Physics & Astronomy
The phenomena of light involved in the appearance of rainbow with red on top and violet on the bottom are dispersion and refraction of light.
The Sun light comprises of photons of different wavelengths. The different wavelengths of photons are dispersed, when incident on the front surface of raindrop, since the refraction angles are different for different colors. Out of seven colors the red light possesses high wavelength, so it deviates more and violet light possess low wavelength, so it refractive least.
When the light is incident on the rain drop, it gets reflected from the back surface of the drop and again gets refracted from the front end of the drop's surface while leaving the drop. The two lights red and violet emerge outside from the raindrop with different angles. Since, red light deviates most relative to the incident angle, it reaches the observer and the drop appears as red. Similarly, the violet light passes over the observer to appear lower in the sky due to least deviation. Hence, a rainbow appears red on the top and violet on the bottom.
The speed of light during its course of travel is defined as the ratio of distance traveled by the light to the time taken. Here, is the distance travelled and is the time taken.
If D is the center to center distance from the Earth to the Moon, is the radius of the Earth, and is the radius of the Moon, then the total distance travelled by the light is, Thus, the speed of the light is, Substitute for D , for , for , and 2.51 s for . Therefore, the speed of the light is .
The light rays bend at the interface of the two media, when it transmits from one medium to another medium. The velocity of light is different in different media and the speed of light is more in rarer medium compared with denser medium. Thus, the light deviates from its path, when it travels from one medium to another medium.
The property of the medium, which describes the how light propagates through it is called index of refraction and it is defined as the ratio of speed of light, c in vacuum to the speed of light, v in medium. The index of refraction is inversely proportional to speed of light. So a higher index of refraction means less velocity and a lower index of refraction means higher velocity. Thus, the ray moves from a material in which it travels slowly to a material in which it travels more rapidly, it bends away from the normal. Hence, the light ray is moving from a material of high index of refraction into a material of lower of refraction bends away from the normal.
The relation between speed of light, v and its wavelength, is, Here, is the frequency.
The speed of light is reduced by a factor of index of refraction, when it transmitted from one medium into another medium. Similarly, the wavelength of light also reduces by a factor of index of refraction, since wavelength is directly proportional to speed of light.
Thus, the wavelength of light increases in the lower index of refraction, when light of wavelength, 600 nm moves from the material with a high index of refraction.
The refractive index varies with the wavelength of light, but frequency of light not affected by refractive index. The frequency of the light doesn't change, when it moves from one medium to another medium. Hence, the frequency remains constant , when the light travels between two media.