ScienceCBSE Class 10 · The Human Eye and the Colourful World

Scattering of Light and the Tyndall Effect

Updated 14 October 2026 · Fact-checked

Scattering is the spreading of light in different directions by tiny particles in its path. When the particles are colloid-sized, the beam's path becomes visible: the Tyndall effect. Very fine particles scatter mainly blue light, so the clear sky looks blue; red is scattered least, so danger signals are red; without an atmosphere the sky would look dark.

Notice the beam shows only in the colloid (Tyndall effect), and tiny particles scatter blue far more than red, so the sky looks blue.

Illustration drawn for QuestionBuddy.

Understand Scattering of Light and Tyndall Effect

What scattering is. Light travelling through a medium that contains tiny particles is sent off in different directions by those particles. We see such scattered light when it reaches our eyes. The earth's atmosphere is a heterogeneous mixture of minute particles: smoke, tiny water droplets, suspended dust and molecules of air.

Tyndall effect. In a true solution, the path of a light beam is not visible. In a colloidal solution, where particles are relatively larger, the path becomes visible because the particles reflect light diffusely in all directions. This scattering of light by colloidal particles is the Tyndall effect. You see it when a narrow shaft of sunlight comes through a small opening into a smoky room, or when sunlight passes through the canopy of a dense, misty forest, where tiny water droplets scatter the light.

Colour of scattered light depends on particle size.

Very fine particles (air molecules and other particles smaller than the wavelength of visible light) scatter mainly blue light (shorter wavelengths).
Larger particles scatter light of longer wavelengths as well.
If particles are large enough, the scattered light may appear white.

Why is the clear sky blue? Air molecules and fine particles are smaller than the wavelength of visible light, so they scatter shorter wavelengths (the blue end) much more effectively than longer wavelengths (the red end). Red light has a wavelength about 1.8 times that of blue light. As sunlight passes through the atmosphere, the blue part is scattered strongly in all directions and reaches our eyes from every part of the sky, so the sky looks blue.

If there were no atmosphere, there would be no scattering, and the sky would look dark. This is why the sky appears dark to passengers flying at very high altitudes, where scattering is not prominent, and why it would look dark to an astronaut in space.

Why danger signals are red. Red light is scattered least by fog or smoke, so a red signal keeps its colour and can be seen from a distance.

Scope note. The 2026-27 syllabus covers scattering and its everyday applications but excludes the colour of the Sun at sunrise and sunset, so you will not be asked to explain the reddish Sun. The twinkling of stars and advance sunrise are due to atmospheric refraction, a different process (bending of light in air layers of changing refractive index), not scattering.

Scattering of Light and Tyndall Effect: formulas and key facts

Size and scattered colour: $very fine particles \to mainly blue; larger particles \to longer wavelengths; large enough \to white$; Particle size decides the colour scattered.
Wavelength comparison: $λ_{red} \approx 1.8 \times λ_{blue}$; Shorter wavelengths (blue) scatter more strongly from fine particles.
Tyndall effect condition: $colloidal particles \Rightarrow visible beam; true solution \Rightarrow beam not visible$; Scattering makes the particles, and the beam, visible.

How to solve Scattering of Light and Tyndall Effect questions

For scattering questions, identify the particle size and the colour that reaches the eye.

1Decide what the scattering particles are: air molecules (very fine), dust or smoke, water droplets (larger).
2Apply the rule: very fine particles scatter blue most; larger ones scatter longer wavelengths too; large ones scatter white.
3Say which light reaches the observer: scattered light (sky) or the light that is least scattered (red signal through fog).
4For 'no atmosphere' or 'high altitude' questions: no or little scattering, so the sky looks dark.
5For Tyndall effect: name the colloid (smoke, mist, milk in water) and say the beam becomes visible.
6Do not mix scattering with atmospheric refraction (twinkling stars) or dispersion (rainbow).

Scattering of Light and Tyndall Effect in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Least scattering of red light

Suppose on a foggy winter morning you are waiting at a railway crossing. You notice that the red stop lamp ahead is still clearly visible as red, while other details in the distance have faded into a grey haze.

Explain why signal lamps meant to warn of danger are red, and why the distant scene looks hazy and grey.

Try it first, then show the solutionSolution

Fog consists of tiny water droplets suspended in air, which scatter light.
Red light has the longest wavelength in the visible range and is scattered least by fog or smoke.
So most red light from the lamp travels straight through to your eyes, and the lamp is seen in the same colour from a distance.
Light from distant objects is scattered many times by the droplets on the way; since these droplets are relatively large, they scatter light of many wavelengths, and the scene looks grey or whitish and hazy.

Answer: Red is scattered least by fog or smoke, so red lamps remain visible from afar; the larger fog droplets scatter many colours, so the distant scene looks grey and hazy.

Memory hook: Fog grabs blue; red gets through: stop lamps are red.

Tricks to remember Scattering of Light and Tyndall Effect

Short memory aids. Say each one aloud once, then use it on a question the same day.

Small scatters Short
Small particles scatter Short wavelengths (blue).
Use it when: Blue sky questions.
Red runs through
Red is scattered least, so it runs through fog.
Use it when: Danger signals.
No air, no blue
No atmosphere, no scattering: a black sky.
Use it when: Astronaut and high-altitude questions.

Scattering of Light and Tyndall Effect: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Blue sky · 2 marks · Very short answerBoard level

Why is the colour of the clear sky blue?

Try it first, then show the solutionSolution

Air molecules and other fine particles in the atmosphere are smaller than the wavelength of visible light, so they scatter shorter wavelengths (blue) more effectively than longer ones (red) (1 mark).
This scattered blue light reaches our eyes from all directions, so the sky appears blue (1 mark).

Answer: Fine particles scatter blue light much more than red; the scattered blue reaches our eyes.

Example 2: Sky for an astronaut · 2 marks · Very short answerBoard level

Why does the sky appear dark instead of blue to an astronaut? What would the sky look like from the earth's surface if the earth had no atmosphere?

Try it first, then show the solutionSolution

In space (or at very high altitude) there is little or no atmosphere, so there are no particles to scatter sunlight towards the observer (1 mark).
Without an atmosphere, there would be no scattering at all, so even from the earth's surface the sky would look dark (1 mark).

Answer: No scattering particles, so no scattered light: the sky looks dark; it would be dark from the earth too without air.

Example 3: Tyndall effect examples · 3 marks · Short answerHarder board level

What is the Tyndall effect? Give two everyday examples and explain why a beam of light is visible in a smoke-filled room but not in a glass of salt solution.

Try it first, then show the solutionSolution

Scattering of light by colloidal particles, which makes the path of a light beam visible (1 mark).
Examples: a beam of sunlight entering a smoke-filled room through a small hole; sunlight passing through the canopy of a misty forest (1 mark).
Smoke particles are colloid-sized and scatter light into our eyes; salt solution is a true solution whose particles are too small to scatter noticeably, so the beam's path is not seen (1 mark).

Answer: Scattering by colloids makes a beam visible (smoke-filled room, misty forest); a true solution's particles are too small.

Example 4: Particle size and colour · 3 marks · Short answerHarder board level

Explain how the colour of scattered light depends on the size of the scattering particles. Use this to explain why clouds, made of water droplets much larger than air molecules, generally look white.

Try it first, then show the solutionSolution

Very fine particles scatter mainly blue light (shorter wavelengths) (1 mark).
Larger particles scatter light of longer wavelengths as well; if particles are large enough, all colours are scattered and the scattered light looks white (1 mark).
Cloud droplets are large compared with air molecules, so they scatter all colours roughly together, and clouds appear white (1 mark).

Answer: Fine → blue; larger → longer wavelengths too; large enough → white. Cloud droplets are large, so clouds look white.

Example 5: Sorting the phenomena · 5 marks · Long answerHOTS / competency

Classify each as scattering, dispersion or atmospheric refraction, giving a one-line reason: (a) blue sky, (b) rainbow, (c) twinkling of stars, (d) red danger lights seen through fog, (e) beam visible in a misty forest.

Try it first, then show the solutionSolution

(a) Scattering: fine air particles scatter blue light most (1 mark).
(b) Dispersion: water droplets split sunlight into its colours (with internal reflection) (1 mark).
(c) Atmospheric refraction: starlight bends through air layers of changing refractive index, and the apparent position flickers (1 mark).
(d) Scattering: red is scattered least by fog, so it passes through (1 mark).
(e) Scattering (Tyndall effect): mist droplets scatter light, making the beam visible (1 mark).

Answer: (a) Scattering (b) Dispersion (c) Atmospheric refraction (d) Scattering (e) Scattering (Tyndall).

Where students lose marks in Scattering of Light and Tyndall Effect

The slips examiners see most often, and what to write instead.

Mistake: Saying blue is scattered least
Students reverse the rule.
Wrong working
The sky is blue because blue light is scattered least and reaches us directly.
Right working
Blue is scattered most by fine particles; the scattered blue fills the sky.
Do this: Small particles scatter short wavelengths most.
Usually costs: Full marks for the question.
Mistake: Explaining twinkling by scattering
Both are atmospheric effects.
Wrong working
Stars twinkle because of scattering.
Right working
Twinkling is due to atmospheric refraction.
Do this: Scattering: colours of sky and signals; refraction: bending and twinkling.
Usually costs: 1-2 marks.
Mistake: Answering the excluded sunset question
Old guides include it.
Wrong working
Long answer on the red Sun at sunset.
Right working
The colour of the Sun at sunrise and sunset is excluded from the 2026-27 syllabus.
Do this: Focus on blue sky, Tyndall effect, red signals and dark sky at altitude.
Usually costs: Time.
Mistake: Vague Tyndall effect definition
Students write 'light becomes visible'.
Wrong working
Tyndall effect is when light is visible.
Right working
Scattering of light by colloidal particles, making the path of the beam visible.
Do this: Name colloidal particles and scattering.
Usually costs: 1 mark.

Exam tips

Use the words 'shorter wavelength' and 'scattered more' together in blue-sky answers.
Link particle size to scattered colour; it is the key idea in HOTS questions.
Keep scattering, dispersion and atmospheric refraction separate in your answers.
Give two Tyndall effect examples from NCERT: smoke-filled room and misty forest canopy.
Colour of the Sun at sunrise and sunset is excluded from the 2026-27 syllabus.

Practice questions on Scattering of Light and Tyndall Effect

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Danger signals are red because red light:
(a) has the shortest wavelength (b) is scattered most by fog (c) is scattered least by fog or smoke (d) is absorbed by air
Hint
Which colour reaches far through fog?
Answer
(c).
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The sky appears dark to passengers in aircraft flying at very high altitudes.
Reason (R): Scattering is not prominent at such heights.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
What makes the sky look bright and blue?
Answer
(a).
Q3 · 2 marks · Very short answerBoard level
A student shines a laser pointer through (i) a glass of water with a few drops of milk and (ii) a glass of clear sugar solution. In which is the beam's path visible? Explain.
Hint
Which is a colloid?
Answer
In (i). Milk in water forms colloidal particles that scatter light (Tyndall effect), making the beam visible. Sugar solution is a true solution; its particles are too small, so the path is not visible.
Q4 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
Suppose a hiker photographs morning sunlight streaming as visible shafts between the trees of a dense, misty forest. Later, at the top of a very high mountain, she notices the sky looks a deeper, darker blue than at the foot.
(a) Name the phenomenon that makes the sun's shafts visible. (1 mark)
(b) What particles cause it in the forest? (1 mark)
(c) Explain why the sky looks darker at the mountain top, and what it would look like with no atmosphere at all. (2 marks)
Hint
Less air above means less scattering.
Answer
(a) Tyndall effect (scattering by colloidal particles). (b) Tiny water droplets in the mist. (c) At high altitude there is less air above the observer, so less sunlight is scattered towards her and the sky looks darker. With no atmosphere there would be no scattering, and the sky would look dark.
Q5 · 3 marks · Short answerHOTS / competency
Red light has a wavelength about 1.8 times that of blue light. Using this fact, explain why blue, not red, dominates the light scattered by the clear atmosphere, and state one consequence other than the blue sky.
Hint
Fine particles scatter shorter wavelengths more effectively.
Answer
Air molecules are smaller than the wavelength of visible light and scatter shorter wavelengths far more effectively. Blue has a much shorter wavelength than red, so blue is scattered much more strongly and dominates the scattered light. Consequence: red, being least scattered, passes through fog or smoke and is used for danger signals visible from a distance.
Q6 · 3 marks · Short answerHarder board level
Distinguish between scattering of light and dispersion of light, with one example of each.
Hint
One sends light in many directions; the other splits it into colours by refraction.
Answer
Scattering: tiny particles send light off in different directions; the colour scattered depends on particle size (example: blue sky, Tyndall effect). Dispersion: white light is split into its component colours because different colours bend by different amounts when refracted (example: spectrum from a glass prism, rainbow).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is the Tyndall effect?Show answer
Scattering of light by colloidal particles that makes the beam's path visible.
2.Which colour do very fine particles scatter most?Show answer
Blue (shorter wavelengths).
3.Why are danger signals red?Show answer
Red is scattered least by fog or smoke.
4.How many times longer is red's wavelength than blue's (NCERT)?Show answer
About 1.8 times.
5.Why does the sky look dark at very high altitude?Show answer
Scattering is not prominent there.
6.When does scattered light appear white?Show answer
When the scattering particles are large enough.

Scattering of Light and Tyndall Effect: frequently asked questions

Why is the sky blue?

Air molecules and fine particles scatter shorter wavelengths, the blue part of sunlight, much more than longer wavelengths. The scattered blue light reaches our eyes from all over the sky.

What is the Tyndall effect with an example?

It is the scattering of light by colloidal particles that makes a light beam's path visible, for example a beam of sunlight in a smoke-filled room.

Why are traffic stop lights and danger signals red?

Red light is scattered least by fog and smoke, so it can be seen in the same colour from far away.

Why would the sky look dark without an atmosphere?

There would be no particles to scatter sunlight towards us, so no light would come from the sky's direction except from the Sun and stars.

Is the colour of the Sun at sunrise and sunset in the syllabus?

No. The 2026-27 CBSE syllabus excludes it.