Light

Light has the reflection property; it can be reflected from all kinds of surfaces. This Reflection of light which comes from the objects on which it falls, enters our eyes, due to which we can see the object.

Reflection:

A glass polished on one side is called a mirror when the rays of light fall on it, the direction of the rays are changed.

•  The change in the direction of light rays is called Reflection of light.
•  A light ray is termed an incident ray when it strikes a surface.
• The ray which returns from the surface struck by it is called the reflected ray.
• A line is making a 90° angle with another line perpendicular to it, representing a mirror. An incident ray strikes on the meeting points of these two lines. Where the incident ray strikes, the mirror is known as the normal to the reflecting surface at that point, as shown in the figure given below.
• An angle of incidence is formed by a normal ray and incident ray.
• The angle of Reflection is formed by a normal and a reflected ray.

Law of Reflection:

Following are two laws of reflection:

• The angle of incidence and the angle of reflection are always certain to be equal.
• Incident ray, reflected ray and the normal ray lie in the same plane when drawn at the same point of incidence.

There is lateral inversion in the image which is formed by any mirror which means the image is just opposite to the real object.

Type of Reflection: Following are two main types of reflection:

• Regular reflection: It takes place when light is incident on smooth, polished, and regular surfaces. Images are formed by regular reflection.
• Diffused or irregular reflection: Such reflection happens from rough surfaces. . Incident parallel rays reflected from a rough or irregular surface are not parallel.

Human Eye:

Eye is an important organ. Objects are seen when light coming from them enters our eyes.

 Structure of an Eye: An eye consists of parts such as a cornea, iris, pupil, lens, retina, and optic nerve.

• An eye has a roughly spherical shape.
• The outer coat of the eye is white. The front part of the eye is called the cornea which is transparent. 
• Behind the cornea, a dark muscular structure is present called iris.
• Pupil is a small opening which is located in the iris. Iris controls the size of the pupil. Iris gives our eyes their distinctive colour. Not only this but also the iris has control over the amount of light which enters the eye.
• Beside the pupil of the eye is a lens which is thicker in the centre.
• Retina is a layer which is found on the back of the eye, the lens focuses light on it. It consists of many nerve cells.
• Sensations felt by the nerve cells and these are transmitted to the brain through the optic nerve. There are two kinds of cells:

1. Cones: These are sensitive to bright light. Cones sense colour.
2. Rods: These are sensitive to dim light.

The junction of optic nerve and retina is known as blind spot because no sensory verb is located here. 

• The impression of an image persists there just for the time period of 1/16th of a second. Any object moving at a rate faster than 16 per second is perceived by the eye as moving.
• Eyelids are provided with the eyes to prevent any object from entering the eye. It shuts out light when it is not required.

Eyes of the Animals:

Animals have eyes shaped in different ways:

1. Eyes of a crab are quite small, however they made  the crab look all around. 
2. Some of the insects like butterflies which possess large eyes that appear consisting of thousands of little eyes inside them. They can see in all directions, front, back and sides.
3. An owl is a species of bird which can clearly see at night but not in day.
   1. The owl is featured with a large cornea and a large pupil to allow more light in its eye.
   2. Also, it has on its retina many rods and only a few cones.
4. Daylight birds such as eagles, parrots can see well in the day  but not at night. More cones and fewer rods are found in them. 
5. Normal Human eye and its defects: A normal eye can see nearby and distant objects clearly. 25 cm is the most suitable distance from which anyone can read comfortably with normal eyes.  However, there could be error in vision resulting into a defective eye:
6. Some persons cannot see distant or near or both objects. With suitable corrective lenses, such defects of the eye can be corrected. 
7. In old age as the eye lens becomes cloudy which leads to foggy eyesight. It is known as Cataract. A new artificial lens is inserted to correct such defects.
8. Lack of Vitamin A is responsible for many eye defects such as night blindness. Vitamin A rich sources such as carrots, green vegetables, cod liver oil, etc. must be a part of diet.
9. Some persons can be visually impaired (limited vision) or blind (no vision) since birth or by injury.
10.  Visually impired people de ko pe their other senses more sharply and thus they can catch the things soon using non-optical aids.
11. Visual aids, tactual aids, auditory aids  and electronic aids come under non-optical aids.

• Visual aids: These can magnify words, providing suitable light intensity.
• Tactual aids: These include Braille writer slate and stylus.
• Auditory aids: These include cassettes, tape recorders, talking books, etc.
• Electronic aids: These include talking calculators and computers, audio CDs, etc.
• Bifocal lenses, contact lenses, tinted lenses, magnifiers, and telescopic aids come under optical-aids.

 Braille System:

Visually impaired persons can read and write using the Braille system. Louis Braille developed this system in 1821. The system in its present format was adopted in 1932.

• There is Braille code for common languages, mathematics, and scientific notation. Many regional languages of India can be read using the Braille system.
• The Braille system consists of sixty three dot characters.
• In the Braille system each dot represents a letter, or combinations of letters or any grammatical signs etc.
• In braille systems dots behave like alphabetical characters arranged in cells of two vertical rows consisting of three dots each.
• Patterns of dots to represent some English letters and some common word as shown below: 

• These patterns are embossed on Braille sheets and help visually challenged persons to recognise words by touching. Dots are raised slightly for convenience.

Blind people learn with the Braille system which begins with letters. The letters are followed by special characters and the special characters are followed by letter combinations. The learning system is based on touching  characters. 

In conclusion: 

Reflected light is responsible for nearly everything you see around you. The moon, for example, receives and reflects sunlight. That is how we perceive the moon. Objects that shine in the light of other objects are referred to as illuminated objects.