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Wave Theory of Light states that a source of light sends out disturbances in all directions. These waves, when carrying energy, reaches the eye, excite the optic nerves, and produce the sensation of vision. Light or noticeable light is electromagnetic radiation inside the piece of the electromagnetic range that is seen by the human eye. Noticeable light is normally characterized as having frequencies in the scope of 400–700 nanometres (nm), between the infrared and the bright regions.
| Table of Contents |
Key Terms: Diffraction, Single Slit Diffraction, Young’s Single Slit Experiment, Fringe Width, Huygens' Guideline, Interference of Light Waves, Electromagnetic radiation, Light
What is Wave Theory?
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Wave theory of light is the arrangement of light as an electromagnetic wave. In this hypothesis, light is addressed as a range of frequencies, some of which we can see (apparent light) and some of which we can't (bright and infrared light)".
Christian Huygens defined the principal point-by-point wave hypothesis of light, with regards to which he was likewise ready to determine the laws of reflection and refraction.
According to him, light is an unsettling influence from the source that movements that as longitudinal mechanical waves through an ether medium as mechanical waves require a mode for its engendering. Afterwards, the presence of ether in all spaces ended up being incorrect. Henceforth, this hypothesis couldn't clarify the proliferation of light through spaces.
Frequently Asked Questions About Wave Theory of LightQues. State Max Planck’s proposition about the nature of light. (1 mark) Ans. Max Planck claimed and proposed that light is composed of finite packets of energy which is also known as a light quantum. It usually depends on the frequency and velocity of light. Ques. State Albert Einstein’s conclusion about the nature of light. (1 mark) Ans. Albert Einstein, in the year 1905, proposed that light consisted the characteristics of both particle and wave. |
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History of Wave Theory of Light
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Light always has triggered the curiosity of thinkers as well as scientists. Sir Isaac Newton introduced the fact that light was made of tiny particles called photons while Christian Huygens assessed the fact that light was made of waves moving perpendicular to the direction of its movement.
It was in the year 1678, that Huygens proposed that every point that a luminous disturbance meets converts into a source of the spherical wave itself. And, what form will the new wave take is determined by the sum of the secondary waves, which are the consequences of the disturbance. This theory of light is called the ‘Huygens’ Principle’.
Wave Theory
Huygens was successful in deriving the laws of reflection and refraction of light with the help of the above-stated principle. Moreover, He also successfully explained the linear and spherical propagation of light.
Although, he failed to explain the Diffraction effects of light. Later, in 1803, the experiment conducted by Thomas Young on the interference of light proved that the Huygens wave theory was correct. In 1815, Fresnel provided mathematical equations for Young’s Experiment.
Electromagnetic Wave Propagation
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The spread of an electromagnetic wave, which has been created by a releasing capacitor or a wavering sub-atomic dipole, is represented in the figure given below. The flash current wavers at a frequency (ν), which is a trait of the circuit. The electromagnetic aggravation that outcomes are spread with the Electric (E) and Magnetic (B) vectors vibrating oppositely to one another and furthermore to the direction of propagation (Z).
The connection among frequency and wavelength is given by the situation:
⇒ f = \(\frac{1}{T}\) (Hz)
And,
⇒ f = \(\frac{c}{\lambda}\) (I)
The speed of light in a vacuum is a widespread steady that is,
⇒ c = 3×108 m/s. (II)
- Electromagnetic fields oppositely sway to the heading of wave venture out and are opposite to one another. Because of this, they are known as Transverse Waves.
- While managing light waves, we manage the sine waveform. The time of the waveform is one full 0 to 360-degree clear.
- Light waves have two significant qualities known as frequency and wavelength.
- The distance between the pinnacles of the wave is known as the frequency. On account of a light wave, the frequencies are in the request for nanometers.
- Wavelength is the number of waves that will cross beyond a point in a second.
As proposed by Einstein, light is made of small parcels of energy known as photons. The formula developed by Planck decides the energy of a photon and it likewise shows that the energy is corresponding to the frequency of the light i.e.
| E = hf |
Where, h is Planck's constant = 6.63×10−34 Joule-Second.
- The electric field in an electromagnetic wave vibrates with its vector power. At a similar recurrence, the attractive field moves opposite to the electric field.
- The electric and attractive vectors, mirroring the abundantly and the vibration bearings of the two waves, are situated opposite to one another and to the heading of wave spread.
- The speed of the subsequent electromagnetic wave can be found in the connections characterizing the electric and attractive field associations.
- Maxwell's conditions demonstrate that speed rises to the speed of light in a vacuum (c; equivalent to 300,000 kilometres each second) isolated by the square base of the dielectric consistent (ξ) of the medium occasions the attractive penetrability (μ) of the medium.
Hence,
⇒ v=c/(ξ*μ)1/2 (1)
For most materials that happen in living cells (some of which are non-leading), the attractive penetrability is equivalent to a worth of solidarity, so that:
⇒ v=c/(ξ)1/2 (2)
Exactly, the speed of light is known to be contrarily relative to the refractive record (n) of the material through which it spreads, in this way:
⇒ υ = \(\frac{c}{n}\)(3)
From conditions (2) and (3), the end can be drawn that the refractive file is equivalent to the square foundation of the dielectric consistent of that material if the estimations are made at a similar recurrence:
⇒ N(v) = (ξ)1/2 (4)
Condition (4) unveils that optical estimations are, truth be told, estimations of the electrical properties of the material. The dielectric properties, thus, straightforwardly mirror the spatial three-dimensional game plan of particles and atoms that characterize the construction of a substance.
Electromagnetic Wave Diagram
The vector depicting the cooperation between an electromagnetic field and a substance lies in a similar way as the electric vector. This is valid whether or not the electric or attractive vectors are thought of, on the grounds that what is important is the impact of the electric or attractive fields on the electrons in the material medium (the attractive field influences those electrons that move in a plane opposite to the attractive field).
Things to Remember
- Robert Hooke developed “Pulse Theory” and compared the spreading of light to the waves of water in 1665 and in 1672, he suggested that light’s vibrations are perpendicular to the direction of propagation whereas Christian Huygens worked out on the mathematical theory of light in 1978.
- The spread of an electromagnetic wave, which has been created by a releasing capacitor or a wavering sub-atomic dipole. The flash current wavers at a frequency (ν), which is a trait of the circuit. The electromagnetic aggravation that outcomes are spread with the Electric (E) and Magnetic (B) vectors vibrating oppositely to one another and furthermore to the direction of propagation (Z).
- Imp Formulas are: f = \(\frac{c}{\lambda}\); E=hf; v=c/(ξ*μ)½; v=c/(ξ)½; υ = \(\frac{c}{n}\); N(v) = (ξ)½
- At a similar recurrence, the attractive field wavers opposite to the electric field. The electric and attractive vectors, mirroring the abundantly and the vibration bearings of the two waves, are situated opposite to one another and to the heading of wave spread.
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Previous Year Questions
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- At the first minimum adjacent to the central maximum of a single-slit diffraction… [NEET 2015]
- In a diffraction pattern due to a single slit of width 'a'… [NEET 2016]
- In a Young' double slit experiment if there is no initial phase difference… [NEET 2019]
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- 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]
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Sample Questions
Ques: Answer the accompanying questions: (2 marks)
(a) If the earth didn't have a climate, would its average surface temperature be higher or lower than what it is currently? Clarify.
(b) An EM wave applies tension on a superficial level on which it is occurring. Legitimize?
Ans: a) Average surface temperature will be lower. Since there will be no nursery impact without the climate.
b) Since electromagnetic waves convey both energy and force, accordingly, they apply tension on a superficial level on which they are episode.
Ques: A plane electromagnetic flood of recurrence 25 MHz goes in free space along the x-course. At a specific point in existence, E=6j V/m. What is B now? (3 marks)
Ans: Now
B0=E0/c=2×10−8 T
To discover the bearing, we note that E is along the y-heading and the wave spreads along the x-pivot. In this manner, B ought to be toward a path opposite to both x-and y-tomahawks. Utilizing vector variable-based math, E X B ought to be along x-heading. Since,
(j+k)=i, B is along the z-heading. Hence,
B=2×108k T
Ques. In a plane E.M wave, the electric field sways sinusoidal at a frequency of 2*1010 Hz and wavelength 48 V/m: (3 marks)
(a) what is the frequency of a wave?
(b) What is the sufficiency of the swaying attractive field
(c) Show that the normal energy thickness of the E field rises to the normal energy thickness of the B field.
Ans: a)Wavelength is given by
lambda ={3* 108}/{2* 1010 = 1.5* 10-2 m
b) B0 = E0/c = 48/3×108 =1.6×10-7 T
c) Average energy thickness of E is given by
u(E)=1/4∈0*E02
The normal energy thickness of B is given by
u(B)=1/4μ0*B02
Presently, E0 = cB0 or c2 = 1μ0 ∈ 0
Subsequently
u(E) = 1/4∈0E02 = 1/4∈0c2B02 = 1/4.∈0.1μ0*∈0.B02 = 1/4μ0*B02 = u(B)
Ques: How do the electric and attractive fields of an e.m. wave have constancy during the progress either from one medium to another or inside its medium? (4 marks)
Ans: During the time spent on radiation one can pleasantly imagine that the fields are "independent "from the sources (electric or attractive dipole).In an exceptionally worked on the way you can contend along these lines: You turn on a dipole receiving wire and the field is spreading out into space and afterward a short second after the fact you rearrange the extremity and in a semi methodology, the field would return the to radio wire and afterward alter. Yet, the piece of the field which is now far enough away doe not know (when you upset the extremity) that it was rearranged and is "drove away" =forced to transmit from the all-around modified field. You don't require sources like electrons for the EM field of a wave just on the grounds that the field lines are constantly shut circles.
Ques: If the electron is a wave, then, at that point what is its inclination? Is it an electromagnetic wave? (3 marks)
Ans: All subatomic particles have wave work. Then again, when we say that a photon has both a wave nature and molecule nature, the wave idea of a photon is most certainly electromagnetic in nature. The molecule idea of the photon is interlinked with the way that it's anything but a ceaseless wave yet must have an on and off condition. We typically consider that a "wave bundle", yet what's difficult to accommodate with traditional electromagnetic is that you can't actually make a burst waveform that doesn't have higher-request sounds in the Fourier range.
Ques: Why doesn't the way of an electromagnetic wave mutilate when It goes through a solid either electric or attractive field? (1 mark)
Ans: Because electromagnetic power fields apply powers on electrical charges, not on other power fields. Electromagnetic waves are without charge.
Ques: Huygen’s wave theory of light could not explain ___________. (1 mark)
(a)Refraction
(b)Reflection
(c)Interference
(d)Photoelectric effect
Ans: The correct option is d. Photoelectric effect.
Ques: According to Huygen’s wave theory, every point on the wavefront behaves as a source of: (1 mark)
(a)Stationary waves
(b)Secondary waves
(c)Surface waves
(d)Beats
(a)Provides us the magnifying power of a microscope.
(b)Allows us to find the focal length of a thick lens.?
(c)Is a geometrical method to find a wavefront.
(d)Is used to determine the velocity of light.
Ans: The correct option is c. Is a geometrical method to find a wavefront.
Ques: State the demerits of Huygen’s wave theory of light. (4 marks)
Ans: The demerits of Huygen’s wave theory of light are:
- The rectilinear propagation of light can not be explained by this theory.
- It can not explain the Compton effect, photoelectric effect, Raman effect, and so on.
- Also, it fails to explain the propagation of light through a vacuum as ether has a high elastic constant and zero density which gives contradictory results.
- Based on Huygen’s theory, luminiferous ether medium is present everywhere in the universe and even in the vacuum which is treated as a material medium for the propagation of light waves. However, Michelson’s and Morley’s theories did not approve of the existence of ether medium.
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