Light is the form of energy that helps us see the world around us. We see objects because light reflected from them enters our eyes. Light travels in a straight line, and this property is the basis of many phenomena like shadows and eclipses. In this chapter we study the nature of light, how images are formed by mirrors and lenses, and how our eyes work.
We cannot see an object if no light reaches our eyes from it. For example, we can see the moon because sunlight falls on it and is reflected toward us, but we cannot see the moon at night when the sun is on the other side. Similarly, a wall in a dark room cannot be seen until light falls on it. Light itself is invisible; we see it only when it enters our eyes.
When light falls on an object, it bounces back. This bouncing back of light from a shiny or smooth surface is called reflection. We can see ourselves in a mirror because of reflection. The light that comes from our face falls on the mirror and is reflected back to our eyes, and we see an image of ourselves in the mirror.
An image is formed when light rays coming from an object are reflected by a mirror. When an object is placed in front of a plane mirror, the image formed is erect, same in size as the object, and it appears as far behind the mirror as the object is in front of it. The image in a plane mirror is a virtual image because it cannot be obtained on a screen. The image is also laterally inverted, meaning that the left and right sides are reversed; this is why the word written on an ambulance is written in a mirror-reversed manner.
A lens is a piece of transparent material, usually glass, that has one or both surfaces curved. Lenses are of two main types: convex lenses and concave lenses. A convex lens is thicker in the middle and thinner at the edges; it is also called a converging lens because it converges parallel rays of light to a point. A concave lens is thinner in the middle and thicker at the edges; it is called a diverging lens because it spreads out parallel rays of light.
Convex lenses are used in magnifying glasses, cameras, telescopes and in correcting long-sightedness. Concave lenses are used in spectacles for short-sighted people, in flashlights and in some magnifying devices. The image formed by a convex lens can be real or virtual depending on the position of the object, while a concave lens always forms a virtual, erect and diminished image.
The human eye is a natural optical instrument that helps us see objects. Light enters the eye through the cornea, passes through the pupil, which is the black opening in the centre of the iris, and is focused by the lens onto the retina at the back of the eye. The retina contains light-sensitive cells that convert light into electrical signals, and these signals are sent to the brain through the optic nerve, where the image is perceived.
The iris is the coloured part of the eye, and it controls the size of the pupil. In bright light, the pupil becomes small to allow less light in, and in dim light, the pupil becomes large to allow more light in. The lens of the eye changes its thickness to focus objects at different distances, a process called accommodation. We blink our eyes about 15 to 20 times per minute, which keeps them moist and free from dust. The blind spot is the point on the retina where the optic nerve leaves, and no image is formed there. Care must be taken to protect our eyes from injury and to rest them while reading.
The eyes are among the most delicate organs of our body, and we must take good care of them. We should read in proper light - not too bright and not too dim - and hold books at a comfortable distance from the eyes. We should not rub the eyes, and if dust enters the eyes, we should wash them with clean water. If there is any problem like itching, redness or difficulty in seeing, a doctor (ophthalmologist) should be consulted, and suitable spectacles should be worn if prescribed.
Defects of vision like myopia (short-sightedness) and hypermetropia (long-sightedness) are corrected with spectacles using concave and convex lenses respectively. Some people may suffer from night blindness, and reading in dim light can also strain the eyes. Loss of vision is called blindness, and people who cannot see use other senses to live independently; Braille is a system of raised dots that helps blind people read by touch. The eyes should be protected from bright sunlight and sharp objects, and we must avoid looking directly at the sun, as this can damage the retina permanently.
| Term | Meaning |
|---|---|
| Reflection | Bouncing back of light from a surface |
| Image | Picture formed by light rays from an object |
| Virtual image | Image that cannot be obtained on a screen |
| Retina | Light-sensitive screen at the back of the eye |
| Pupil | Opening in the iris that controls light entering the eye |
| Lens | Shape | Use |
|---|---|---|
| Convex | Thicker in middle | Magnifying glass, camera, telescope |
| Concave | Thinner in middle | Spectacles for short-sightedness |
Light is essential for vision, and understanding its properties helps us understand mirrors, lenses and the human eye. Light travels in straight lines and is reflected by surfaces; a plane mirror forms a virtual, erect and same-size image. Convex and concave lenses have different shapes and uses, and the human eye works like a natural camera, focusing light onto the retina. Proper care of the eyes, correct lighting while reading and regular check-ups are necessary to keep our vision healthy. This chapter gives us the foundation of optics and helps us appreciate the gift of sight.