light- part 2
TRANSCRIPT
Gurudatta K Wagh
Standard/ Class/ Grade - X SSC, CBSE; - VIII ICSE
Light – Part 2
Summary
Refraction of lightDefinition, Why does the direction of light rays change?
Appreciation of refractionRefraction of light through a glass slab
Laws of refraction
Refractive index (n)Rare to dense, Dense to rare, Special effect, Mirage, Hot and cool air
Twinkling of starsPlanets, Sunrise and sunset,
Dispersion of lightRainbow
Scattering of lightTyndall effect, Scattering of light through atmosphere, Blue sky and red signal, Blue sky and dark signal, Sunrise and sunset
Refraction of light
Light travels along a straight line path in a transparent medium.Light does not travel in the same direction in all media.
Refraction When light travels from one medium to another obliquely, the direction of propagation of light in the second medium changes.
Definition The phenomenon of change in the direction of light when it passes from one transparent medium to another.
Why does the direction of light rays change?The direction of light rays changes when it enters from one medium to another as the velocity of light is different in different media.
Appreciation of refractionRefraction of light through a glass slab Refraction of light takes place twice in the glass slab.
Due to this the ray emerges parallel to incident ray.
But the light ray is shifted slightly to the left side.
Following are the reasons.
Laws of refraction
1. The incident ray and the refracted ray are on the opposite sides of the normal at the point of incidence and all three lie in the same plane.
2. For a given pair of media, the ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is constant.
sin i/ sin r = constant (n)constant (n) = refractive index of the second
medium with respect to the first medium
Refractive index (n)
Values of ‘n’ are given with respect to vacuum
‘n’ depends upon the relative speed of propagation of light in different media
If v1 is the velocity of light in medium 1 and v2 in medium 2 then ‘n’ of the 2nd medium with respect to the 1st is given by the ratio of the magnitude of velocity of light in medium 1st to that in medium 2nd and is represented as 1n2 = v1/v2
Similarly n of 1st medium with respect to 2nd is2n1 = v2/v1
If medium 1st is vacuum or air then ‘n’ of medium 2nd is considered with respect to vacuum. It is the absolute ‘n’ of medium and represented as n2
When the ray of light moves from an optically rarer medium to an optically denser medium, the ray bends towards the normal, e.g. from air to glass.
Rare to dense‘n’ of the 2nd medium is greater than the ‘n’ of the 1st. Greater the value of ‘n’, the more is the bending of refracted ray towards normal. ‘i’ > ‘r’
Dense to rare‘n’ of the 1st medium is greater than the ‘n’ of the 2nd. Greater the value of ‘n’, the lesser is the bending of refracted ray away from normal. ‘i’ < ‘r’
Special effectWhen the ray is incident normal to the interface between any two media in any way i.e. rarer to denser or denser to rarer, then ‘i’ = 0 and ray of light propagates undeviated from one medium to another.
MirageIt is observed on a very hot summer afternoon. It is the apparent random wavering of the hot air rising above the heated roads.
The air just above the fire becomes hotter than the air further up. The hot air is lighter and rarer than the cool air, which is denser, above it. ‘n’ of hot air is less than cool air.
As the physical conditions of air are not stationary, the apparent position of object fluctuates. It is the effect of atmospheric refraction on a small scale in our local environment.
Hot and cool airAt the time of holi festival, we see flickering of an object through a turbulent stream of hot air rising above the holi fire.
During refraction of star light from atmosphere as star light bends towards normal, the apparent position of stars is slightly higher than its actual position.
This apparent position of the stars isn't stationary but changes slightly.
Twinkling of starsThe large scale effect is of twinkling of stars. The stars emit their own light at night. The stars are point sources of light as they are very far away.
Because of the mobility of the air and in the temperature, the atmosphere is not steady. Hence ‘n’ of air in the given region goes on changing continuously and randomly.
When the atmosphere refracts more light towards us, the star is seen bright. When the atmosphere refracts less lights towards us the star is seen dim.
Thus due to change in ‘n’ of atmosphere, stars appear twinkling at night.
PlanetsThe rays of light from a planet also pass through the atmosphere of earth. But planets do not twinkle.
As compared to stars, planets are nearer to the earth. So a planet can be considered as a collection of a large number of point sources of light.
If the intensity of light from one point source decreases it increases from the other source. Hence average intensity remains the same. Therefore planets do not twinkle.
Sunrise and sunsetAdvanced sunrise and delayed sunset can also be explained in the basis of atmospheric refraction.
The observer should see the sun when it reaches the horizon; but it is seen two minutes before that. As a ray of light from the sun enters the earths atmosphere it follows a curved path due to refraction before reaching the observer.
Hence advanced sunrise and delayed sunset increases duration of day by 4 minutes.
It appears to the observer as if the rays are coming from the position where the sun is seen by the observer.
Hence the sun is seen earlier before the sun reaches the horizon.
Dispersion of lightSir Isaac Newton was the first to use a glass prism to obtain the spectrum of sunlight.
A prism is a transparent medium bound by two plane surfaces inclined at an angle. When white light is dispersed into seven colours by a prism, different colours of light bends through different angles with respect to incident ray.
Out of these seven colours, red light bends the least while violet light bends the most. So the rays of each colour emerge along different paths and become distinct. Hence we get a spectrum of seven different colours.
RainbowThe beautiful phenomenon of the rainbow is a combination of different phenomena - dispersion, refraction and reflection of light.
The water droplets act as small prisms.
When sunlight enters the water droplets, they refract and disperse the incident sunlight. Then they reflect it internally inside the droplet and finally again refract it.
Scattering of light
Tyndall effect When light incident on very small particles in the solution in a medium is scattered, we can see the path of light.
Scattering of light When a ray of light is incident on extremely small particles, the particles deflect the light in different directions.
When a parallel beam of light is incident on a plane polished surface like a mirror, the angle of incidence is the same for all the rays and so we get a parallel beam after reflection of this light.
But when the same parallel beam of light is incident on a rough surface like bricks, walks or ground, the angles of incidence are different for different rays of the beam. Hence the reflected rays travel in different directions leading to scattering of light.
Scattering of light through the atmosphere The atmosphere is a homogeneous mixture of minute particles like dust, smoke, tiny water droplets and molecules of air. When the sunlight passes through a canopy of dense forest, scattering of light can be observed.
Particles of larger size can scatter white light also. In the visible range of light, maximum scattering of blue light and least scattering of red light take place.
As red light is scattered the least by atmosphere, it can travel larger distance. Hence danger signals are red coloured.
Blue sky and red signal
The size of scattering particles plays an important role related to colours of scattered light. Very fine particles mainly scatter blue light.
Blue sky and Dark skyBlue colour of sky is related to scattering of light through atmosphere. The molecules of air and other particles of atmosphere have very fine size as compared to the wavelength of visible light. When sunlight passes through the atmosphere, these fine particles scatter light of different wavelengths in the same proportion of wavelengths of respective colours. The scattered resultant light enters or eyes and the sky appears blue.
When astronauts fly at higher altitude, the same sky appears dark to them. This is because there is no atmosphere at high altitude and so the scattering of light does not take place.
Sunrise and sunsetEven though the sky looks blue, it appears reddish or orange at sunrise or sunset. This is related to scattering of light.
At the time of sunrise or sunset, the sun is very close to the horizon. Sunlight has to travel a longer path through the atmosphere to reach the observer.
The blue and violet colours are scattered in a greater amount than red colour. They are scattered away from the path of light as thickness of the atmosphere is more between the horizon and the observer. The light that reaches the observer is mostly red and yellow. Hence the colour of the sky is reddish orange.
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