Some Natural Phenomenon Due to Sunlight

You see the blue sky, the white clouds, the red sky at the time of

sunset and sunrise. All these are nothing but natural wonders. Do you

know the sky appears to be blue or the clouds to be white? This is all

due to the sunlight. Let us study about one such natural phenomenon

due to sunlight that is the rainbow.

Scattering of Light

When we enter a dark room, usually we cannot see the path of light.

But with the help of the laser beam, we are able to see the path. This is

due to the scattering of light by small particles of air in the path of the

laser beam. The phenomenon in which path of the light incident on the

particle is redirected in a different direction this is called scattering of

light. Light of shorter wavelengths is scattered much more than the

light of longer wavelengths.

Sometimes we do wonder why does the sky appear blue? Since the

wavelength of the blue color is smaller than the wavelength of the red

color, the scattering of the blue light by the particles in earth’s

atmosphere is very large. Although the violet light is scattered more

than the blue light, our eyes are not very sensitive to violet light. This

is the reason we see the sky as blue.

The cloud is composed of dust particles and molecules of water. These

particles are large and do not obey the law of scattering. Therefore all

the colors are scattered equally. So the clouds are white. At sunrise or

sunset, sunlight has to travel through the atmospheric air for longer

distance. As a result, a large number of air particles come in its way.

These particles scatter most of the blue light and make the sun look

orange and red.

The Rainbow

Everyone must have seen the rainbow. The rainbow that appears in the

sky is the most beautiful optical phenomenon. The sunlight passing

through the water droplets present in the atmosphere undergo

reflection and refraction to form a rainbow. Sometimes after the rains

two rainbows are seen. The two rainbows are the primary rainbow and

secondary rainbow.

If the sunlight undergoes one internal reflection in the raindrops

before emerging than the rainbow formed is the primary rainbow.

Primary rainbow is brighter and narrow. It has red color at the outer

edge and violet at the inner edge. The red light has a longer

wavelength and is bent at least. The violet with a shorter wavelength

is bent most.

While forming a secondary rainbow, light rays undergo two internal

reflections inside the water drops and due to this, it appears to be faint.

The colors are reversed in the secondary rainbow with red at the inner

edge and violet at the outer edge.

Question For You

Q. Very fine particles mainly scatter _______ colored light.

A. Red

B. Blue

C. Green

D. Yellow

Answer: B. Very fine particles mainly scatter blue light as the

wavelength of blue color light is lowest as compared to red color. Fine

particles being smaller in size scatter the shorter wavelength light.

Total Internal Reflection

You must have seen diamonds sparkling. Do you know why do the

diamonds sparkle? Or suppose you put a coin below the glass and put

some water in the glass. You cannot see the coin from the sideways

but the same coin is visible to you if you look at it from the top. Why

does this happen? The answer to this is an internal reflection. Let us

study the total internal reflection in detail. 

Total Internal Reflection

To understand the concept of total internal reflection let us carry out a

small activity. Take a coin and a glass. Now put the coin below the

glass and fill the glass with water. Why did the coin disappear? We

can see the coin from the top of the glass but if we try to see it

sideways, we cannot see it. The coin is invisible to us. Why did this

happen?

This is because when there is water in the glass, light from the coin

travels through the glass to our eyes at a particular angle. Further on

adding water to the glass the light from the coil hits the inside of the

glass at an angle greater than the critical angle. All the light from the

glass is internally reflected. This is what internal reflection is.

Total internal reflection is defined as the complete reflection of a light

ray at the boundary of two media when the ray is in the medium with

greater refractive index. Total internal reflection takes place in two

following conditions:

● When the light is in the more dense medium and approaching

the less dense medium.

● If the angle of incidence is greater than the critical angle.

How is the sparkling of diamond-related to total internal reflection?

A diamond sparkles as when the rays of light enter the diamond they

suffer total internal reflection at its various faces. The critical angle for

the ray of light traveling from the diamond to air is very low about

24º. Therefore most of the incident rays suffer total internal

reflections.

Also, the diamonds are usually cut in such a way that whenever the

ray of light enters it, it suffers total internal reflections at various

faces. When the angle of incidence at any face is less than 24º, the

light comes out from it and diamond appears to be bright. Total

internal reflection is also used in optical fibres. Optical fibres are used

in transmitting video and audio signals to a long distance.

Questions For You

Q. A ray of light, traveling in water, is incident on its surface open to

air. The angle of incidence θ which is less than the critical angle. Then

there will be:

A. only a reflected ray and no refracted ray

B. only a refracted ray and no reflected ray

C. a reflected ray and refracted ray and the angle between them

would less than π- 2θ

D. a reflected ray and refracted ray and the angle between them

would greater than π- 2θ

Answer: C

The ray will be partly reflected and partly refracted. ∠MOB = π – 2θ.

But the angle between refracted and reflected ray is ∠POB. Clearly,

∠POB is less than ∠MOB that means less than π – 2θ

Reflection of Light by Spherical Mirrors

When you look in the mirror have you noticed something interesting

about you and your image in the mirror? Let us carry out a small

activity. Stand in front of the mirror and move your right hand. Now

lift your left hand. Did you notice that in the mirror the right appears

left and vice versa? Let us study in detail about a spherical mirror. 

Spherical Mirror

Suppose you are sitting at the dining table and you don’t like the food,

you start playing with the spoon. You look yourself in the spoon and

you notice that you look pretty funny. The moment you get the spoon

closer you get a magnified image and when taken far, you see an

inverted image.

Do you know what’s really happening? To understand what is

happening lets us talk about the special class of mirrors known as

spherical mirrors. Let us first understand the terms of spherical

mirrors.

● The radius of Curvature (c): It is the distance between Pole and

the Center of curvature.

● Center of Curvature (r): The Center of Curvature of a spherical

mirror is the point in the center of the mirror which passes

through the curve of the mirror and has the same tangent and

curvature at that point.

● Aperture: It is a point from which the reflection of light

actually happens.

● Pole (p): Pole is the midpoint of a mirror. It’s twice the focus.

● Focus: It is any point, where light rays parallel to the principal

axis, will converge after reflecting from the mirror.

● Principal axis: An imaginary line passing through the optical

center and the center of curvature of the spherical mirror.

● Focal Length: It is on the axis of a mirror where rays of light

are parallel to the axis converge after reflection or refraction.

Spherical mirrors are of two types

● Convex Mirror

● Concave Mirror

Concave Mirror

A concave mirror is also known as the converging mirror as in these

type of mirrors light rays converge at a point after they strike and are

getting reflecting back from the reflecting surface of the mirror.

Convex mirror

The convex mirror has a reflective surface that curves outward. These

mirrors are “always” form virtual, erect and diminished regardless of

the distance between the object and mirror.

When parallel rays of light strike the mirror, they are reflected in a

way wherein they spread out or diverge. For this reason, a convex

mirror is also a diverging mirror too. If these reflected rays are

extended behind the mirror by dotted lines, they meet at a point.

This point is the focus of the convex mirror. The concave mirror is

used in the vehicle so that the driver is aware of the vehicle coming

from behind. They are also used in street light reflectors.

Video on Ray Optics

Reflection of Light

Any polished or shiny surface like that of an water can act as a mirror.

When a ray of light falls on such smooth or shiny object light from the

object bounces back those rays of light to our eyes and this

phenomenon is Reflection of Light.

Laws of Reflection

In the diagram given above, the ray of light that approaches the mirror is the “Incident Ray”. The ray that leaves the mirror is the “Reflected Ray”. At the point of incidence where the incident ray strikes the mirror, a perpendicular line is drawn is the “Normal”. This normal is

what divides the incident ray and the reflected ray equally and gives us the “Angle of Incidence”

θ

i

and “Angle of Reflection”

θ

r

.

The Laws of Reflection are :

● The angle of incidence is equal to the angle of reflection.

● The incident ray, the normal and the reflected ray, all lie in the

same plane.

● The reflected ray and the incident ray are on the opposite sides

of the normal.

Types of Reflection

● Regular Reflection

● Diffused Reflection

Regular Reflection

It is a mirror-like reflection of rays of light. Here the rays of light

which are reflected from a smooth and shiny object such as a mirror,

are reflected at a definitive angle and each incident ray which is

reflected along with the reflected ray has the same angle to the normal

as the incident ray.

Diffused Reflection

This is a non-mirror-like reflection of light. In this type of reflection

rays of light hit an irregular object with a rough surface, and reflects

back in all directions. Here, the incident ray which is reflected along

with reflected ray doesn’t have the same angle to the normal as the

incident ray.

Reflection of Rays From a Spherical Mirror

Learn more about the Image Formation of Spherical Mirror here.

Question For You

Q. When the object is the focus of a concave mirror, the image is

formed at

A. focus

B. the center of curvature

C. within a focus

D. infinity

Answer: D. When the object is the focus of a concave mirror, the

image is at infinity.

Refraction

Suppose there is only one fish in the aquarium. But still, it appears to

be many. Or while traveling on a road on a hot summer day, distantly,

water appears in the middle of the road out of no-where. Do you know

how this happens? This is all due to the refraction. Let us study more

about refraction. 

Refraction

The change in direction or bending of a light wave passing from one

transparent medium to another; caused by the change in wave’s speed

is known as “Refraction”.

An example to understand this better is that of placing a straw/stick in

a glass of water wherein it to be bent when viewed from any other

angle than 900 to the surface. This happens because of the bending of

light rays as they move from air to glass. This bending of light

depends on the speed of light in air and glass and the speed is

dependent on the wavelength.

Another example of refraction is if you take a pencil and dip it in

water, the pencil appears to be bent. It does not appear straight. Why

is the pencil appearing bent even though it is a straight nice pencil?

This is because of a phenomenon of refraction. The medium involved

here is air and water. As soon as the light waves enter the water, the

light rays bend and because of this bending of light waves, we see the

pencil as broken.

The extent of bending of light rays entering from one medium to

another is the “Refractive Index”. It is denoted by the letter ‘n’. It is

represented as :

n = c/v

where c = velocity/speed of light of a certain wavelength in the air and

v = velocity of light in any medium.

Learn about the Laws of Reflection here

Snell’s Law

It gives the amount of bending of light rays. It also determines the

relationship between the angle of incidence, the angle of refraction

and relative indices of a given pair of media. It is the ratio of the sine

of the angle of incidence to the sine of the angle of refraction is a

constant for the light of given color and for the given pair of media.

Laws of Refraction

The angle of incidence is the angle between the incident ray and the

normal; denoted as ‘i’. The angle of refraction is the angle between the

refracted ray and the normal; denoted as ‘r’. Laws of refraction state

that:

● The incident ray, reflected ray and the normal, to the interface

of any two given mediums; all lie in the same plane.

● The ratio of the sine of the angle of incidence and sine of the

angle of refraction is constant.

What is Total Internal Reflection?

Solved Example For You

Q. A biconvex lens of focal length ‘f ‘forms a circular image of the

sun of a radius ‘r’ in a focal plane. Then,

A. πr² ∝ f

B. πr² ∝ f²

C. If the lower part is covered by a black sheet, the area of the

image is equal to πr²/2

D. If ‘f ‘ is doubled, the intensity will increase.

Answer: B

from the above image, we can say that, r = f tan α

Hence, πr² ∝ f²

Refraction at Spherical Surface and By Lenses

Every one of us has used the spherical lenses in some way or the

other. The lens we use in spectacles, the mirror of our vehicle, is

nothing but the spherical surface. But do you know what causes

refraction in the spherical surface? Let us study in detail about

spherical surface and also about refraction in lenses. 

Refraction at Spherical Surface

Let us now see the refraction of light at the spherical surface. Now, the

change in direction or bending of a light wave passing from one

transparent medium to another caused by the change in wave’s speed

is the Refraction. Suppose the above figure is a spherical surface.

There is one medium with refractive index n1 and second medium

with refractive index n2.

There is an object O and a ray of light from the object O is incident on

the spherical mirror. Since it is moving from a rarer medium to a

denser medium, the ray bends towards the normal. An image is

formed and radius of curvature of a spherical surface is R with the

center C of the spherical surface.

● ”u” is the object distance from a pole of a spherical surface

● ”v” is the image distance from a pole of the spherical surface

Now as we know that,

● n1 is the refractive index of a medium from which rays are

incident.

● n2 is the refractive index of another medium

We get,

● tanα = ● MN

● OM

●

● tanγ = ● MN

● MC

●

● tanβ = ● MN

● MI

●

Now, for Δ NOC, i is the exterior angle.

i = ∠ NOM + ∠ NCM

i=

MN

OM

+

MN

MC

…….1

Similarly,

r =

MN

MC

–

MN

MI

…….2

Now by using Snell’s law we get

n1 sin i = n2sin r

Substituting i and r from Eq. (1) and (2), we get

n

1

OM

+

n

2

MI

=

n

2

−

n

1

MC

As, OM = -u, MI = +v, MC = +R

Hence, the equation becomes

n

2

v

–

n

1

u

=

n

2

−

n

1

R

Types of Lenses

I. Convex or Convergent Lenses

II. Concave or Divergent Lenses

A lens is a part of a transparent thick glass which is bounded by two

spherical surfaces. It is an optical device through which the rays of

light converge or diverge before transmitting. Thus spherical lenses

are of two major kinds called Convex or Convergent lenses and

Concave or Divergent lenses. The point from which these rays

converge or appear to diverge is called the Focus or Focal point.

Convex or Convergent Lenses

A convex lens is thicker in the middle and thinner at the edges. A

convex lens is also known as a “biconvex lens” because of two

spherical surfaces bulging outwards.

Concave or Divergent Lenses

A concave lens is thicker at the edges and thinner in the middle. A

concave lens is also known as a “biconcave lens” because of two

spherical surfaces bulging inwards

Ray diagrams for image formation by lenses

Lens Formula

Lens formula relates the image distance (v), object distance(u) and the

focal length (f) of the lens.

1

v

–

1

u

=

1

f

Question For You

Q1. A fish sees the smiling face of a scuba diver through a bubble of

air between them, as shown. Compared to the face of the diver, the

image seen by the fish will be:

A. small and erect

B. smaller and inverted

C. larger and erect

D. larger and inverted

Answer: A. The image will be smaller and erect since the air bubble

act as a concave lens which always forms smaller and erect images.

Refraction Through a Prism

Have you ever observed that the people standing in the pool always

look shorter then they are? Also, the spoon in the glass of water

appears to be bent. Why does this happen? This is because of

refraction. Let us now study about ”refraction through a prism”. 

What is Refraction?

Before studying how refraction takes place through a prism, let us see

what refraction is. The change in direction or bending of a light wave

passing from one transparent medium to another caused by the change

in wave’s speed is the refraction. The extent of bending of light rays

entering from one medium to another is the refractive index and is

denoted by the ‘n’.

It is represented as n = c/v, where c = velocity/speed of light of a

certain wavelength in the air and v = velocity of light in any medium.

What is a Prism?

It is a solid figure having two triangular bases and three rectangular

surfaces and is the closed surface. The angle between each surface is

the angle of the prism. Here the opposite surfaces are equal surfaces

and are parallel. We notice that here there are two refracting surfaces

which means the surface where refraction of light takes place.

Learn more about Human eye and Defects of Vision here.

Refraction Through a Prism

Let A, B, C be the glass of the prism. Suppose BC is the base and AB

and AC are its two refracting surfaces. From the above figure, we can

say that OP is the incident. The ray traveling through the rarer medium

and than the refractive index of the prism is the incident ray. As the

ray PQ strikes the surface of the and it is called as the refracted ray.

OR is the emergent ray which comes out.

When the ray light enters the glass, it bends towards normal and when

ray comes out, it bends away from the normal. Now the angle between

the emergent ray and incident ray is the angle of deviation. For a

single refracting surface, δ = |i – r|

In this case, δ = (i1 + i2) – (r1 + r2)

δ = i1 + i2 – A, A is the angle between the prism between two lateral

surfaces. We know that ∠A and ∠Q is 180º and Angle of the prism

of (A) is r1 + r2

r1 is the angle of refraction inside the prism and r2 is the angle of

refraction outside it. For an angle of minimum deviation, δ is

minimum and i1 = i2 = i

δmin = 2i – A

For small A, δ = (µ – 1) A

Minimum Angle of Deviation for a Prism

At the minimum deviation, Dm the refracted ray inside the prism

becomes parallel to its base, i.e. i = e⇒ r1 = r2 = r, then r = A/2 and

Dm = 2i – A, where i is the angle of emergence, r1 and r2 are the

angles of refraction and A is the angle of the prism.

Learn more about Atmospheric Refraction and Scattering of Light

here.

Question For You

Q. A prism made up of flint glass is such that the incident ray does not

emerge from the second surface. The critical angle for flint glass is

36º. Then, refracting angle A must be

A. 37º

B. 54º

C. 71º

D. 73º

Answer: D. In the prism to occur the total internal reflection, the

reflecting angle must be more than twice the critical angle of the

material. So here the critical angle for flint glass is 36º so the

refracting angle must be greater than 72.

Dispersion By Prism

Suppose on a rainy day when the roads are wet and you are driving a

car or riding a bike sometimes you see that the petrol spills on the

road. When the petrol mixes with the water we can see different layers

of colors on the road. Why does this happen? What is the phenomenon

behind this? The answer to this is dispersion. Now let us study about

the dispersion of prism. 

Prism

It is a solid figure having two triangular bases and three rectangular

surfaces and is the closed surface. The angle between each surface is

the angle of the prism. In a prism, the opposite surfaces are equal

surfaces and are parallel. There are two refracting surfaces which

mean the surface where refraction of light takes place. We are not

concerned about other faces.

Dispersion

With the help of a narrow beam of light, a glass prism, and a white

wall it is possible to produce the band of seven colors using white

light. Keep this arrangement near the window. Place the glass prism in

such manner that the sunlight through the window falls on one side of

the prism and then on the white wall.

You can see that the light reflected on the wall has several colors. The

prism splits the white light into seven different colors. This splitting of

white light into many colors is called as a dispersion of light.

Dispersion is nothing but splitting of white light into its constituents

colors.i.e into seven different colors.

The seven colors are violet, indigo, blue, green, yellow, orange, and

red (VIBGYOR). The pattern of color which is obtained is called as a

spectrum. Sometimes in the rainbow, you may not see all the seven

colors. This is because of the colors overlap each other.

Let us study about Refraction through a Prism in detail.

What Causes Dispersion of Prism?

Dispersion of prism takes place because white light entering the prism

consists of so many different colors. Each of these different colors has

a different wavelength. According to Cauchy’s formula, refractive

index (μ) of a material depends upon wavelength (λ) and is given by,

μ = a+

b

(λ²)

+ \( \frac{c}{(λ4)} \), where a, b, c are constants of the material.

Since the wavelength of violet light is smaller than of the red light μv >

μr, therefore the violet light has a larger angle than the red light. As a

result, the dispersion of white light takes place on the second surface

of the prism.

Questions For You

Q1. The dispersion of white light occurs because the angle of

………………….. of different colors is different when passing

through the glass prism.

A. Reflection

B. Refraction

C. Incidence

D. None of these

Answer: B. The dispersion of white light occurs because the angle of

refraction of light of different colors is different when passing through

the glass prism.

Q2. Which of the following is not a primary color?

A. Yellow

B. Red

C. Green

D. Blue

Answer: C. The colors which cannot be obtained by mixing of two or

more colors are said to be primary colors. Red, yellow and blue are

primary colors while green is not.

Optical Instruments

How do see the objects around us? Yes, with our eye. It is a most

important organ of the human body which enables us to view all

things around us. An eye is one of the most important optical

instrument. Let us study in detail about other optical instruments used

by us. 

The Eye

The human eye is an optical instrument that enables us to view all the

objects around us is a very complex organ. Let us study the structure

of the human eye. The white protective membrane seen when looked

into the eye directly is a Sclera. It is tuff, opaque and fibrous outer

layer of the eyeball.

The circular part is the Iris. The color of the eye is determined by the

color of the iris. The center transparent area of the iris is the Pupil.

The iris works like the shutter of the camera. It absorbs most of the

light falling on it and allows it to pass through the pupil.

The amount of light that enters the inner part of the eye depends on

the size of the pupil. In bright light, the iris contracts the pupil to

restrict the light, whereas in low light it widens the pupil to emit more

light into the eye. The eyeball is spherical in shape. The retina of the

eye is able to detect the light and its color because of the presence of

senses known as rods and cones.

Light entering the human eye is first refracted by the cornea. The

refracted light is then incident on an iris. The lens is just behind the

iris and light after refracted through the pupil falls on it and forms a

sharp image. Image formation exactly on the retina enables us to see

the object clearly.

Defects in Human Eye

Like the ability to focus diminishes with the age of a person and this

defect is Presbyopia. This defect is corrected by using converging

lenses. The other defect is Hypermetropia. It is observed in the people

of any age. The person suffering from this defect will have a normal

vision while looking at the far object. But the vision is blurred for

nearby objects. This is corrected by using a convex lens.

The Microscope

As we all know Microscope is an optical instrument used to view

small object. Let us first talk about the simple microscope.

A simple microscope is an optical instrument, we use for the

magnification of small objects to get a clear image or vision. It is a

convex lens having a short focal length. This microscope is at a small

distance from the object for the magnification and hence this forms a

virtual image. The simple microscope enables us to view very small

letters and figures. Watchmakers also make use of these. Now let us

see what compound microscope is.

With a compound microscope, we get very large values of

magnification. We use this microscope to see microscopic objects like

microorganisms. It comprises of two convex lenses and magnification

occurs in both of these lenses. the components of a compound

microscope are eyepiece, objective lens, fine and rough adjustment

screw.

Video on Optics

Telescope

The telescope is of two types. One is the reflecting type and another

one is the refracting type. Reflecting telescopes are the ones which do

not use lenses at all. They use mirrors to focus the light together. The

type of mirror used is a concave mirror.

Mirrors also bend the light together, except that they do it by

reflecting the light instead of bending it. Refracting telescopes work

by using two lenses to focus the light and make it look like the object

is closer to you than it really is. Both the lenses are in a shape of

‘convex’. Convex lenses work by bending light inwards.

Question For You

Q1. In a compound microscope, the intermediate image is:

A. Virtual, erect and magnified

B. Real, erect and magnified

C. Real, inverted and magnified

D. Virtual, erect and reduced.

Answer: C. As shown in the figure, the intermediate image P’Q’ is

real, inverted and magnified.