Hello, and welcome to today's lesson on Light Energy. I'm delighted to have you with me as we explore one of the most fascinating topics in physics. Today, we will discover how light travels, how it bounces off surfaces, how mirrors create images, and how the beautiful colours around us are formed. Let us begin this illuminating journey together.
Light is a form of energy that gives us the sensation of sight. Without light, we would live in complete darkness, unable to see the world around us. Light travels in straight lines, a property called rectilinear propagation. Because of this, light casts shadows when it hits opaque objects, and we can predict exactly where light will go.
Light can travel through empty space. In fact, it reaches us from the sun across millions of kilometres of vacuum. Materials that allow light to pass through easily are called transparent media. Glass, clear water, and air are all transparent. But light behaves differently when it hits a shiny surface. It bounces back, and this bouncing back of light is what we call reflection.
Reflection is the process where light strikes a surface and returns into the same medium. Imagine throwing a ball against a wall. It rebounds back to you. Light behaves similarly when it hits a polished surface like a mirror. Different surfaces reflect light to different extents. A highly polished mirror reflects almost all the light that falls on it, while a transparent object reflects very little. Black objects absorb all light and reflect none, which is why they appear dark.
A plane mirror is made by silvering one side of a thin glass plate. The silvered surface is coated with an opaque material to protect it. Light enters through the glass side and reflects from the silvered surface behind. This is why you always see your reflection from the front of the mirror, not through the back.
To understand reflection properly, we need to learn some important terms. First, the incident ray, this is the ray of light that travels toward and strikes the mirror surface. The point where it hits is called the point of incidence. The reflected ray is the light that bounces back from the surface.
The normal is a line drawn perpendicular to the mirror surface at the point of incidence. It is an imaginary reference line that helps us measure angles. The angle of incidence, represented by the letter i, is the angle between the incident ray and the normal. The angle of reflection, represented by the letter r, is the angle between the reflected ray and the normal. Both angles are always measured from the normal, not from the mirror surface.
Now we come to one of the most important principles in this chapter, the laws of reflection. These laws govern how light behaves when it strikes any reflecting surface.
First law: the angle of incidence is always equal to the angle of reflection. In symbols, ∠i = ∠r. This means if light hits a mirror at thirty degrees to the normal, it reflects at thirty degrees on the other side.
Second law: the incident ray, the reflected ray, and the normal all lie in the same plane. This means all three are flat together, like lines drawn on a single sheet of paper. They do not twist out of alignment with each other.
Here is a special case to remember. When light strikes a mirror normally, that is, perpendicular to the surface, the angle of incidence is 0°, not ninety degrees. Therefore, the angle of reflection is also 0°. The light simply retraces its path and comes straight back.
Let us see how a plane mirror forms an image. When an object is placed in front of a mirror, light rays from every point on the object strike the mirror and reflect according to the laws of reflection. These reflected rays enter our eyes, but our brain traces them backward. The point where these backward extensions appear to meet is where we see the image.
Images formed by plane mirrors have several distinct characteristics. First, the image is virtual, meaning it cannot be captured on a screen placed behind the mirror. Second, the image is erect, or upright, not upside down. Third, the image is exactly the same size as the object. Fourth, the image is as far behind the mirror as the object is in front of it.
If you stand two metres from a mirror, your image appears two metres behind it.
Fifth, and most interestingly, the image shows lateral inversion. This means the left side of the object becomes the right side of the image, and vice versa. If you raise your right hand, your mirror image raises its left hand. This is why the word AMBULANCE is written in reverse on emergency vehicles, so drivers see it correctly in their rear-view mirrors.
There are two types of reflection we should distinguish. Regular reflection occurs from smooth, polished surfaces like mirrors. Parallel light rays remain parallel after reflection, giving a clear, sharp image.
Irregular or diffused reflection occurs from rough surfaces like walls, paper, or wood. Here, parallel light rays scatter in many different directions. This is actually why we can see objects around us. If all surfaces were perfectly smooth like mirrors, we would only see images, not the objects themselves.
Plane mirrors have many practical uses in daily life. We use them as looking glasses for grooming. Opticians use them to make their examination rooms appear longer. Barbers use two facing mirrors to show the back of your head. Plane mirrors are essential components in periscopes, kaleidoscopes, and solar cookers. They also serve as rear-view mirrors in vehicles, though slightly curved mirrors are often preferred for a wider field of view.
Now let us talk about how fast light travels. Light is incredibly fast. In vacuum or air, its speed is approximately three times ten to the power of eight metres per second. In symbols, 3 × 10⁸ m/s. This equals three lakh kilometres per second. This is the maximum speed light can achieve. That is about three hundred million metres every second, or roughly three lakh kilometres per second. At this speed, light could circle the earth more than seven times in one second.
However, light slows down when it passes through transparent materials. In water, the speed drops to two point two five times ten to the power of eight metres per second. In symbols, 2.25 × 10⁸ m/s. In glass, it slows further to two times ten to the power of eight metres per second. In symbols, 2 × 10⁸ m/s. When light moves from air into water or glass, it slows down. When it emerges back into air, it speeds up again to its maximum velocity.
White light from the sun is actually a mixture of many colours. When passed through a glass prism, it separates into a beautiful spectrum. The seven prominent colours can be remembered by the word VIBGYOR: violet, indigo, blue, green, yellow, orange, and red.
Among these, three are special. They are called primary colours. These are red, green, and blue. These three colours are fundamental because they cannot be created by mixing other colours of light. However, when combined in equal intensities, they produce white light.
Secondary colours are formed by mixing two primary colours. Red plus green gives yellow. Green plus blue gives cyan, sometimes called peacock blue. Blue plus red gives magenta, also known as purple. Your television and computer screens use this principle. They contain tiny dots of red, green, and blue phosphors that combine in various ways to create all the colours you see.
Finally, let us understand why objects appear coloured. When white light falls on an opaque object, the object reflects some colours and absorbs others. The colour we see is the colour of light that reaches our eyes after reflection.
A red rose appears red because it reflects red light and absorbs all other colours. A leaf appears green because it reflects green light and absorbs the rest. White objects reflect all colours equally, while black objects absorb all colours and reflect none.
This explains some interesting observations. If you shine green light on a red rose, it will appear black. Why? Because the rose absorbs green light and has no red light to reflect. Similarly, a red rose seen in red light appears bright red because it reflects that red light strongly.
Let us quickly recap the key points from today's lesson. First, light travels in straight lines and can reflect off surfaces. Second, the laws of reflection state that the angle of incidence equals the angle of reflection, and that the incident ray, reflected ray, and normal all lie in one plane. Third, plane mirrors form virtual, erect, same-sized images that are laterally inverted and located as far behind the mirror as the object is in front. Fourth, light travels at 3 × 10⁸ m/s in air or vacuum, but slows down in water and glass. Fifth, red, green, and blue are primary colours that combine to form white light, while yellow, cyan, and magenta are secondary colours formed by mixing primary colours in pairs. Sixth, the colour of an object depends on which colours of light it reflects and which it absorbs.
I hope this journey through light energy has brightened your understanding of how we see the world. Light is all around us, reflecting and revealing the beauty of our universe. Keep observing, keep questioning, and keep exploring. Until next time, stay curious and keep shining.