What is Light?
Light rays, also known as visible light, is electromagnetic radiation that can be detected by the human eye. The electromagnetic spectrum as a whole is extremely broad, ranging from low energy radio waves with wavelengths measured in metres to high energy gamma rays with wavelengths less than 1x10-11 metres.
Light can also be described as a stream of photons, which are massless packets of energy that travel at the speed of light with wavelike properties. A photon is the smallest packet (quantum) of energy that can be transported, and the discovery that light travels in discrete quanta was the impetus for Quantum Theory.
Light Definition
Light, also known as visible light, is electromagnetic radiation that falls within the portion of the electromagnetic spectrum that the human eye can perceive. Visible light is typically defined as having wavelengths in the 400–700 nm range, which lies between infrared (which has longer wavelengths) and ultraviolet (which has shorter wavelengths) (with shorter wavelengths).
Basic Properties of Light
Light radiation travels at a speed of approximately 299 792 458 metres per second (m/s) in a vacuum (a container with no air). This is referred to as the speed of light.
The speed of light is the same for all electromagnetic waves.
The wavelengths and frequencies of electromagnetic waves distinguish them from one another (wavelength is inversely related to frequency).
с = f
c = speed of light =3108 in a vacuum = 300,000 km/s = 186,000 miles/s
The electromagnetic spectrum is a continuous representation of all electromagnetic waves organized by frequency and wavelength. Visible light constitutes only a small portion of the electromagnetic spectrum.
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The intensity or brightness of light decreases as the square of the distance from the source increases (inverse square law).
Ray Model of Light-
To predict the path that waves will take, they are frequently approximated as rays. The ray model of light assumes that light moves in a straight line in a vacuum or uniform medium. A ray is a straight line that depicts the path of a very narrow light beam. Ray diagrams are diagrams that show the path of light rays. Despite ignoring the wave nature of light, ray diagrams are useful in describing how light behaves at boundaries (reflection or refraction) and are frequently used to locate the image formed by a mirror or a lens.
The Behaviour of Light at a Boundary-
When a light ray (or any wave) incident on a medium, one of three things can happen. Light can be absorbed by the new medium and converted into internal energy and/or heat, transmitted through the new medium, or reflected into the original medium. In reality, a combination of the three fates is more likely.
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The Three Possible Results of Light Energy when it interacts with Matter/Substances:
Reflected-
Light reflection returns energy to the same medium from which it originated.
Transmitted Through-
Light can pass through the new material with varying degrees of interaction with the molecules of the substance.
Absorbed-
The light energy can be completely absorbed by the substance's molecules and converted to heat.
We Can Classify Substances Based on How Light Interacts With Their Molecules.
Light cannot pass through opaque objects because they absorb and/or reflect all light.
Transparent objects allow light to travel in straight lines through them. Objects can be transparent to certain colours or frequencies of light while being opaque to others. Normal glass is transparent to visible light but opaque to UV and IR light.
As light passes through translucent substances or objects, it scatters in all directions. Visible light can pass through our atmosphere.
Propagation of Light
The process by which an electromagnetic wave transfers energy from one point to another is referred to as light propagation. When light passes between boundaries from one medium to another, three major processes occur; Transmission, Reflection and Refraction.
Polarisation of Light Definition:
Polarization of transverse waves is possible.
The alignment of a transverse wave along a single plane is known as linear polarisation. Light can be linearly polarised by using polarising film or filters to allow only waves vibrating along the axis of the filter to pass through.
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Light's linear polarisation provides experimental evidence that light is a transverse wave. Polarized light can be created by nature through reflection or scattering. The glaring light emitted by roads and bodies of water is horizontally polarised due to reflection.
Polarizing sunglasses eliminate glare by vertically orienting the transmission axis. Polarized light and polarising lenses are used in a variety of situations, such as creating a 3-D image from a specially created flat image, analysing metallic and plastic materials for stress points and weak spots in structures, eliminating glare from water or other vehicles while boating, fishing, or driving, and creating colour in films that either reflects or transmit light.
Optical Nature of Light
Ray Optics-
Reflection of light: The turning back of an electromagnetic wave at the surface of a substance is referred to as reflection. According to the Law of Reflection, the angle of incidence equals the angle of reflection. Angles are typically measured with respect to the normal. At the point of incidence, the normal is a line drawn perpendicular to the surface.
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Types of Reflection-
The size of the irregularities on the surface in relation to the wavelength of the light incident on the surface determines whether rays are reflected in a regular or irregular pattern.
When light strikes a "smooth" surface, the rays are reflected parallel to each other, resulting in specular (regular) reflection.
When light strikes a "rough" surface, the rays are reflected in a variety of directions, resulting in diffuse reflection.
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Laws of Reflection-
First Law: According to the first law of reflection; when a ray of light strikes a mirror and gets reflected back then the angle of incidence is equal to the angle of reflection
I=R
Where,
I: Angle of incidence
R: Angle of reflection
Second Law: According to the second law of reflection the incident ray, reflected ray, and the normal lie on the same plane on the surface of reflection.
Refraction of Light:
Refraction of light is the phenomenon of bending a wave when it enters a medium with a different speed. When light passes from a fast medium to a slow medium, the light ray bends toward the normal to the boundary between the two media. Snell's Law describes the amount of bending as a function of the indices of refraction of the two media.
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The Amount of Bending is Determined by Two Factors:
Change in Speed – If a substance causes light to refract (bend) more, it will refract (bend) more.
The Angle of the Incident Ray – the amount of refraction will be more noticeable if the light enters the substance at a greater angle. If, on the other hand, the light enters the new substance from the side (at 90° to the surface), it will slow down but not change direction.
Index of Refraction-
The index of refraction (refractive index) is defined as the difference between the speed of light in a vacuum and the speed of light in the medium.
n = cv
n = Index of refraction
c=velocity of light in vacuum
v= velocity of light in the medium
The Refractive Index of Some Transparent Substances are-
Light slows down when it enters a substance with a higher refractive index (for example, from air into glass). The light bends in the direction of the normal line.
When light enters a substance with a lower refractive index (such as water into the air), it accelerates. The light deviates from the straight line.
Light will slow down and change direction more as it enters the substance with a higher refractive index.
Snell’s Law
Snell's Law describes the relationship between the indices of refraction ‘n’ of two media and the propagation directions in terms of angles to the normal. Fermat's Principle or the Fresnel Equations can be used to derive Snell's law.
\[\frac{n_{1}}{n_{2}}\] = \[\frac{Sinθ_{2}}{Sinθ_{1}}\]
Application of Light Waves
Light is used in various industries such as medical, automobile, manufacturing and scientific research. It is mainly used in telescopic equipment used for research purposes.
Uses of Light-
Cleaning robots detect obstacles.
Atmospheric observations are made using laser beams
Early detection of cancer and dementia.
Multifunction sensors for smartphones.
Exploring the interior of objects without destruction
Product manufacturing.
Eyesight correction
FAQs on Light
1. What is light according to the CBSE Class 12 Physics syllabus, and how is it classified in the electromagnetic spectrum?
Light is a form of electromagnetic radiation that is visible to the human eye, typically with wavelengths between 400 and 700 nanometres. In the electromagnetic spectrum, light lies between infrared (longer wavelengths) and ultraviolet (shorter wavelengths), making it the only part of the spectrum we can naturally see.
2. Explain the three main ways light interacts with different types of substances, as per the NCERT Physics curriculum.
Light can interact with substances in the following three main ways:
- Reflection: Light bounces back into the original medium.
- Refraction: Light changes direction when passing into a new medium with a different refractive index.
- Absorption: Light energy is absorbed and converted into heat or internal energy within the material.
3. How does the speed of light change when it moves from one medium to another, and what is the significance of the refractive index?
The speed of light decreases when it enters a medium with a higher refractive index and increases in a medium with a lower refractive index. The refractive index quantifies how much the light slows down and bends in a particular medium, which is critical for understanding phenomena like refraction and total internal reflection.
4. What is Snell's Law and how is it used to calculate the direction of a refracted light ray?
Snell's Law relates the angles of incidence and refraction for light passing through two different media. It is given by: n1 sin θ1 = n2 sin θ2, where n is the refractive index and θ is the angle with the normal. This law helps determine the path of light during refraction.
5. State the laws of reflection and explain their significance with respect to ray diagrams in physics.
The two laws of reflection are:
- The angle of incidence equals the angle of reflection.
- The incident ray, the reflected ray, and the normal to the surface all lie in the same plane.
These laws form the basis for constructing ray diagrams, which help predict image formation in mirrors and optical devices.
6. What is polarization of light and what does its occurrence confirm about the nature of light waves?
Polarization is the alignment of vibrations of light waves along a single plane. It proves that light is a transverse wave, not a longitudinal one, as only transverse waves can be polarized. This principle is used in sunglasses, stress analysis in materials, and 3D movie technology.
7. How does light propagation differ in transparent, opaque, and translucent materials?
- Transparent materials let light pass through clearly, allowing objects to be seen clearly (e.g., glass).
- Opaque materials absorb or reflect all light, blocking vision (e.g., wood).
- Translucent materials scatter light in various directions, only allowing partial visibility (e.g., frosted glass).
8. Why does the angle of refraction not always match the angle of incidence, and what factors determine the direction in which the light bends?
The angle of refraction differs from the angle of incidence due to the change in the speed of light when entering a medium with a different refractive index. The amount and direction of bending depend on:
- The difference in refractive indices of the two media
- The angle at which light strikes the boundary
9. What is meant by the emission and absorption spectra of light, and how do they arise in the context of atomic structure?
Emission spectra are created when electrons in an atom fall from higher to lower energy levels, emitting light at specific frequencies. Absorption spectra occur when electrons absorb energy and jump to higher levels, leaving dark bands in the spectrum due to certain light frequencies being absorbed.
10. In what ways is the study of light and its properties important in real-life applications and technology?
Understanding light enables innovations in fields such as:
- Optical instruments (microscopes, telescopes)
- Medical diagnostics (endoscopy, laser surgery)
- Communications (fiber optics)
- Industries (barcode scanning, quality control)
- Day-to-day devices (cameras, eye correction lenses, screens)
11. How does the ray model of light simplify the study of optical phenomena, and what are its limitations?
The ray model treats light as straight lines (rays) to predict paths and image formation efficiently. It works well for mirrors and lenses, but ignores wave effects like interference and diffraction, so it cannot explain all optical phenomena.
12. What is the inverse square law with respect to light intensity, and how does it explain brightness with distance?
According to the inverse square law, the intensity or brightness of light decreases as the square of the distance from the source increases. This means that doubling the distance from a light source makes the light appear just one-fourth as bright.
