The colorful world

colloid particle size

1-1000 nm

How does the color of the scattered light depend on the size of the colloidal particles

• Very fine particles- Scatter light of short wavelength i.e., blue • Large particles- Scatter light of longer wavelength i.e., red • Very large particles- Scatter white light

Angle of prism

Angle at which the two refracting faces are inclined to each other.

Dispersion of light

The phenomenon of splitting of white light into its seven constituent colors when it passes through a glass prism.

Red light

• Maximum speed/wavelength • Least deviated

Angle of emergence

Angle between emergent ray and normal

Recombination of white light

Newton found that when an inverted prism is placed in the path of dispersed light then after passing through the prism, they recombine to form white light.

Example of monochromatic light

Sodium light

Example of polychromatic light

white light

Refraction in a prism

A ray of light, after suffering refraction through a prism, bends towards the base of the prism. For any ray of light, Angle of incidence + angle of emergence = Angle of prism + Angle of deviation ∠ i + ∠ e = ∠ A + ∠ D

The Sun appears white at noon because

At noon, the sun is overhead and sunlight would travel shorter distance relatively through the atmosphere. Hence, at noon, the sun appear white as only little of the blue and violet colours are scattered.

Where is a rainbow formed?

Direction opposite to that of he sun

Apparent flattering of the Sun's disc at sunset and sunrise cause

Due to atmospheric refraction.

Explain Newton's conclusion and how he obtained it

He was the first, who obtained spectrum of sunlight by using glass prism. He tried to split the spectrum of white light more by using another similar prism, but he could not get any more colors. He repeated the experiment using second prism in inverted position with respect to the first prism. It allowed all the colors of spectrum to pass through second prism. He found white light emerges on the other side of second prism. He concluded that Sun is made up of seven visible colour VIBGYOR.

Why are traffic signals of red color?

In visible spectrum, the red color has the largest wavelength. Red is least scattered by fog or dust particles. So, we can observe red color easily even in foggy and dusty conditions.

Colloids

Intermediate state of true solution and suspension Heterogenous system in which one substance is dispersed (disperse phase) as very fine particles in another substance called dispersion medium

example of tyndall effect

It can be observed when sunlight passes through a canopy of a dense forest.

Rainbow

It is the spectrum of sunlight in nature. It is formed due to the dispersion of sunlight by the tiny water droplet, present in the atmosphere.

Polychromatic light

Light consisting of more than two colors or wavelengths

Monochromatic light

Light having a single wavelength or frequency

Scattering of light

Phenomenon in which a part of light incident on a particle is redirected in different directions

Why don't planets twinkle?

Planets are closer to earth and are seen as extended source of light i.e. the collection of large number of point sized sources of light. Therefore the total amount of light entering our eyes from all individual point source will nullify the twinkling effect.

Prism

Portion of transparent medium bonded by 2 plane faces inclined to each other at a certain angle.

Angle of deviation

The angle between incident ray and emergent ray.

Why is the sky blue?

The blue colour of sky is due to the scattering of the sunlight by the molecules of the atmosphere. The molecules of air such as N2 and O2 have sizes smaller than the wavelength of visible light. As sunlight passes through the atmosphere, these molecules absorb some amount of sunlight and re-emit it. They scatter blue light of shorter wavelength more strongly than red light of longer wavelength. The scattered blue light enters our eyes.

Atmospheric refraction

The deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of altitude.

Relation between angle of incidence and emergence

The ray of light travels down at an angle through the air and enters the glass. The light refracts or bends as it enters the glass, then carries on and emerges at a new angle back into the air. The ray of light coming out is parallel to the incident ray and the angle of emergence is the same as the angle of incidence. ∠ i = ∠ e

Cause of tyndall effect

The size of the colloidal particle is relatively larger than the solute particles of a true solution. The colloidal particles first absorb energy from the incident light and then scatter a part of energy from their surfaces. Thus tyndall effect is due to scattering of light by colloidal particles and the colloidal particles are seen as points of light moving against a dark background.

Cause of blue color of the ocean

The water molecules of the ocean scatter blue light more strongly than light of other colors.

Tyndall effect

When a beam of light strikes the minute particle of earth's atmosphere, suspended particles of dust and molecules of air, the path of the beam becomes visible. The phenomenon of scattering of light by the colloidal particles gives rise to Tyndall Effect.

Color of sunrise and sunset

While sunset and sunrise, the colour of the sun and its surroundihg appear red. During sunset and sunrise, the sun is near to horizon, and therefore, the sunlight has to travel larger distance in atmosphere. Due to this, most of the blue light (shorter wavelength) is scattered away by the particles. The light of longer wavelength (red colour) reaches our eye. This is why sun appear red in colour.

Composition of white light

White light consists of seven colors i.e., violet, indigo, blue, green, yellow, orange and red. VIBGYOR

Cause of dispersion of white light

White light dispersion occurs because, as they pass through a prism, different colors of light bend through different angles with respect to the incident ray.

Spectrum

colored band produced when a beam of light passes through a prism

Why the duration of day becomes approximately 4 minutes shorter if there was no atmosphere?

• Actual sun rise happens when it is below the horizon in the morning. • The rays of light from the sun below the horizon reach our eyes because of refraction of light. • Similarly, the sun can be seen about few minutes after the actual sun set. • Thus the duration of day time will increase by 4 minutes. This is due to atmospheric refraction. Because of this, sun is visible about 2 minutes earlier than actual sunrise and about 2 minutes after the actual sun set.

Properties of colloidal solutions

• Heterogenous mixtures • Size ranges from 1-1000 nm • Path of a beam of light becomes visible while passing through a colloidal solution (tyndall effect) • Colloidal particles cannot be separated by simple sedimentation or filtration.

Explain the twinkling effect of stars

• It is also due to atmospheric refraction. • Distant star acts like a point source of light. • As the beam of starlight keeps deviating from its path, the apparent position of star keeps on changing because physical condition of earth's atmosphere is not stationary. • Hence, the amount of light enters our eyes fluctuate sometimes bright and sometime dim. • This is the "Twinkling effect of star".

The appearance of a star's position

• Light from a star is refracted when it leaves space and enters the earth's atmosphere. • Air higher up in the sky is rarer but that near the earth's surface is denser. • So as the light from the star comes down, the dense air bends the light more. • Due to this refraction, the star appears to be at a higher position

Violet light

• Minimum speed/wavelength • Most deviated

Phenomena of light responsible for formation of a rainbow (3)

• Refraction • Dispersion • Internal reflection of light

Explain the formation of a rainbow

• The water droplets act like small prism. • They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop. • Due to the dispersion of light and internal reflection, different colors reach the observer's eye.