Class 10, Science, Chapter-9, Lecture-8, Dispersion (Notes)
DISPERSION OF LIGHT:
The splitting up of white light into its component colours on passing through a transparent medium like a glass prism, due to difference in refractive index for different components, is called dispersion of light.
CAUSE OF DISPERSION OF LIGHT:
Light rays of different colours travel at the same speed in vacuum but with different speeds in a refractive medium.
So the refractive index of different coloured lights for the same medium is different and thus they deviate at different angles.
ANGLE OF DEVIATION:
The angle through which a ray of light bends with respect to the incident ray after passing through a prism is termed as the angle of deviation.
SPECTRUM:
The range of radiations of different wavelengths obtained by dispersion of some light is called the spectrum.
SPECTRUM OF WHITE LIGHT:
The band of seven colours (VIBGYOR) obtained by splitting up of white light on passing through a prism (dispersion of light) is called the spectrum of white light.
MEANING OF VIBGYOR:
VIBGYOR is the acronym for the seven colours of white light in decreasing order of frequency.
V stands for Violet
I stands for Indigo
B stands for Blue
G stands for Green
Y stands for Yellow
O stands for Orange
R stands for Red
FORMATION OF RAINBOW:
The water droplets present in the atmosphere act like small prisms. The light rays entering into the water droplets undergo refraction, internal reflection and refraction.
This causes the dispersion of the white light and different colours reach the observer’s eye from different points.
ATMOSPHERIC REFRACTION:
The refraction of light by the earth’s atmosphere due to change in refractive indices with change in density or temperature of different layers of atmosphere is called the atmospheric refraction.
CAUSE OF ATMOSPHERIC REFRACTION:
The gradual change in refractive indices of different layers of atmosphere due to change in density or temperature of air causes atmospheric refraction.
TWINKLING OF STARS:
The light from a star undergoes continuous refractions as it enters the atmosphere and the star appears at a position different from its original position.
The apparent position of star keeps on changing gradually due to changing physical conditions of the atmosphere and thus the amount of starlight entering the eye flickers.
The combined effect of the fluctuating position of star and the change in light flux causes twinkling of stars.
Planets Do Not Twinkle.
Reason:
The apparent size of planets is bigger than that of the stars because the planets are nearer to us than stars.
This makes planet a collection of large number of point-sized sources of light which nullify the twinkling effect of one another.
Hence, the atmosphere is unable to cause variations in light flux entering our eye from the planet.
The Sun Is Visible to Us About Two Minutes Before the Actual Sunrise.
Reason:
When the sun is slightly below the horizon, the rays coming from the sun undergoes atmospheric refraction and bends. The apparent position of the sun appears to be slightly above the horizon due to bending of light and thus becomes visible to us two minutes before it actually reaches the horizon.
The Sun Is Visible to Us About Two Minutes After the Actual Sunset.
Reason:
When the sun is slightly below the horizon, the rays coming from the sun undergoes atmospheric refraction and bends. The apparent position of the sun appears to be slightly above the horizon due to bending of light, and becomes visible to us even after two minutes of actual sunset.