Optician’s Guide (A Manual for Opticians) Ajay Kumar Bhootra
Chapter Notes

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LightChapter 1

Light is a form of radiant energy, arising out of a source and always travels in a straight line. A straight line represents the path of a ray of light. A few such rays taken together is called the beam of light. It is not possible to produce a single ray of light. However the small source may be, it always produces a beam of light, consisting of several rays.
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Fig. 1.1: Parallel rays
A beam of light may be of three different types (Figs 1.1 to 1.3):
  1. Parallel.
  2. Divergent, and
  3. Convergent.
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Fig. 1.2: Covergent rays
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Fig. 1.3: Divergent rays
2In a parallel beam of light the rays are parallel to one another (Fig. 1.1). Rays of light coming from a very distant source, are regarded as Parallel. When the rays of light are produced from a point and are spread out is called the Divergent rays (Fig. 1.3) and when it travels towards a point is called Convergent rays (Fig. 1.2).
When the light falls upon the retina, it stimulates certain highly specialised cells, which produces nerve impulses. These produce perception of light, colour, form, size and motion. It has been found that light travels 1,86,000 miles per second in the air and takes about 8.3 minutes to reach the earth from the sun.
A substance or any portion of space through which light can pass is called Optical Medium. All the objects or substances around us are either self-luminous or non-luminous. A body is called self-luminous when it emits light by itself. It may be used as a source of light, such as sun, burning candles, etc. Bodies which are not self-luminous are not visible unless light falls upon them and is scattered in all directions. Each point then behaves like a luminous source. Most of the objects around us are non-luminous, such as table, chair, etc.
Non-luminous bodies may be transparent, translucent and opaque. A transparent body is one which transmits the light according to the straight line law whereas a body is opaque when it does not transmit the light at all. There are substances which transmit the light but not in straight line. They are called translucent bodies. Light in passing through them is scattered: Translucent bodies may be used as screen on which optical images are projected.
When the rays of light passes from one medium to another, some of the light is reflected or bent back to the original medium. The phenomenon is known as reflection of light.
3If the ray of light falls on a very smooth and polished surface, all the reflected rays will travel in a definite direction, keeping a relation with the incident rays like in the Figure 1.4.
The reflection occurs in accordance with the following two laws:
  1. The incident ray, the reflected ray and the normal ray or the ray perpendicular to the reflecting surface – all lie in the one plane.
  2. The angle of incidence is equal to the angle of reflection.
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Fig. 1.4: Reflection of light
The Figure 1.5 shows the reflection of a single ray of light. Here AO is the incident ray which is reflected along OB by a plane mirror MN. The ray AO is the direction in which the light falls on the surface. O is the point of incidence and OB is the reflected ray. PO is the normal ray which is perpendicular to the surface MN at the point of incidence. The angle made by the incident ray with the normal ∠AOP is called the angle of incidence which is equal to the ∠BOP which is the angle of reflection.
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Fig. 1.5: Laws of reflection
4If the surface is not plane, the reflected rays will have various directions and the light will be scattered. But at each point the law of reflection is obeyed. When the surface is rough or matt, the reflected light is scattered or diffused.
When a ray of light travels from one medium to another, it changes its path. The change in direction of light is known as refraction.
Suppose a ray of light traveling along AB in the air falls obliquely on the block of glass. The ray will now enter into the glass block. But the direction of the AB now takes the path BC (Fig. 1.6).
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Fig. 1.6: Refraction of light
When a ray of light passes from rare medium (i.e. air) to a dense medium (i.e. glass) it bends towards the normal and when it passes from a dense medium to rare medium, it bends away from the normal. But when a ray of light from one medium falls normally on another, it passes straight into the second medium without any change of its direction.
The velocity of light is different in different media. It is greater in optically rare medium and less in optically dense medium. So when a light passes from a rare medium to a dense medium its velocity reduces and vice versa. For this reason the ratio of velocity of light in air to that in a given medium is taken as the refractive index {(μ) pronounced as mew} of the medium (air being taken as standard).
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Example 1: To find the refractive index of crown glass, (if the velocity of light through it is 1,22,000 miles per second):
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Refractive index of crown glass = 1.52.
Example 2: To find the refractive index of flint glass (if the velocity of light through it is 1,13,000 miles per second):
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Refractive index of flint glass is 1.65.