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Transmission, absorption or reflection happen when a light wave falls incident on an object. Light is the only electromagnetic radiation that can be seen by the naked eye. It is made up of photons, which are little packets of energy. In this article, we will discuss the wave nature of light to understand transmission, absorption and reflection of light along with illustrations to understand the concepts better.
Keyterms: Transmission, Absorption and Reflection of Light, Explanation, Wave Nature Of Light, Electromagnetic waves, Huygen’s Wave Theory, Light, Photon, Energy, Human Eye
Wave Nature Of Light
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Light is a transverse, electromagnetic wave that the average person can see with naked eyes. Like any electromagnetic wave, light obeys:
c = f λ
Where,
c → 3 × 108 m/s is the speed of light in a vacuum
f → Frequency of the electromagnetic waves
λ → Wavelength.
Visible Light Spectrum
The visible wavelength range is from 380 nm to 760 nm. When light interacts with objects that are several times larger than its wavelength, it travels in straight lines and behaves like a ray, as all waves do. When it interacts with smaller items, it shows off its wave properties.
Characteristics of a light wave
- In all reference frames, the speed of light in a vacuum is a universal constant.
- The intensity of a light wave is related to its amplitude.
- The colour of a light wave is related to its frequency.
- A light wave's wavelength is inversely proportional to its frequency.
Characteristics of a Light Wave
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Huygen’s Wave Theory
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Christiaan Huygens (1629–1695) released Traité de la Lumière in 1678, in which he asserted that light is a wave. Light is a type of disruption, according to this idea. The medium particles vibrate in a direction that is perpendicular to the direction of disturbance propagation.
Huygen’s Wave Theory
Principle of Huygens’ Construction
- It highlights the notion that every point on a wavefront is a secondary wavefront source.
- The new wavefront is defined by the envelope of these wavefronts, i.e. the surface tangent to all secondary wavefronts.
Reflection Of Light
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When the inherent frequency of electron vibrations differs from the frequencies of the incident light, light is reflected. When a light wave strikes an object in this way, the object's electrons begin to vibrate. The energy is re-emitted as a light wave after the electrons oscillate for a short time with a small amplitude.
Reflection of Light
Reflection is determined by the smoothness or texture of the surface. When surface defects are smaller than the incident light's wavelength (as in a mirror), nearly all of the light is reflected evenly.
The surface of smooth water in the lake, where incident light is reflected in an organised manner to generate a clear image of the surroundings surrounding the lake, is the most basic example of visible light reflection. When you throw a pebble into a lake, it causes the water to ripple, disrupting the reflection by spreading the reflected light rays in all directions.
Reflection of Light
Laws of Reflection
- The first law of reflection states that the incident ray, the reflected ray, and the normal to the surface of the mirror, all lie in the same plane.
- The second law of reflection states that the angle of reflection is equal to the angle of incidence. Both angles are measured with respect to the normal to the mirror.
∠i = ∠r
Where,
∠i → Angle of Incidence
∠r → Angle of Reflection
Laws of Reflection
Total Internal Reflection
When light travels from an optically denser medium to a rarer medium at the interface, it is partly reflected back into the same medium and partly refracted to the second medium. This reflection is called internal reflection.
Refraction of Light
The formula of Total Internal Reflection is given by,
n1 / n2 = sin r / sin i
Where,
- r → Angle of Refraction
- i → Angle of Incidence
- n1 → Refractive index in medium 1
- n2 → Refractive index in medium 2
The two requirements of total internal reflection are as follows:
- The light ray passes through a rarer medium to a denser medium.
- The incidence angle must be larger than the critical angle.
Transmission Of Light
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When the light passes through an object, it produces this effect. The vibrations of the electrons are passed on to the adjoining atoms and re-emitted from the opposite side of the item if the object through which light is travelling is transparent. The light is said to be transferred in these instances.
Transmission of Light
Because blue light is transmitted while all other colours are absorbed, you see the colour blue light in the fish tank.
Absorption Of Light
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The electrons in an atom tend to vibrate at specific frequencies. When an atom is struck by a light wave with the same natural frequency, the atom's electrons are set into vibrational motion. When electrons interact with neighbouring atoms during vibration, the vibrational energy is converted to thermal energy. As a result, we can deduce that selective light absorption happens when the frequency of the light matches the frequency of the electrons in the atoms. Because different atoms and molecules have distinct inherent vibration frequencies, they will absorb different frequencies of the visible light selectively.
Absorption of Light
Our Perception Of Colour
The interaction of the object with the incident light is largely responsible for the colour of the objects we see. The colour of an object is not contained within the object. It is in the light that shines upon it that it is found. When a specific frequency of light falls on an object, the light frequencies that are absorbed never reach our vision. The colour of an item is determined only by the frequencies of incident light that are reflected or transmitted. If an object absorbs all of the light wave's frequencies except the one associated with red, the object appears red.
Applications of Absorption of light in real life
- Global and local temperatures are influenced by the absorption of radiation by atmospheric gases (such as the greenhouse effect) and land and ocean surfaces in meteorology and climatology.
- X-rays are absorbed to varying degrees by different tissues (particularly bone) in medicine, which is the basis for X-ray imaging.
Xray Imaging
- Sunglasses, coloured filters, dyes, and other optical materials are designed with specific regard to which visible wavelengths they absorb and in what proportions.
Previous Year Questions
- In a YDSE bi-chromatic light of wavelengths 400 nm and 560 nm are used… [JEE Advanced]
- In a hall, a person receives direct sound waves from a source 120m away… [BHU UET]
- Two light sources are said to be coherent if they are obtained from… [VITEEE 2010]
- The Brewsters angle ib for an interface should be… [NEET 2020]
- Unpolarized light of intensity I is incident on a system of two polarizers, A followed by B…? [JEE Mains 2018]
- Light of wavelength 550nm falls normally on a slit of width 22.0×10−5cm. The angular position…? [JEE Mains 2018]
- In an interference experiment, the ratio of amplitudes of coherent waves is…? [JEE Mains 2019]
- Diameter of the objective lens of a telescope is 250cm. For light of wavelength 600nm…? [JEE Mains 2019]
- An observer is moving with half the speed of light towards a stationary microwave source…? [JEE Mains 2017]
- In Young's double slit experiment, the slits are 2mm apart and are illuminated… [NEET 2013]
- Young's double slit experiment is first performed in air and then in a medium… [NEET 2017]
- When the angle of incidence on a material is 60∘… [KEAM]
- The ratio of the intensities of two waves is 16:9… [KEAM]
- In Young’s double slit experimental setup… [KEAM]
- A monochromatic source of wavelength 60nm was used in Young’s… [KEAM]
- A narrow slit of width 2mm is illuminated by monochromatic light of wavelength… [KEAM]
- A slit of width a is illuminated by red light of wavelength… [KEAM]
- Huygen's wave theory of light could not explain… [KEAM]
- In a double slit experiment, the screen is placed at a distance of… [KEAM]
- In a Fraunhofer diffraction at single slit of width d with incident light… [KEAM]
Things To Remember
- Our eyes respond to visible light, which is an electromagnetic wave. In a vacuum, c = fλ, where c = 3 × 108 m/s is the speed of light, f is the frequency of the electromagnetic wave, and λ is its wavelength in a vacuum.
- Light travels in a medium in the shape of wavefronts, according to Huygen's Principle. A wavefront is a point where all particles vibrate at the same time. Every particle on a wavefront acts as a secondary light source, emitting secondary wavelets.
- Electromagnetic radiation is reflected at the boundary between two media (surface reflection) or at the interior of a medium (volume reflection), whereas transmission is the transit of electromagnetic radiation through a medium.
- Absorption is the conversion of radiant energy into another type of energy, most commonly heat, through interaction with matter.
- Light is reflected and absorbed by all objects. Some emit visible light of their own, but the majority of them are visible because they reflect light into our eyes. The colour of an object is determined by the wavelengths of light it absorbs and which wavelengths it reflects.
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Sample Questions
Ques. Why does Sun appear red at sunrise and sunset? (2 marks) (CBSE 2016)
Ans. It is due to the least scattering of red light as it has the longest wavelength.
As per Rayleigh’s scattering, the amount of light
scattered ∝ \(\frac{1}{\lambda^4} ]\)
Ques. (i) State the principle on which the working of an optical fibre is based.
(ii) What are the necessary conditions for this phenomenon to occur? (2 marks) (CBSE 2016)
Ans. (i) Working of an optical fibre is based on the principle of total internal reflection.
(ii) (a) Light should travel from a denser to a rarer medium.
(b) Angle of incidence should be more than critical angle given by ic = sin-1 (\(\frac{1}{\mu}\)).
Ques. What colour would a blue shirt appear when the lights are turned off and the room is entirely dark? (2 marks)
Ans. When the room lights are turned off, any object present in the room appears black. The colour of the object depends on the frequency of the light that is reflected in the observer’s eye. Without light, there is no reflected light as a result of which the green shirt appears black.
Ques. Unpolarised light is an incident on a plane glass surface. What should be the angle of incidence so that the reflected and refracted rays are perpendicular to each other? (2 marks)
Ans. For i + r to be equal to π/2,
we should have tan iB = µ = 1.5
This gives iB = 57°.
This is Brewster’s angle for the air to glass interface.
Ques. What are the two kinds of reflection observed? (2 marks)
Ans. The reflection of light can be roughly categorized into two types of reflection. Specular reflection is defined as light reflected from a smooth surface at a definite angle, whereas diffuse reflection is produced by rough surfaces that tend to reflect light in all directions
Ques. How are we able to see objects around us? (2 marks)
Ans. We are able to see objects in front of us due to the light reflected by them. When light falls on an opaque object, which is an object that does not allow light to pass through it, so these objects have absorption zones present in them which allows them to absorb most of the wavelength falling on them and reflect other wavelengths which are absorbed.
Ques. What is Huygen’s Principle? (2 marks)
Ans. Huygens’ principle is essentially a geometrical construction, which gives the shape of the wavefront at any time, allows us to determine the shape of the wavefront at a later time.
According to Huygens’ principle,
(i) Every point on a wavefront behaves like a light source and emits secondary wavelets.
(ii) The secondary wavelets spread in all directions in space (vacuum) with the velocity of light.
(iii) The envelope of the wavefront of secondary wavelets, after a given time, along forwarding direction gives the new position of the wavefront.
Ques. Prove Laws of reflection using Hyugen’s Principle. (5 marks)
Ans. As shown in the figure below, consider a plane wavefront AB incident on the reflecting surface XY, both the wavefront and the reflecting surface being perpendicular to the plane of the paper.
First, the wavefront touches the reflecting surface at B and then at the successive points towards C. In accordance with Huygens’ principle, from each point on BC, secondary wavelets start growing with the speed c. During the time the disturbance from A reaches point C the secondary wavelets from B must have spread over a hemisphere of radius
BD = AC = ct,
where t is the time taken by the disturbance to travel from A to C.
The tangent plane CD drawn from the point C over this hemisphere of radius ct will be the new reflected wavefront. Let angles of incidence and reflection be i and r, respectively.
In AABC and ADCB, we have
∠BAC = ∠CDB [each is 90o]
BC = BC [common]
AC = BD [each is equal to ct]
∴ ΔABC ≅ ΔDCB
Hence, ∠ABC = ∠DCB
or, i = r
i.e. the angle of incidence is equal to the angle of reflection. The proves the first law of reflection.
Further, since the incident ray SB, the normal BN and the reflected ray BD are respectively perpendicular to the incident wavefront AB, the reflecting surface XY and the reflected wave front CD (all of which are perpendicular to the plane of the paper) therefore, they all lie in the plane of the paper i.e. in the same plane. This proves the second law of reflection.
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