CBSE & NCERT Chemistry Solutions for Class 11 Nature of Light
Chemistry Notes for Class 11 : Visible light, X-rays, and microwaves are some of the types of electromagnetic radiations. All electromagnetic radiations consist of energy propagated by electric and magnetic fields that increase or decrease in intensity as they move through space.
Wave Nature of Light
The wave properties of light (electromagnetic radiation) are described in terms of the following parameters:
- Frequency (ν, spell as mu, in Greek): The frequency of a wave is the number of cycles it undergoes per second. Its unit is 1/second (sec-1, it is also called Hertz, Hz)
- Wavelength (λ, termed as lambda in Greek): The wavelength is the distance between any points on the wave and the corresponding point on the next crest or trough of the wave. It is the distance the wave travels during one cycle. The units of wavelength are meters (m), for very short distances it is nanometers (10-9m), picometers (10-12m) or non-SI unit is Angstrom (Å, 10-10m).
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- Speed: The speed of the wave is the distance it moves per unit time i.e. meters per second (m/s). It is the product of frequency (cycles per second) and wavelength (meters per cycle).
In a vacuum, the electromagnetic radiations move at 3.00x 108m/s and it is constant, represented as the speed of light, c. Thus c = ν x λ
Since c is constant, therefore ν and λ have a reciprocal relationship. The radiations with high frequency have low wavelength and vice versa.
- Amplitude: It is the height of the crest or depth of troughs. For electromagnetic waves, the amplitude is related to the intensity of the radiation. It is the measure of brightness of the visible light. Light of low amplitude means it is less intense and dim, while the light of high amplitude means it is more intense and bright.
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Light as a wave exhibits the phenomenon of-
- Refraction and Dispersion: Light of a given wavelength travels at different speeds through various transparent media- vacuum, air, water, quartz, etc. Therefore, when a light wave passes from one medium to another, the speed of light wave changes, this changes the direction of light and the wave continues at a different angle.This process is known as refraction. The angle of refraction depends on the two media and wavelength of light.
In the process of dispersion, the white light separates (disperses) into its component colors when it passes through the prism or another refracting object, because each incoming wave is refracted at a slightly different angle.
Refraction Dispersion
- Diffraction and Interference: When a wave strikes the edge of an object, it bends around it, the process known as It is defined as the bending of light around the corners of an obstacle or aperture into the region of the geometrical shadow of the obstacle. If the wave passes through the slit about as wide as the wavelength of the light wave, it bends around both the edges of the slit and forms a semi-circular wave on the other side of the opening.
When waves of light pass through two adjacent slits, the nearby emerging circular waves interact through the process of Interference. If the crests of the wave coincide (in phase), they interfere constructively, i.e. the amplitudes add together to form brighter region. If the crests coincide with the troughs (out of phase), they interfere destructively i.e. the amplitudes cancel to form the darker region. This gives rise to diffraction pattern.
Diffraction of pattern
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Particle nature of light
- Photoelectric Effect: The explanation of the photoelectric effect was given by Max Planck in 1900. Max Planck wasn’t actually studying the photoelectric effect himself. Instead, he was studying something known as black-body radiation. Black-body radiation is the light produced by a black object when it is heated up (for example, stove element that glows red when it is turned on). Like the photoelectric effect, scientists couldn’t explain black-body radiation using the wave theory of light either. Max Planck, however, realized that black-body radiation could be understood by treating light like a stream of tiny energy packets (or particles). These packets of energy are now termed as “photons” or “quanta”, and hence, light is quantized.
According to Plank’s quantum theory, the energy of photon is given by-
E = hν, where h = Plank’s constant which is equal to 6.63 x 10-34J.s
Since, c = νλ(explained above, in definition of speed of light)
E = hc/λ
λ = wavelength of the photon
ν = frequency of the photon
Albert Einstein applied the theory of quantized light to the photoelectric effect and found that the energy of the photons, or quanta of light, did depend on the light’s frequency. In other words, all of a sudden Einstein could explain why the frequency of a beam of light and the energy of a beam of light were related.
The particle theory of light explained black-body radiation and the photoelectric effect, but it could not explain double-slit experiment, it could only be explained by the double-slit diffraction experiment. So it was not clear till then weather the light behaved as a wave or as a particle. Hence, light could be either a wave or a particle. In 1905 Albert Einstein (1879-1955) developed a theory stating that light has a dual nature. Light acts not only as a wave but also as a particle. Albert Einstein’s theory is known as the wave-particle duality of light and is now fully accepted by modern scientists.
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Could you make separate topic for black body radiation