स्पष्टीकरण
Reflection of Light at Plane Surfaces
Light is a form of energy that travels in straight lines until it encounters a boundary between different optical media. When light falls on a highly polished, smooth surface, such as a plane mirror, a regular bouncing back of the light rays occurs into the same medium. This phenomenon is known as reflection of light.
To analyse reflection accurately, scientists use standard geometric terms. The ray of light approaching the mirror is called the incident ray, and the point where it strikes the reflecting surface is the point of incidence. The ray that bounces back from the surface is the reflected ray. An imaginary line drawn perpendicularly to the mirror surface at the point of incidence is known as the normal.
All reflection is governed by two strict fundamental laws, collectively known as the laws of reflection. The first law states that the incident ray, the normal to the mirror at the point of incidence, and the reflected ray all lie in the same plane. The second law states that the angle of incidence is always equal to the angle of reflection. The angle of incidence is the angle formed between the incident ray and the normal, denoted by the symbol i. The angle of reflection is the angle formed between the reflected ray and the normal, denoted by the symbol r. Therefore, mathematically, angle i equals angle r. These laws hold true for any flat, smooth surface, ensuring predictable image formation.
Refraction of Light and Speed Changes
When light travels from one transparent medium into another transparent medium, its direction of propagation changes at the boundary separating the two media. This bending of light is called refraction. Common examples include looking at a straight stick immersed obliquely in water, which appears bent at the water surface, or reading text through a glass block placed flat on a page.
Refraction is caused by a change in the speed of light as it passes from one medium to another. Light travels at its maximum possible speed in a vacuum, approximately three hundred million metres per second. When light enters a denser optical medium, such as water or glass, its speed decreases. Conversely, when light emerges from a denser medium into a less dense medium, its speed increases.
When a ray of light travels obliquely from an optically rarer medium, like air, to an optically denser medium, like water or glass, it slows down and bends towards the normal. In this case, the angle of refraction is smaller than the angle of incidence. When a ray travels from an optically denser medium to an optically rarer medium, it speeds up and bends away from the normal. If a ray of light strikes the boundary perpendicular to the surface, it passes straight through without any deviation in its path, even though its speed still changes.
Laws of Refraction and Refractive Index
Refraction is governed by quantitative rules known as the laws of refraction. The first law states that the incident ray, the refracted ray, and the normal to the interface of two transparent media at the point of incidence all lie in the same plane. The second law, commonly known as Snell's law of refraction, states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media. Mathematically, this is expressed as the sine of angle i divided by the sine of angle r equals a constant value, which is known as the refractive index of the second medium with respect to the first.
The refractive index is a dimensionless quantity that expresses how much the optical density of a medium alters the speed of light. The absolute refractive index of a medium compares the speed of light in a vacuum to the speed of light in that specific medium. It is calculated by dividing the speed of light in a vacuum by the speed of light in the medium. Because the speed of light in air is very close to its speed in a vacuum, the absolute refractive index of air is taken as approximately one point zero.
When light passes from medium one to medium two, the relative refractive index is expressed as the ratio of the speed of light in medium one to the speed of light in medium two. A higher refractive index indicates an optically denser medium where light travels more slowly and bends more significantly towards the normal.
Refraction through a Prism and Dispersion of Light
A glass prism is a transparent optical element with flat polished surfaces that refract light. Unlike a rectangular glass slab, a prism has two triangular bases and three rectangular lateral surfaces inclined at an angle called the angle of the prism. When a ray of light enters a prism, it refracts towards the normal upon entering from air to glass, and then refracts away from the normal upon emerging from glass back into air. Because the refracting surfaces are not parallel, the emergent ray is deflected at an angle to the direction of the incident ray, creating an angle of deviation.
When a narrow beam of white sunlight is incident on a glass prism, it splits into a band of distinct colours. This phenomenon of splitting white light into its component constituent colours is known as dispersion. The orderly sequence of colours obtained on a screen is called a spectrum, remembered by the acronym VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, and Red.
White light is composed of different wavelengths of light. Within a glass prism, different colours travel at slightly different speeds. Violet light has the shortest wavelength and experiences the maximum bending, whereas red light has the longest wavelength and experiences the minimum bending. Consequently, each colour emerges at a slightly different angle, producing the visible spectrum.
Atmospheric Refraction and Scattering of Light
The Earth is surrounded by an atmosphere where air density varies with altitude and temperature. As light from celestial objects travels through layers of air with differing optical densities, it undergoes continuous atmospheric refraction. This bending of light explains astronomical phenomena such as the early sunrise and delayed sunset, where the sun remains visible for a few minutes after it has actually moved below the horizon, and the apparent twinkling of stars caused by fluctuating air currents.
When sunlight strikes microscopic particles, gas molecules, dust, and water droplets in the atmosphere, it undergoes scattering of light. Molecules and small particles scatter shorter wavelengths of light much more effectively than longer wavelengths, a principle known as Rayleigh scattering. Because blue light has a shorter wavelength than red light, it is scattered in all directions by atmospheric gases, which is why the clear sky appears blue.
At midday, sunlight travels through a shorter path through the atmosphere, so minimal blue light is lost and the sun appears white. In applications of scattering, danger signals are painted red because red light has the longest wavelength in the visible spectrum and is least scattered by fog, smoke, or rain, allowing signals to be seen clearly over long distances.
