Spectrum

The cause of dispersion of white light is the change in speed of light with wavelength.

When white light enters the first surface of a prism, light of different colours due to their different speeds in glass, gets deviated through different angles towards the base of the prism i.e., the dispersion (or splitting) of white light into its constituent colours takes place at the first surface of prism.

Violet colour is deviated the most and red colour the least.

Therefore, light of different colours present in white light follows different paths inside the glass prism and then strikes the second surface of prism

On the second surface of prism, only refraction takes place (from glass to air) and different colours are deviated through different angles i.e., violet is deviated the most and red the least. As a result colours get further separated on refraction at the second surface.

The light emerging out of prism, thus has different colours that spread out to form a spectrum on the screen.

When white light enters the first surface of a prism, light of different colours due to their different speeds in glass, gets deviated through different angles towards base of the prism i.e., the dispersion of white light into its constituent colours takes place at the first surface of prism.

On the second surface of the prism, only refraction takes place and different colours are deviated through different angles i.e., violet colour is deviated the most and red colour the least. Hence, the colours get further separated on refraction at the second surface.

(i) On the first surface, there is both the dispersion and deviation of light.

(ii) On the second surface, there is only the deviation of light.

Below diagram shows the effect of the prism on the white light:

(a) Below diagram shows the effect of the prism on the beam and the spectrum seen on the screen:

A coloured patch like a rainbow is seen on the screen. This is called the spectrum.

(b) When a slit is placed in between the prism and the screen to pass only the light of green colour then only green light is observed on the screen.

(c) With the help of the above experiment we can say that the prism does not produce colours, but it only splits the various colours present in the light incident on it.

(a) When a beam of monochromatic light undergoes minimum deviation through an equiangular prism, then the beam passes parallel to the base of the prism.

(b) If white light is used instead of a monochromatic light, then the white light splits into its constituent colours i.e., spectrum is formed.

(c) With the help of the experiment given above, we conclude that the white light is polychromatic in nature.

both deviation and dispersion

Reason — When a white light enters the first surface of a prism, light of different colours due to their different speeds in glass, gets deviated through different angles towards the base of the prism i.e., dispersion of white light into its constituent colours takes place at the first surface of prism.

Hence, both deviation and dispersion takes place at the first surface.

All of the above.

Reason —

• The dispersion of white light occurs at the first surface of the prism, where white light separates into its constituent colors due to differences in their refractive indices.
• The deviation of light rays occurs at both surfaces of the prism, where each color of light bends or deviates as it enters and exits the prism due to changes in refractive index.
• The prism does not produce colors but only splits the various colors present in white light.

Hence, all the statements are true.

Violet < Green < Red

Reason — The above sequence represents the order of increasing wavelengths of light, with red having the longest wavelength, green in the middle, and violet having the shortest wavelength.

all have the same speed

Reason — All electromagnetic waves, including light, travel at the same speed in a vacuum, which is the speed of light denoted by 'c'. Therefore, regardless of their colour, red, blue, and violet light all travel at the same speed in vacuum.

Q

Reason — As the critical angle for yellow colour is 45°, so it suffers refraction along the line AC of the prism.

400 nm

Reason — As we know,

Speed of light (c) = Frequency (f) x Wavelength (λ)

and

C = 3 x 10⁸ m s^-1

Frequency range = 7.5 × 10¹⁴ Hz

Substituting the values in the formula,

$3 \times 10^8 = 7.5 × 10^{14} \times λ \\[0.5em] \text {λ} = \dfrac {3 \times 10^8 }{7.5 × 10^{14}} \\[0.5em] \Rightarrow \text {λ} = 0.4 \times 10^{-6} \text {m} \\[0.5em] \Rightarrow \text {λ} = 400 \times 10^{-9} \text {m} \\[0.5em]$

Therefore,

λ = 400 nm

δ_I > δ_G > δ_Y > δ_R

Reason — The order of deviation is : δ_I > δ_G > δ_Y > δ_R as red light deviates the least and indigo the most. A

Both A and R are true and R is not the correct explanation of A.

Explanation

Assertion (A) is true. When a ray of light passes through a rectangular glass slab, there is no dispersion of light. After refraction at two parallel faces of a glass slab, the ray of light emerges in a direction parallel to the incident ray. As rays of all colours emerge in the same direction as incident ray, hence there is no dispersion, but only lateral displacement

Reason (R) is true. Dispersion of light is the phenomenon of splitting of white light into its constituent colors but this does not justify assertion.

assertion is false but reason is true

Explanation

Assertion (A) is false. A beam of white light does give a spectrum when passing through a prism, but it is not a "hollow" prism that causes this dispersion. A hollow prism contains air in it, so when a beam of white light is passed through it, it does not give a spectrum. Because light is passed to air through air, so no spectrum will be produced.

Reason (R) is true. The speed of light inside a medium, such as a glass prism, is generally slower than the speed of light in vacuum but here prism is hollow so refractive indices of air and hollow is are same . So the speed of light outside the prism is same as the speed of light inside the glass prism.

As we know,

Speed of light (c) = Frequency (f) x Wavelength (λ)

Given,

λ = 550 nm

= 550 × 10^-9 m

= 0.55 x 10^-6 m

C = 3 x 10⁸ m s^-1

Substituting the values in the formula, we get,

$3 \times 10^8 = \text {f} \times 0.55 × 10^{-6} \\[0.5em] \text {f} = \dfrac {3 \times 10^8 }{0.55 × 10^{-6}} \\[0.5em] \Rightarrow \text {f} = 5.45 \times 10^{14} \text {Hz} \\[0.5em]$

Hence, we get,

Frequency = 5.4 × 10¹⁴ Hz

As we know,

Speed of light (c) = Frequency (f) x Wavelength (λ)

Given,

C = 3 x 10⁸ m s^-1

Frequency range = 3.75 × 10¹⁴ Hz to 7.5 × 10¹⁴ Hz.

Let, f₁ = 3.75 × 10¹⁴ Hz

Substituting the values in the formula,

$3 \times 10^8 = 3.75 × 10^{14} \times λ \\[0.5em] \text {λ} = \dfrac {3 \times 10^8 }{3.75 × 10^{14}} \\[0.5em] \Rightarrow \text {λ} = 0.8 \times 10^{-6} \text {m} \\[0.5em] \Rightarrow \text {λ} = 8000 \times 10^{-10} \text {m} \\[0.5em]$

As, 1 Å = 10^-10 m

Therefore,

λ₁ = 8000 Å

Let, f₂ = 7.5 × 10¹⁴ Hz

Substituting the values in the formula,

$3 \times 10^8 = 7.5 × 10^{14} \times λ \\[0.5em] \text {λ} = \dfrac {3 \times 10^8 }{7.5 \times 10^{14}} \\[0.5em] \Rightarrow \text {λ} = 0.4 \times 10^{-6} \text {m} \\[0.5em] \Rightarrow \text {λ} = 4000 \times 10^{-10} \text {m} \\[0.5em]$

As, 1 Å = 10^-10 m

Therefore,

λ₂ = 4000 Å

Therefore, the wavelength range is 4000 Å to 8000 Å.

The three factors on which the deviation produced by a prism depends are as follows —

1. The angle of incidence (i) — As the angle of incidence increases, the angle of deviation first decreases, reaches to a minimum value for a certain angle of incidence and then on further increasing the angle of incidence, the angle of deviation begins to increase.
2. The angle of prism (A) — Angle of deviation increases with increase in the angle of prism (A).
3. Refractive index of the material of prism — For a given angle of incidence, the prism with a higher refractive index produces a greater deviation than the prism which has a lower refractive index.
   For example — A flint glass prism produces more deviation than a crown glass prism for same refracting angle since μ_flint > μ_crown

Speed of light in a transparent medium decreases with the decrease in wavelength of light. Therefore, the refractive index of glass (material of prism) increases with the decrease in wavelength of light.

Hence, the deviation produced by a prism also increases with the decrease in the wavelength of light incident on it.

For example — In visible light, violet colour ( wavelength λ = 4000 Å) is deviated the most and red colour (wavelength λ = 8000 Å) is deviated the least.

(a) The approximate wavelengths for blue light is 4800 Å and red light is 8000 Å.

(b) The colour of light with shortest wavelength has the highest frequency.
When we compare the wavelength of violet colours (4000 Å) and red (8000 Å), we observe that the wavelength of violet colour is shorter than that of red.
Hence, Violet light of 4000 Å has higher frequency.

The seven prominent colours present in white light in the order of increasing wavelength are as follows —

1. Violet
2. Indigo
3. Blue
4. Green
5. Yellow
6. Orange
7. Red

The seven prominent colours of the white light spectrum in order of their increasing frequencies are —

1. Red
2. Orange
3. Yellow
4. Green
5. Blue
6. Indigo
7. Violet

(a) In vacuum, both have the same speed.

(b) As speed of light increases with increase in wavelength of light. When we observe the given values, we find that the wavelength of red colour (7.8 x 10^-7 m) is more than that of blue colour (4.8 x 10^-7 m). Hence, in glass, red light has greater speed.

The phenomenon of splitting of white light by a prism into its constituent colours is known as dispersion.

On passing white light through a prism, the band of colours seen on a screen is called the spectrum.

(a) The seven colours of the spectrum in order are Violet, Indigo, Blue, Green, Yellow, Orange and Red.

(b) No, all the seven colours have different widths in the spectrum.

(c) Violet colour of the white light is deviated the most, as it's wavelength is least i.e., 4000 Å and Red colour of the white light is deviated the least, as it's wavelength is the longest i.e., 8000 Å.

When a prism is submerged in water instead of air, the dispersion of light decreases. This is because dispersion depends on the difference in refractive index between the prism material and the surrounding medium.
In air, the difference is large, so light bends more at the surfaces of the prism, causing greater dispersion. In water, the difference is smaller, so light bends less as it enters and exits the prism. As a result, the spread of the constituent colours is reduced.

On increasing the wavelength of light in glass, the speed of light will also increase.

For example — In visible light, the speed of violet colour ( wavelength λ = 4000 Å) is least and red colour (wavelength λ = 8000 Å) is most.

(a) Red colour of white light travels fastest in glass as it has longest wavelength i.e., λ = 8000 Å.

(b) Violet colour of white light travels slowest in glass as it has shortest wavelength i.e., λ = 4000 Å.

Colour of light is the subjective property of light related to its wavelength.

Different colours differ in their wavelength. In fact, wavelength is the characteristic of colour, irrespective of its origin i.e., the light of the same colour, obtained from different sources will have same wavelength.

(i) The range of wavelength in Å is 4000 Å to 8000 Å.

(ii) The range of wavelength in nm is 400 nm to 800 nm.

The four colours of the spectrum of white light which have wavelength longer than blue light are green, yellow, orange and red.

If the prism is made of diamond instead of glass, the dispersion of light will increase. This is because diamond has a much higher refractive index than glass. A higher refractive index causes light of different colours to bend more and spread out more, leading to greater separation of colours in the spectrum.

The electromagnetic waves of wavelength λ = 8000 Å to 10⁷ Å are called infrared radiations.

For the detection of infrared radiations —

If a thermometer having it's bulb blackened is moved from the violet end towards the red end of the spectrum of visible light, it is observed that there is a very slow rise in temperature. But when this thermometer is moved beyond the red extreme, a rapid rise in temperature is noticed.

It means that the part of spectrum beyond the red extreme of the visible light has certain radiations which produce a strong heating effect, but they are not visible. These radiations are called the infrared radiations.

The infrared radiations are used for therapeutic purposes by doctors.

The electromagnetic radiations of wavelength λ = 100 Å to 4000 Å are called the ultraviolet radiations.

For the detection of ultra violet radiations —

If silver chloride solution is exposed to the electromagnetic waves starting from the red to the violet end and then beyond it, it is observed that from the red end to the violet end, the solution remains almost unaffected.

But just beyond the violet end, the solution first turns violet and then finally it becomes dark brown (or black).

It shows that there exists certain radiations beyond the violet extreme of the visible part, which are chemically more active than the visible light. These radiations are called the ultra violet radiations ( or actinic rays).

Ultraviolet radiations are used for sterilizing purposes. Type

frequency

Reason — When an electromagnetic wave passes from one medium to another, the frequency of the wave remains unchanged. This is a fundamental property of electromagnetic waves.

Gamma wave and infrared wave respectively

Reason — Wave A with a very short wavelength is characteristic of gamma rays and wave B with a much longer wavelength falls into the infrared range.

gamma rays, X-rays, infrared rays, microwaves

Reason — The arrangement of radiations in increasing order of their wavelengths is

• Gamma rays,
• X-rays,
• Ultraviolet,
• Visible light,
• Infrared rays,
• Microwaves and
• Radio waves.

infrared radiation

Reason — Mercury in the thermometer shows a rapid increase due to the heating effect of the radiation. Hence, we can say that the radiation P is the infrared radiation.

gamma rays

Reason — Gamma rays are the most energetic electromagnetic radiations of wavelength shorter than 0.01 nm and frequency above 10¹⁹ Hz.

A - (i), B - (ii), C - (iii), D - (iv)

Reason — The correct pairing is :

(a) As we know,

Velocity of wave (c) = frequency (f) x wavelength (λ)

Given,

f = 500 MHz

Hence, f = 500 x 10⁶ Hz or

f = 5 x 10⁸ Hz

λ = 60 cm

As 100 cm = 1m

Therefore,

60 cm = $\dfrac{60}{100}$ m

Hence, λ = 0.6 m

Substituting the values in the formula above we get,

$c = (5 \times 10^8) \times 0.6 \\[0.5em] \Rightarrow c = 3 \times 10^8$

Hence,

Speed of the electromagnetic wave = 3 x 10⁸ ms^-1

(b) The electromagnetic wave is travelling through air.

Assumption — Speed of X-rays = 3 x 10⁸ ms^-1

As we know,

Speed of wave (c) = frequency (f) x wavelength (λ)

Given,

c = 3 x 10⁸ ms^-1

λ = 0.01 Å

As 1 Å = 10^-10 m

Hence,

λ = 0.01 Å = 0.01 x 10^-10 m

Substituting the values in the formula above we get,

$3 \times 10^8 = f \times (0.01 \times 10^{-10} ) \\[0.5em] \Rightarrow f = \dfrac{3 \times 10^8}{0.01 \times 10^{-10}} \\[0.5em] \Rightarrow f = \dfrac{3 \times 10^8}{1 \times 10^{-12}} \\[0.5em] \Rightarrow f = 3 \times 10^{20} Hz\\[0.5em]$

Hence,

Frequency of x - rays = 3 x 10²⁰ Hz

The list of radiations of electromagnetic spectrum in increasing order of their wavelength is as follows —

(i) Gamma rays

(ii) X – rays

(iii) Ultraviolet rays

(iv) Visible light

(v) Infrared radiations

(b) Gamma rays of the electromagnetic spectrum has the highest penetrating power.

(a) Wavelength of X-ray waves is in the range of 0.1 Å to 100 Å. Hence, the electromagnetic wave having wavelength 50 Å is X-ray.

(b) As all the waves move with the speed of 3 x 10⁸ ms^-1 in vacuum. Hence, the speed of x-ray in vacuum is also equal to 3 x 10⁸ ms^-1.

(c) X-ray waves are used for the detection of fracture in bones, teeth etc (i.e., radiography).

The three radiations which are invisible and beyond the violet end of the visible spectrum, (in decreasing order of wavelength) are as follows —

The part of spectrum beyond the red extreme and the violet extreme to which our eyes do not respond is called the invisible spectrum.

(a) Ultraviolet rays — wavelength range 10 to 400 nm

(b) Visible light — wavelength range 400 to 800 nm

(c) Infrared radiations — wavelength range 800 to 10⁶ nm.

Two electromagnetic waves of wavelength smaller than that of violet light ( λ = 400 nm ) are —

(a) Ultraviolet radiations ( λ = 100 to 400 nm ) and

(b) X-rays ( λ = 0.01 to 10 nm )

Ultraviolet radiations are used for sterilizing purposes.

X-rays are used for detection of fracture in bones, teeth, etc. ( i.e.,radiography ) and for diagnostic purposes such as CAT scan in medical science.

(a) Microwaves are used for analysis of atomic and molecular structure.

(b) Ultraviolet radiations are used for sterilizing purposes.

(c) Infrared radiations are used for therapeutic purposes by doctors.

(d) Gamma rays are used in medical science to kill cancer cells (i.e., radio therapy).

(a) The waves of lowest wavelength are gamma rays ( λ = below 0.01 nm).

(b) The waves used for taking photographs in dark are infrared rays.

(c) The waves produced by the changes in the nucleus of an atom are gamma rays.

(d) The waves having wavelength nearly 0.1 nm are X-rays.

(a) Wave A is Gamma rays (as the wavelength range of gamma rays is shorter than 0.1 Å). Wave B is Infrared radiations (as the wavelength range of infrared radiations is 8000 Å to 10⁷ Å).

(b) All electromagnetic waves travel with the speed of light (i.e., 3 x 10 ⁸ ) in vacuum. Thus, ratio of speeds of these waves in vacuum is 1 : 1.

All red hot bodies such as a heated iron ball, flame, fire, etc. are the sources of infrared radiations. The Sun is the natural source of infrared radiations.

The electric arc and sparks give ultraviolet radiations. A mercury vapour lamp emits radiations, a part of which has ultraviolet radiations along with the visible light.

Two properties of ultraviolet radiations which are similar to the visible light are as follows:

1. Ultraviolet radiations travel in a straight line like visible light, with a speed of 3 x 10⁸ m s^-1 in air or vacuum.
2. They obey the laws of reflection and refraction like visible light.

Two properties of ultraviolet radiations which differ from the visible light are as follows:

1. Ultraviolet radiations have wavelength in the range of 100 Å to 4000 Å whereas wavelength of visible light is the range of 4000 Å to 8000 Å.
2. Ultraviolet radiations cause fluorescence on striking a zinc-sulphide screen whereas visible light does not cause such fluorescence.

Two properties of infrared radiations which are similar to the visible light are as follows:

1. Infrared radiations travel in a straight line like visible light, with a speed of 3 x 10⁸ m s^-1 in air or vacuum.
2. They obey the laws of reflection and refraction like visible light.

Two properties of infrared radiations which differ from the visible light are as follows:

1. Infrared radiations have wavelength in the range of 8000 Å to 10⁷ Å whereas wavelength of visible light is the range of 4000 Å to 8000 Å.
2. Infrared radiations do not affect ordinary photographic plate whereas visible light affects photographic plate.

Harmful effects of the given radiations are as follows —

(a) Ultraviolet radiations cause health hazards like skin cancer if human body is exposed to them for a long period.

(b) Infrared radiations causes skin burns.

(i) Infrared radiations are used for photography in fog because they are not much scattered, and so they can penetrate appreciably through it.

(ii) Infrared radiations are used for signals during the war because they are not visible and they are not absorbed much in the medium.

(iii) The photographic darkrooms are provided with infrared lamps because they provide some visibility without affecting the photographic film.

(iv) A rock salt prism is used instead of a glass prism to obtain the infrared spectrum because the rock salt prism does not absorb the infrared radiations, whereas a glass prism absorbs them.

(v) A quartz prism is required for obtaining the spectrum of the ultraviolet light because ultraviolet radiations can pass through quartz whereas ordinary glass absorbs them.

(vi) Ultraviolet bulbs have a quartz envelope instead of glass because ultraviolet radiations are not absorbed by quartz whereas ordinary glass absorbs the ultraviolet light.

Thermometer A (with red light) would show a higher reading.

Reason — Red light has the longest wavelength and the lowest frequency among the three. Hence, it carries more heat energy compared to violet or indigo light. As a result, it causes a greater rise in temperature, making thermometer A show a higher reading.

(a) The radiations in the order of their increasing wavelength are follows —

gamma rays, x-rays, infrared rays, micro waves, radio waves.

(b) The radiation used for satellite communication is micro waves.

(a) The wave which has a wavelength of 10^-3 nm is gamma rays.

(b) The property of gamma rays different from light is that, it has strong penetrating power. Hence, gamma rays can pass through human body and can cause immense damage.

(a) X-rays are used in the study of atomic arrangement in crystals as well as in complex molecules.

(b) As X-rays can penetrate through human flesh, but they are stopped by bones. Hence, they are also used to detect fracture in bones.

The invisible electromagnetic waves beyond the red end of visible spectrum are called the infrared (or heat )radiations.

Range of wavelength of infrared radiations is 8000 Å to 10⁷ Å.

The range of wavelength of the electromagnetic waves visible to us is 4000 Å to 8000 Å.

They are called the visible radiations (or visible light) because in the presence of these radiations, other objects are seen by us.

(i) The region just beyond the red end of the spectrum is known as the infrared.

(ii) The region just beyond the violet end of the spectrum is known as the ultraviolet.

(a) Infrared

(b) Ultraviolet

(i) The radiations of wavelength just longer than 8 x 10^-7 m are infrared radiation.

(ii) The radiations of wavelength just shorter than 4 x 10^-7 m are Ultraviolet radiations.

(a) Ultraviolet radiations are absorbed through glass, but can pass through quartz. Therefore, to obtain the ultraviolet spectrum from its source, a quartz prism is used.

The green house gases such as carbon dioxide, present in the earth's atmosphere absorb the low energy infrared radiations and keep the earth's surface warm.

smaller than

Reason — Scattering of light occurs when the size of the particles or molecules in the medium through which light travels is smaller than the wavelength of the light. This condition allows the incident light to interact with the particles, causing it to scatter in various directions.

I ∝ $\dfrac{1}{λ^4}$

Reason — Intensity of scattered light is found to be inversely proportional to the fourth power of the wavelength of light.

(i.e., I ∝ $\dfrac{1}{λ^4}$ ).

blue colour

Reason — Intensity of scattered light is found to be inversely proportional to the fourth power of the wavelength of light.

(i.e., I ∝ $\dfrac{1}{λ^4}$ ).

As blue light has lowest wavelength, hence, the blue light is scattered the most.

16

Reason — According to Rayleigh scattering, the amount of scattering is inversely proportional to the fourth power of the wavelength of the light.

We know,

λ_violet = 4000 Å and

λ_red = 8000 Å.

According to Rayleigh scattering,

$\dfrac{\text{Intensity of scattering of violet}}{\text{Intensity of scattering of red}}$ = $(\dfrac{λ_{red}}{λ_{violet}})^4$

So,

$\dfrac{\text{Intensity of scattering of violet}}{\text{Intensity of scattering of red}}$ = $(\dfrac{8000}{{4000}})^4$ = 2⁴

= 16

Hence, Intensity of scattering of violet = 16 time Intensity of scattering of red.

blue

Reason — When an astronaut goes above the atmosphere of the earth in a rocket, he sees the sky black, but to him the earth appears blue due to the blue colour of sunlight scattered by the earth's atmosphere reaching him.

scattering

Reason — During sunrise and sunset, the sun is positioned at a lower angle in the sky. This causes sunlight to travel through a thicker layer of Earth's atmosphere, which scatters shorter wavelengths of light (such as blue and green) more strongly than longer wavelengths (such as red and orange). As a result, the shorter wavelengths are scattered out of the line of sight, leaving predominantly red and orange hues to be observed.

black

Reason — Unlike earth, which has an atmosphere that scatters sunlight in various directions, the moon lacks an atmosphere. As a result, there is no scattering of sunlight to create the blue sky that we see on earth. Hence, the sky in a direction other than that of the sun appears black to an observer on the moon.

longest, scattered

Reason — When light passes through a medium, shorter wavelengths (such as blue and violet) are scattered more strongly than longer wavelengths (such as red and orange). Hence, red light is often used for danger signals to ensure visibility over longer distances.

Scattering is the process of absorption and then re-emission of light energy by the dust particles and air molecules present in the atmosphere.

The scattering of light is not same for all wavelengths of incident light.

The intensity of scattered light is found to be inversely proportional to the fourth power of the wavelength of light

( i.e., I ∝ $\dfrac{1}{λ^4}$ ).

The above relation holds true, when the air molecules are smaller in size than the wavelength of incident light.

The air molecules of size bigger than the wavelength of incident light, scatter the light of all wavelengths of white light to the same extent.

For example — The wavelength of violet is least (λ = 4000 Å) and that of red light is most (λ = 8000 Å), therefore, from the incident white light, violet light is scattered most and the red light is scattered the least. (violet light is scattered nearly 16 times more than the red light).

Red colour of white light is scattered the least because wavelength of red light (λ = 8000 Å) is longest and intensity of scattered light (I) ∝ $\dfrac{1}{λ^4}$.

In the visible light, the wavelength of red light is longest, therefore the light of red colour is scattered least by the air molecules of the atmosphere.

Hence, the light of red colour as compared to the light of other colours can penetrate to a longer distance without becoming weak.

Thus, red light can be seen from the farthest distance in comparison to the light of other colours having the same intensity.

Hence red light is used for danger signal, so that the signal may be visible from the far distance even in fog, etc.

When seen from the space (or moon), the sky would appear black in colour.

As there is no atmosphere on moon, therefore, no scattered Sun light reaches the moon's surface. Hence, to an observer on the surface of moon, no light reaches his eyes, except the light reaching directly from the Sun.

Thus, the sky in direction other than the that of the Sun will appear black.

Similarly, when an astronaut goes above the atmosphere of the earth in a rocket, he sees the earth black.

The characteristic property of light responsible for the blue colour of sky is scattering.

The light from the Sun has to travel a long distance of the earth's atmosphere before reaching us. As light travels through the atmosphere, it gets scattered in different directions by the air molecules present in it's path.

The blue (or violet) light due to its short wavelength is scattered more as compared to the red light of longer wavelength.

Thus, the light reaching our eye directly from Sun is rich in red colour, while the light reaching our eye from all other directions is the scattered blue light.

Therefore, the sky in the direction, other than the direction of Sun, is seen blue.

At the time of sunrise and sunset, the light from Sun has to travel the longest distance of atmosphere to reach the observer.

Since, the blue light of short wavelength is scattered more, much of it is lost, while the red light of long wavelength is scattered a little, so it is not much lost.

Thus, blue light is almost absent in sunlight reaching the observer and only the red (white - blue = red) light reaches us.

As a result, the Sun and the region near by it, is seen red.

At noon, the Sun is directly above our head, so we get the light rays directly from the Sun after travelling the shortest distance, without much scattering of any particular colour. Hence, the sky is seen white.

The clouds are nearer the earth surface and they contain dust particles and aggregates of water molecules of size bigger than the wavelength of visible light.

Therefore, the dust particles and tiny ice particles present in clouds scatter all colours of incident white light from Sun to the same extent and hence when the scattered light reaches our eyes, the clouds are seen white.

The smoke from a fire looks white because the molecules of smoke are bigger than the wavelength of light, so they scatter lights of all colours equally and the scattered light appears white.

(i) Violet is scattered the most.

(ii) Red is scattered the least.

(a) Yellow light is scattered the least as it has the highest wavelength.

(b) Blue light is scattered the most as it has the lowest wavelength.