Light is a form of energy. It helps in the sense of vision. When light falls on an object, some of the light is reflected back to our eyes. Thus, we are able to see an object because of light being reflected from the object.
Reflection of Light
When light falls on a shiny surface, most of it bounces back. This phenomenon is called the reflection of light.
Laws of Reflection
There are two laws of reflection which are as follows:
First Law of Reflection: The incident ray, the reflecting ray and normal at the point of incidence; all lie in the same plane.
Second Law of Reflection: Angle of incidence is always equal to the angle of reflection. In the given figure; the angle of incidence is shown by blue colour and the angle of reflection is shown by red colour.
Formation of Image in plane Mirror
In this figure; PQ is a plane mirror and O is an object. OA and OC are the incident rays; coming from the object. The incident rays are reflected from the mirror and reflected rays emerge as AB and CD. If AB and CD are extended behind the mirror, they appear to meet at point ‘I’. This is the point where the image is formed. If OM is object distance and MI is image distance, then in the plane mirror; OM = MI. This means image distance is equal to object distance in case of a plane mirror.
Some features of the image formed by a plane mirror:
- An image is always formed behind the mirror.
- It is a virtual image; which means it cannot be obtained on the screen.
- An image is upright and laterally inverted.
- Image size and distance are equal to object size and distance.
When all the reflected rays are parallel to each other; this case of reflection is called regular reflection. Regular reflection happens from a smooth surface, e.g. a mirror. We get to see clear images when the image is formed by regular reflection.
When the reflected rays are not parallel to each other; this case of reflection is called irregular reflection. Irregular reflection happens from a rough surface. The laws of reflection are obeyed in this case as well but because of the irregularities in the surface, the reflected rays are not parallel. We get to see somewhat blurred images when the image is formed by irregular reflection. The relative clarity of an image depends on the relative smoothness of the reflecting surface.
When two or more mirrors are placed at some angles to each other, we get to see multiple images. Let us take an example in which two mirrors are placed opposite to each other. If an object is placed between them, its image is formed in both the mirrors. The image in one mirror would act as an object for another mirror and this sequence would continue. This will result in the formation of multiple images.
The number of images formed depends on the angle between the two mirrors. This can be calculated by using the following formula:
Number of Images = (360/Angle) – 1
So, if the given mirrors are at a right angle to each other, 3 images will be formed. If the given mirrors are at 30° angle, we shall get 11 images. When the mirrors are kept opposite and parallel to each other, there would be an infinite number of images formed.
Eyes are the sense organs which give us the sense of vision. The human eye is almost spherical in shape. Following are the main structures in the human eye.
Cornea: The eyeball is covered with a tough layer. This layer is transparent on the front. This transparent portion is called cornea.
Iris: Iris is a thin circular structure. It works like the shutter of a camera. It controls the amount of light entering the eye. The colour of iris imparts distinct colour to the eyes of an individual. A person with blue iris has blue eyes.
Pupil: There is a hole in the centre of iris. This is called pupil. Light enters the eye through the pupil. When the light is bright, iris contracts and thus allows less light into the eye. When the light is dim, iris dilates and thus allows more light into the eye.
Lens: Lens is present behind the pupil. The lens in the human eye is a convex lens. The size and thickness of the lens change as per the distance of an object.
Retina: Retina is at the back of the eye and marks the inner layer of the eyeball. Images are formed on the retina and thus it works like a screen. There are photosensitive cells in the retina. These cells are of two types, which are as follows:
- Cone Cells: The cone cells are sensitive to bright light. They also give the sense of colour.
- Rod Cells: The rod cells are sensitive to dim light.
Optic Nerve: The optic nerve emerges from the back of the eyeball. This nerve goes to the brain.
Blind Spot: The junction of the retina and the optic nerve is called the blind spot. There is no photosensitive cell at this spot, and hence no image is formed at this spot on the retina.
Persistence of Vision
An image stays on the retina for about 1/16 of a second. This feature is called persistence of vision. Due to this, when many still images are shown in a sequence; they give the illusion of moving images. Movies and animation are made by exploiting this property of the human eye.
Care of Eyes
Eyes are the very important organ and they are sensitive too. So, proper care of eyes is very important. Some tips for care of eyes are as follows:
- Do not read in too bright or too dim light.
- Do not look directly at a bright object or at the sun.
- Do not keep the book too close to your eyes; while reading. Don’t keep the book too far either.
- If something gets into the eye, do not rub the eye. Wash it with cold water.
- In case of any problem; like itching or burning sensation; consult an ophthalmologist. A doctor who specializes in the disease of eyes is called an ophthalmologist.
Some of the vision defects are as follows:
Myopia: A person suffering from myopia finds it difficult to see a distant object. This disease is also called short-sightedness or near-sightedness. Spectacles with suitable lenses can improve the vision in a myopic person.
Hypermetropia: A person suffering from hypermetropia finds it difficult to see a nearby object. This disease is also called long sightedness. Spectacles with suitable lenses can improve the vision in a hypermetropic person.
Cataract: The cornea becomes cloudy in a cataract patient and thus the person cannot see clearly. Cataract is treated by surgery. In cataract surgery, the cornea is cleaned and an artificial lens is transplanted in the eye.
Visually Challenged Person:
Some people face with the disability of vision. This disability can be partial or complete. Such persons are called visually challenged persons. For a visually challenged person; life can be very difficult. These people usually show a marked development of other senses; like the sense of hearing and sense of touch. Many aids have been devised to make their life easy. They can be divided into two categories, viz. optical and non-optical aids.
Optical Aids: Optical aids can help a person who is partially visually challenged. These aids enlarge an image or a text so that they could be visible. TV monitors, magnifying devices and telescopic devices come under this category.
Non-optical Aids: Non-optical aids are helpful for a person who is completely visually challenged. These aids rely on the senses of hearing and touch. Aids which rely on the sense of touch are called tactual aids. Tactile buttons on the pedestrian light and in public transport are examples of tactual aids. Tactile strips at the edge of the platforms are also meant for visually challenged persons. Even the currency notes have tactile markings so that a visually challenged person can recognize notes of different denominations.
In some countries; specially trained guide dogs are pressed into the service of a visually challenged person.
Braille Script: The Braille scripts are written in the form of raised dots so that a text can be read by the visually impaired and challenged ones. This system is composed of 63 characters. A reader needs to touch the Braille script to read it.