# Light

## Introduction to Reflection

• The balancing of light from a surface is called reflection.
• Or
• The to and fro movement of light in the same medium after striking an opaque surface is called reflection.
• The amount of light that is reflected depends on the material and nature of the surface on which light falls
•  An opaque body absorbs some of the light that falls on it and reflects the remaining light.
•  A transparent body allows almost all the light to pass through it and reflects very little light.
•  A smooth and a highly polished surface such as a shiny stainless steel surface or a mirror reflect almost all the light that falls on it.

## Reflection of light on a plane surface

•  A plane polished surface capable of regularly reflecting almost all the light falling on it is known as a plane mirror.
•  A ray of light which falls on the mirror is called incident ray.
•  The ray which is sent back to the original medium is called the reflected ray.
•  The perpendicular to the reflecting surface at the point of incidence is called the normal.
•  The angle made by the incident ray with the normal of the point of incidence is called the angle of incidence.
•  The angle made by the reflected ray with the normal at the point of reflection is called angle of reflection.

## Laws of Reflection

When reflection takes place, it follows certain rules which are known as the laws of reflection.
The laws followed during reflection are as follows-

### First law

The incident ray reflected ray and the normal at the point of incidence lie in the same plane.

### Second law

The angle of incidence is always equal to angle of reflection, that is
<i = <r

## Formation of image by a plane mirror

•  The characteristics of image formed could vary depending on the type of mirror used.
•  The characteristics of an image formed by plane mirror are as follows-
•  The image formed is erect.
•  The image is of same size as the object.
•  The image is laterally inverted, that is, the right appears as the left side of the image.
•  The image is as far behind the mirror as the object is in front of it.
•  The image formed is virtual, that is, it cannot be caught on a screen

## Regular and diffused Reflection

Reflection of light is of two kinds depending on the nature of the reflection and diffused or irregular reflection.

### Regular reflection

When the reflecting surface is very well polished and smooth, the light that falls on it is evenly reflected in only one direction. This phenomenon is known as regular reflection.

### Diffused reflection

If the reflecting surface is irregular, the ray of light that falls on it are scattered in all directions. This is called irregular or diffused reflection.

## Multiple reflections

Take two plane mirrors and place them such that they make an angle between them. Place an object between the mirrors. You can see several images in both the mirror. This is because the image formed by one mirror act as the object for the second mirror. This phenomenon is referred to as multiple reflections.
The number of image formed depends on the angle between the mirrors.
The number of images formed is found using the formula.
n = (360/O) -1
n = number of images
o = angle between the two mirrors.

## Instrument Based on Multiple reflection

### Kaleidoscope

This instrument works on the principle of multiple reflections. It consists of three plane mirrors inclined at an angle of 60o to each other.
The arrangement is kept in a cylindrical cardboard base with a few pieces of coloured glass at one end. The mirrors form multiple images of these glass pieces, resulting in beautiful pattern when seen from the other end. On rotating the cylindrical box, the pattern change continuously as the position of glass piece shifts.

### Periscope

It is another instrument which uses the principle of multiple reflections. This instrument also needs two plane mirrors that are placed at an angle of 45? to the vertical. In a periscope mirrors are placed at a distance from each other. The image formed by one mirror acts as the object for the other mirror. The main use of Periscope is in submarines that remain under water.

## Refraction of Light

• The change of direction of light rays when they pass from one optical medium into another is called refraction of light.
•  A transparent substance is called optical medium.
•  There are certain rules that are followed by light during refraction.
•  When light travels from a rarer (eg. Air) to a denser (eg. Glass) medium it bends towards the normal.
•  When light travels from a denser to a rarer medium, it bends away from the normal.
•  When light travel along the normal there is no deviation that is the ray suffers no refraction.

## Refraction of light through a Prism

A prism is a transparent solid body, often having triangular bases. When three rectangular surface that it forms two triangular faces at opposite ends, they form prism. The triangular faces are called the principal faces of the prism.

## Dispersion

• The splitting up of light into its constituent colours is called dispersion. Sunlight is a mixture of light of several colours. It is known as white light. When white light passes through a prism, dispersion takes place because rays of different colours are deviated through different angles by a prism. Violet is deviated the most and red the least.
•  The band of colours obtain because of dispersion of light is called a spectrum.
•  The spectrum of sunlight consists of violet, indigo, blue, green, yellow, orange and red.

## Rainbow

The rainbow is a very beautiful natural spectrum on a large scale. It is formed in the direction opposite to the sun. When there is moisture in the air the tiny droplets of water acts as prisms and disperse sunlight.

## Human Eye

The eye is natural optical instrument that forms an image of objects on a screen called the retina. This enables us to see to objects.
The front part of the eye is called the cornea and is made up of a transparent material.
Behind the cornea is the iris the coloured parts of the eye.
In the middle of the iris is a hole called the pupil? It appears black because no light is reflected from it.
The iris adjusts the amount of light entering the eye through pupil.
Behind the iris is a convex lens. The eye lens is held in position by a pair of muscles called ciliary muscles.
The lens forms an Image of an object on the retina which is at the back of the eye.
The retina is covered by a large number of nerve cells sensitive to light which carry the impression to the brain by means of optic nerve.
There are two kinds of nerve cells on retina-
Cones – that sense colour & are sensitive to bright light.
Rods – which are sensitive to dim light

## Power of accommodation of Eye

• The ciliary muscles alter the thickness of the lens enabling the eye to see far and near objects equally clearly.
• The ability of eye to adjust itself to see objects at different distances clearly is called the power of accommodation of the eye.
•  The least distance at which eye can clearly see an object is called the near point or the least distance of distinct vision.
•  The far point of the eye is the maximum distance to which the eye can see. For a normal eye it is at infinity.

### The blind spot

There is one point on the retina where the nerve fibers enter the optic nerve. The image falling on this part of the retina cannot be seen and hence it is known as the blind spot.

### Persistence of Vision

To see and object, its image must be formed on the retina by the eye lens. As soon as the object is removed, the image disappears but the brain remembers the image for 1/10th of a second even after the removal of object. This effect is called Persistence of vision.

## Defects of vision and their correction

The two common \defects of the eye are short- sightedness or myopia, and long – sightedness or hyper- metropia or hyperopic.

### Myopia (Short – sightedness)

•  Myopia is a defect of the eye in which distant objects cannot be seen clearly but nearby objects can be seen clearly.
•  To correct this defect, a diverging or concave lens is used to create an extra divergence so that the images get formed on the retina.

### Hypermetropia (Long- sightedness)

•  Hypermetropia is a defect of the eye in which distant objects can be seen clearly by the eye but nearby objects cannot be seen distinctly.
•  This defect is because the focal length of the eye lens too long or the eyeball is too short. As a result, nearby objects are brought to focus at a point behind the retina.
•  The defect of Hypermatropia may be corrected by using a converse or converging lens since the rays require an extra convergence to meet at a lesser distance.

### Cataract

• This is another defect of the eye generally occurring at old eye. In a person suffering from cataract, the eye lens becomes cloudy leading to loss of vision.
•  Cataract can be treated by surgically removing the eye lens and inserting in its place a new artificial lens.

### Braille

•  The Braille system is a method of writing for the blind.
•  It uses six raised dots. The positions of different dots represent the different letters of the alphabet.
•  Visually challenged people can read them by feeling them with their fingers. The method was developed by Louis Braille of Paris.

### Eye care

• A diet rich in Vitamin A, selenium and vitamin C and E help to maintain good eyesight and to prevent to delay development of cataract.
•  Do not read in very dim or very bright light; avoid reading in a moving vehicle.
•  Avoid watching TV or working on a computer for long sessions.