of the rainbow are painted on seven different sectors. When the disc is rotated at certain speeds, it appears to be white. This shows how the seven colors make the white light.