Why the colour of clear sky is blue?

Why the colour of clear sky is blue?

The atmosphere of earth is full of particles or molecules like N2, O2, O3, H2, H2O, dust particles etc. These are so small that we cannot see them with naked eyes. Even with powerful microscope of 100X or higher available in school labs, air particles are not visible (except dust particles). When sunlight encounters these particles there is a change in the direction of sun rays which lead to a phenomenon known as scattering of light.The sunlight consists of different frequencies from 430 – 770 THz (or wavelengths 390 -700 nanomoetres). The colour of the scattered light depends upon the size of the encountered particles. The process of selective scattering is known as Rayleigh scattering.

The size of the air particles has size comparable to the wavelength of visible light at the blue end. These particles are more suitable in scattering light of blue and violet colours as compared to red colour. This is the reason why sky appears blue in colour.

Due to pollution, large size particles are introduced in the atmosphere. These particles are efficient in scattering light of longer wavelengths also. This causes the pale blue or grey colour of the sky in a polluted atmosphere. 

Watch the video shown below on youtube. This video shows how a laser light is scattered by water and talcom powder. The scattering of laser by water or talcom powder makes its path visible (known as Tyndall's  effect) and this property can be used to study relection or refraction.

Uses of spherical mirrors

Uses of spherical mirrors 

Uses of Concave Mirror

1. A concave mirror is used as a reflector of light in headlights of automobiles to obtain a parallel beam of light. In a similar way concave mirrors are also used in torch lights and search lights.

In such a case, the source of light such as bulb is placed at the focus of the concave mirror. The light rays which fall on the concave mirror are reflected parallel to the principal axis and thus a parallel beam is obtained.

Practically, there are two bulbs one above the focus to obtain low beam (to light the ground nearby) and the other bulb slightly below the focus to obtain a high beam (to illuminate a larger distance but this will blind the driver approaching from opposite side).

In some reflectors the position of the bulb can be shifted slightly to obtain a low beam/high beam.


2. A concave mirror is used as a dentist’s mirror.
Dentists use a concave mirror to obtain the enlarged image of a tooth. The focal length of concave mirror used is large enough so that object (tooth) is placed between Focus and Pole. Thus a virtual, erect and magnified image of the tooth is obtained.

The position of object (tooth) is very important. Imagine what would happen if dentist view an inverted image using a concave mirror if it is placed between C and F. the image obtained will be magnified but inverted. In confusion, dentist may remove a healthy tooth.

3. Concave mirrors are used as concentrator of heat and light in solar furnace.
A solar furnace can be constructed by using a huge concave mirror or an array of plane mirrors mounted on a curved surface giving rise to a concave shape. The huge concave mirror is directed towards the sun. The sun’s rays get focused at F. At focus F, the temperature will be very high as all heat rays (infra red) get converged there. The temperature can reach up to 3500°C which can be used to melt metals.

One such solar furnace is installed in Mount Louis in France.

4. A concave mirror can be used as a shaving mirror.
Such a mirror will have a large focal length say 1m or 1.5m so that the person standing nearby would be placed between F and P. A virtual, erect and magnified image of the face is obtained which helps the person to see an enlarged image of his face while shaving.



Uses of Convex mirrors

1. It is used as a rear view mirror in automobiles.

Since a convex mirror provides a wider field of view and erect image of the object, it is a perfect choice for the rear view mirror.

The driver can view the wide view of traffic behind the vehicle.

The only disadvantage with the convex mirror is that the driver may be confused about the actual distance of the traffic behind his own vehicle. The image formed by the convex mirror is diminished and it gives an illusion that the object is very far. Even a closer object may appear very far in a convex mirror.

You can find this caution in every rear view mirror of automobile “Objects in the mirror are closer than they appear”.

2. Convex mirror are also used at the intersection of a busy traffic and at sharp curve.

Power of a lens

Power of a Lens

The power of a lens tell us its ability to converge or diverge a  beam of light falling on it.

Observe the following two convex lenses

The convex lens (A) with the short focal length converge the light rays by large angles and focus the rays close to the optical centre. Thus the lens with short focal length has a high converging power. 

The other lens (B) with longer focal length converge the light rays by small angle and focus the rays far from the optical centre. This lens with larger focal length has a low converging power.

Similarly for a concave lens, the shorter the focal length more is the diverging power of the lens.

Thus power of a lens may be defined as the reciprocal of its focal length in metres.

\[P=\frac{1}{f}\]
\[P=\frac{100}{f}\]  (if f is expressed in cm)

SI unit of power of a lens is dioptre. It is denoted by the symbol D.

Power of lens is said to be one dioptre if its focal length is 1m.
\[P=\frac{1}{f}\] 
\[P=\frac{1}{1}=1\]

Convex lens has a positive power as the focal length of convex lens is positive.

A concave lens has a negative power as the focal length of concave lens is negative.

Power of a combination of a lens

If lenses are combined then power of the combination is simply the algebraic sum of power of individual lenses.

Spherical lenses

Spherical Lenses
A spherical lens is a transparent medium bounded by two surfaces. Atleast one of the two surfaces must be spherical.
If the lens has one spherical surface then other surface will be plane. This results into two types of lenses.
(a) Plano convex lens
(b) Plano concave lens

If both the surfaces of the lens are spherical then following lens are obtained.
(c) Double convex lens ( or simply known as convex lens)
(d) Double concave lens ( or simply known as concave lens)

(e) Convexo concave
(f) Concavo convex 

Few terms related to spherical lens

Optical centre (O) : The centre of the lens is known as optical centre.

Principal axis : The line passing through the centre of curvatures of spherical surfaces of lens and the optical centre is called principal axis.

Principal focus of a convex lens

When incident rays parallel to the principal axis falls on a convex lens then after refraction through the lens, all rays meet at a point on the other side of the lens. This point is known as the principal focus of a convex lens.

Since a lens is transparent light can enter the lens from either of the two surfaces. Hence a lens has two foci labeled as F1 and F2.

Focal length (f): The distance between optical centre and focus of a spherical lens is called focal length.

2F1 and 2F2 are the points located at twice the distance of focal length from the optical centre of the lens.

Principal focus of a concave lens

When incident rays parallel to the principal axis falls on a concave lens then after refraction through the lens, all rays diverge and appears to coming from a point on the principal axis. This point is known as the principal focus of a concave lens.

Since a lens is transparent light can enter the lens from either of the two surfaces of the concave lens. Hence a concave lens has two foci labeled as F1 and F2.

Watch a video for formation of image by a convex lens from our YouTube partner 'Learn n Hv Fun'.

Working of electric motor

ELECTRIC MOTOR

An electric motor is a rotating device which converts electrical energy into mechanical energy.
It means it takes energy from electricity and using this energy the motor system rotates its rotator. The motion of rotator means that it possesses mechanical energy.
It appears so simple but we have to understand the process by which this energy change take place.

Image Credit -Lookang many thanks to Fu-Kwun Hwang and author of Easy Java Simulation = Francisco Esquembre, Ejs Open Source Direct Current Electrical Motor Model Java Applet ( DC Motor ) 20 degree split ring, CC BY-SA 3.0

The basic principle behind the working of motor is that when a current carrying wire is placed in a magnetic field it experiences a force. The direction of this force can be determined by Fleming’s left hand rule.

Thus using electrical energy we setup an electric current in a coil
and in an electromagnet. The electromagnet thus behaves like a magnet. The current carrying coil when placed in magnetic field of electromagnet experiences a force. Using suitable arrangement and designing, the coil can be made to rotate.

Construction
A simple electric motor consists of a rectangular coil ABCD of insulated copper wire placed between two opposite poles magnets as shown in the figure. The ends of the coil are connected to two half of a split ring (S1 and S2) attached to the axle. The split rings are connected to two carbon brushes B1 and B2 as shown in the figure. The carbon brushes are connected to a battery through connecting wire and a key (or switch).

©Udvita.org

Working:
First Half Cycle

Let the plane of coil is initially placed horizontally as shown in the figure. The direction of current in the coil is along ABCD. The direction of magnetic field is from North pole to the south pole.
By applying Fleming’s left hand rule on arm AB, the direction of force on arm AB is downward. Similarly the direction of force on arm CD is upward. Under the action of two equal and opposite will make the coil mounted on an axle to rotate anticlockwise.

Second Half Cycle
After half a rotation, arms AB and CD will interchange its position. The split ring S1 is now in contact with brush B2 and the split ring S2 is in contact with brush B1. The direction of current in the coil is now DCBA, reversed as compared to first half cycle. A device which reverses the direction of current in a circuit is called commutator. In electric motor, split rings acts as commutator.

©Udvita.org

By applying Fleming’s left hand rule on arm AB, the direction of force on arm AB is upward. Similarly the direction of force on arm CD is downward. Again under the action of two equal and opposite will make the coil mounted on an axle to rotate anticlockwise.

Commercial motor
A commercial motor consists of an electromagnet instead of permanent magnets. The current carrying coil consists of a large number of turns (in thousands). A soft iron core is used on which the coil is wound.
The soft iron core along with the coil is called the armature.

Image credit-Abnormaal, Electric motor, CC BY-SA 3.0

Uses of electric motor
Electric motor is used in electric fans, water pumps, mixer, MP3 player, computer etc

Simplest Electric Motor
Watch the Simplest electric Motor video made by our YouTube Channel partner 'Learn n hv Fun'.
The motor is simply made using a copper coil and few neudymium magnets using a 1.5V  electric cell.

Human Eye - Accomodation

HUMAN EYE – ACCOMMODATION

A human eye with normal vision can see nearby objects as close as 25cm clearly and distinctly and far away objects as far as infinity clearly without any strain in eyes. 

To see an object clearly the eye lens must focus the image on the retina. This sharp focusing is achieved by eye lens with the help of ciliary muscles. The ability of eye lens to adjust its focal length to focus objects at different distances on the retina is called accommodation.

How Accommodation is achieved?

An eye lens is a jelly like material made up of protein. The curvature/shape of the eye lens can be changed to some extent by the action of ciliary muscles. When ciliary muscles contract, curvature of eye lens increases. This decreases focal length of eye lens. This enables us to see nearby objects clearly. 

Similarly when ciliary muscles relax, curvature of eye lens decreases. This increases focal length of eye lens. This enables us to see far away objects clearly. 

The following steps would be easier to understand the process.

To view nearby objects:

Ciliary muscles contracts → curvature of eye lens increases → eye lens becomes thicker → focal length of eye lens decreases → image of nearby objects focused on the retina 

The above action of ciliary muscles and eye lens enables us to see nearby objects clearly.

To view far away objects:

Ciliary muscles relaxes in this situation → curvature of eye lens decreases → eye lens becomes thinner → focal length of eye lens decreases → image of far away objects focused on the retina. 

The above action of ciliary muscles and eye lens enables us to see far objects clearly.

The power of accommodation for a normal vision is 4 dioptre.

The power of eye lens when a nearby objects is viewed is 44 D. The power of eye lens when a far away object is viewed is 40 D. Thus the maximum variation in the power of eye lens achieved is 44D – 40D = 4D. 

Least distance of distinct vision (Near point)

The minimum distance at which the object can be see clearly and distinctly without any strain in the eyes is called the least distance of distinct vision or near point of the eye. 

For a person with normal vision this value is 25 cm.

What would happen if the object is closer than 25 cm? Would any image be formed on eye lens?

Yes, the image would be formed but that will be blurred, not well focused. Try focusing on the object. Some of you may be able to focus the object but if would cause strain in your eye after few seconds. 

Try holding a pen or a pencil close to your eye or nose. At first the object will appear blurred but if you try to focus object may get focused to some extent causing strain in your eye. You have to either shift your focus or have to move away the object from you.

Reason:

The curvature of the eye lens can be change up to some extent only and that with the help of cilicary muscles. Thus focal length of eye lens cannot be decreased beyond a certain minimum limit. Hence there is minimum distance at which objects can be seen comfortably. That minimum comfortable distance is 25 cm. This is called near point or least distance of distinct vision.

Far point

The farthest point up to which a human eye with normal vision can see is infinity and this is called the far point of human eye. 

Try focusing a star. These stars are hundred, thousand light years from us. The nearest star Alpha centuri is 4.2 LY away. ( 1 LY = 9.46 X \(10^{15}\) m)