Scattering of light
Dust specks glitter in a sunbeam entering a window. In fog the beam of a car's headlamps becomes visible.
When light strikes tiny particles and spreads in all directions, it is scattering of light (प्रकाशाचे प्रकीर्णन). We see the beam when scattered rays enter our eyes.
Rule 1: Particles are needed to see a beam — A laser beam through clear water is not seen. After a few drops of milk make the water slightly turbid, the beam shows up. Dissolved salt or sugar leaves water clear with no particles, so no beam is seen.
- Easy : The laser beam is not visible in clear water.
- Medium : The beam is seen in water with a few drops of milk because the milk particles scatter the light.
- Exam-type : A laser through a glass box with incense smoke will show a beam, because smoke particles scatter the light.
Rule 2: Why the sky is bright by day — Sunlight is scattered by dust and gas molecules in the air, so the whole sky looks bright. Without an atmosphere there would be no scattering; the sky would look dark even by day and the Sun would be seen directly.
- Easy : After sunrise the whole surrounding becomes bright.
- Medium : In photographs from satellites the space around looks dark even by day.
- Exam-type : From a rocket outside the atmosphere the sky looks dark because there are no particles to scatter the light.
A beam is visible because scattered light from the particles reaches our eyes.
Thinking a laser beam shows in any liquid. In a clear liquid with no particles it does not.
Memory clue : "Particles present, beam shows" — the particles scatter light towards our eyes.
Spot the difference
| Point | Clear water | Water with milk drops |
|---|---|---|
| Particles | Absent | Present |
| Laser beam | Not seen | Seen |
Practice questions
- 1What is scattering of light?
- 2Why is a laser beam not seen in clear water?
- 3Why does a car's headlamp beam show in fog?
- 4What changes when milk drops are added to water?
- 5Why does the sky look bright in the daytime?
- 6Will a beam be seen in water with dissolved salt?
- 7How would the daytime sky look if there were no atmosphere?
- 8What will you see if a laser passes through a box of incense smoke?
Show answers (try them yourself first)
- 1Light spreading in all directions after hitting tiny particles.
- 2There are no particles to scatter the light.
- 3Fog particles scatter the light.
- 4The water becomes turbid and the laser beam becomes visible.
- 5Particles and molecules in air scatter sunlight.
- 6No, because the water stays clear and has no particles.
- 7Dark, with the Sun seen directly.
- 8The beam will show because smoke particles scatter light.
Colours of the sky: blue sky and red sunset
The sky looks blue by day, while at sunrise and sunset the sky and the Sun look reddish.
Sunlight has many colours. Gas molecules like nitrogen and oxygen scatter blue the most, so the sky looks blue. Red is scattered less.
Rule 1: Blue scatters the most — The blue part of sunlight spreads in every direction and reaches our eyes from all over the sky, so the sky looks blue.
- Easy : By day the sky looks blue.
- Medium : Red scatters less, so it does not fill the sky as blue does.
- Exam-type : In the milk-water experiment, viewed from the side, a bluish tinge appears; this is scattered blue light.
Rule 2: Why the Sun looks red at sunset — At sunrise and sunset, sunlight has to travel a longer path through the atmosphere. On the way most of the blue is scattered away, and red-yellow light reaches us directly, so the Sun looks red.
- Easy : At sunset the Sun looks reddish.
- Medium : Sunlight travels a longer distance, so more blue is scattered.
- Exam-type : At noon the Sun is overhead and the path through the air is short, so it looks whitish-yellow.
Colours of the sky: blue sky and red sunset
In the app you can move this figure with your finger.
Open in the appIn the experiment, more milk drops make the reddish colour from the front more intense; with too many drops it disappears.
Thinking the Sun itself turns red at sunset. Its colour is the same; scattering of blue makes it look red.
Memory clue : "Longer path → more blue scattered → red remains" — the sunset rule.
Spot the difference
| Time | Path of light | Colour seen |
|---|---|---|
| Noon | Short | Blue sky |
| Sunrise/sunset | Long | Reddish Sun |
Practice questions
- 1Which colour is scattered the most?
- 2What is the colour of the sky by day?
- 3How does the Sun look at sunset?
- 4Why does the sky look blue?
- 5Why is the path of light longer at sunset?
- 6What does it mean that red light is scattered less?
- 7Why is it wrong to say the Sun turns red?
- 8Why does the Sun look whitish at noon?
Show answers (try them yourself first)
- 1Blue.
- 2Blue.
- 3Reddish.
- 4Air molecules scatter blue light the most.
- 5The Sun is near the horizon, so light travels farther through the air.
- 6More of it travels straight on.
- 7The Sun does not change colour; scattering of blue makes it look red.
- 8The path is short, so less blue is scattered away.
Shadows: point and extended sources
A hand held near a lamp casts a dark shadow with a faint edge on the wall.
When an opaque object blocks the straight path of light, a shadow forms behind it. A tiny source like a dot is a point source; a larger one is an extended source.
Rule 1: Point source: only a dark shadow — A point source gives only a dark shadow (umbra, प्रच्छाया), because rays touching the edge of the object leave a sharp boundary.
- Easy : Light coming out of a tiny hole in cardboard acts like a point source.
- Medium : A point source puts a single dark shadow of the ball on the screen.
- Exam-type : A point source gives no penumbra because all rays start from the same point.
Rule 2: Extended source: umbra and penumbra — An extended source gives two parts: a dark umbra in the middle and a faint penumbra (उपच्छाया) at the edge. Moving the screen away makes both bigger. If the object is smaller than the source and the screen moves away, the umbra shrinks and finally vanishes.
- Easy : A lamp's shadow is dark in the middle and faint at the edges.
- Medium : On moving the screen away both parts grow.
- Exam-type : The shadow of a high-flying bird is not seen on the ground because the umbra vanishes and only a very faint penumbra remains.
Shadows: point and extended sources
In the app you can move this figure with your finger.
Open in the appUmbra is the dark part; penumbra is the partly dark, faint part.
Thinking the shadow always becomes smaller when the screen moves away. Usually it grows; only the umbra of a small object shrinks and vanishes.
Memory clue : "Umbra = full dark, Penumbra = partial" — the penumbra is the faint shadow around the edge of the umbra.
Spot the difference
| Point | Point source | Extended source |
|---|---|---|
| Umbra | Present | Present |
| Penumbra | Absent | Present |
Practice questions
- 1How does a shadow form?
- 2Which part of a shadow is dark?
- 3Which part of a shadow is faint?
- 4What is a point source?
- 5What happens to umbra and penumbra when the screen moves away?
- 6Why is no penumbra formed by a point source?
- 7If the object is smaller than the source, what happens to the umbra as the screen moves away?
- 8Why is the shadow of a high-flying bird not seen on the ground?
Show answers (try them yourself first)
- 1An opaque object blocks the light.
- 2Umbra.
- 3Penumbra.
- 4A tiny, dot-like source of light.
- 5Both become bigger.
- 6All rays start from a single point.
- 7It shrinks and finally vanishes.
- 8The umbra vanishes and only a very faint penumbra remains.
Eclipses and zero shadow day
Sometimes the Sun is partly covered by day, or a full Moon turns reddish. This is an eclipse.
When the Sun, Moon and Earth come in a straight line, an eclipse (ग्रहण) happens. If the Moon comes between the Sun and the Earth, it is a solar eclipse; if the Earth comes between the Sun and the Moon, it is a lunar eclipse.
Rule 1: Solar eclipse — The Moon's shadow falls on the Earth. A solar eclipse is seen only on a new moon day. If the solar disc is fully covered it is total; if only partly, partial. Never watch a solar eclipse with bare eyes; use special eclipse glasses, because harmful ultraviolet rays reach the Earth.
- Easy : On a new moon day the Moon can come in front of the Sun.
- Medium : If the Sun is partly covered it is a partial eclipse.
- Exam-type : Looking at a solar eclipse with bare eyes can damage the eyes; so use special safe glasses.
Eclipses and zero shadow day
In the app you can move this figure with your finger.
Open in the appRule 2: Lunar eclipse — The Earth's shadow falls on the Moon. A lunar eclipse is seen only on a full moon night. If the whole Moon enters the shadow it is total; if part of it, partial. It can be watched with bare eyes and lasts a few hours.
- Easy : A lunar eclipse happens on a full moon night.
- Medium : A lunar eclipse can safely be watched with bare eyes.
- Exam-type : A lunar eclipse cannot happen on a new moon day because the Earth is not between the Sun and the Moon then.
Eclipses and zero shadow day
In the app you can move this figure with your finger.
Open in the appRule 3: Zero shadow day — On the day when the Sun is exactly overhead at noon, objects cast no shadow; this is a zero shadow day. It happens only between the Tropic of Cancer (23.5° N) and the Tropic of Capricorn (23.5° S).
- Easy : On that day a stick shows no shadow at noon.
- Medium : This happens on different days at different places in this region.
- Exam-type : Kashmir has no zero shadow day because it lies north of the Tropic of Cancer.
An eclipse is a natural event. We should remove myths and superstitions about it. Never look at a solar eclipse with bare eyes.
Thinking going out or eating during an eclipse is harmful. It is just a play of shadows; only looking at the Sun directly is dangerous.
Memory clue : "Solar = new moon = Moon in the middle; Lunar = full moon = Earth in the middle" — learn the middle object together with the date.
Spot the difference
| Point | Solar eclipse | Lunar eclipse |
|---|---|---|
| In the middle | Moon | Earth |
| Day | New moon | Full moon |
| With bare eyes | Never | Safe |
Practice questions
- 1On which day does a solar eclipse occur?
- 2On which day does a lunar eclipse occur?
- 3Which eclipse must not be seen with bare eyes?
- 4Who is between the Sun and the Earth in a solar eclipse?
- 5Differentiate total and partial eclipse.
- 6What is a zero shadow day?
- 7What risk do ultraviolet rays pose during a solar eclipse?
- 8How would you remove superstitions about eclipses?
Show answers (try them yourself first)
- 1New moon day.
- 2Full moon day.
- 3Solar eclipse.
- 4The Moon.
- 5In a total eclipse the disc is fully covered; in a partial one only partly.
- 6The day when the Sun is overhead at noon and no shadow is seen.
- 7Watching with bare eyes can damage the eyes.
- 8By explaining that an eclipse is a natural shadow event, with scientific reasons.