Magnetic Effect Of Electric Current And Light

The magnification produced by a mirror is +1/3. Then the mirror is a convex (concave mirror, convex mirror, plane mirror)

Explanation: The magnification of a mirror is given by

Where v is the image distance and u is the object distance.

The magnification can only be positive if the object and the image are on the opposite sides of the mirror, which is achieved only when we use a convex mirror.

The phenomenon of producing an emf in a circuit whenever the magnetic flux linked with a coil changes is Electromagnetic induction. (Electromagnetic induction, inducing current, inducing voltage, change in current)

Explanation: With the change of magnetic field associated with a coil, an emf is produced in the circuit. This phenomenon is known as electromagnetic induction.

This phenomenon is in accordance with conservation of energy.

An electric current through a metallic conductor produces magnetic field around it. (Magnetic field, mechanical force, induced current)

Explanation: As an electric current passes through a metallic conductor magnetic field is produced around it. The magnetic field lines (which indicate the magnetic field) due to a metallic conductor is shown in the figure below.

The field of view is maximum for convex mirror (plane mirror, concave mirror, convex mirror)

Explanation: The field of view is the size of the largest object that can be seen through a mirror that is located at a fixed position.

This figure shows the wider field of view of a convex mirror than a plane mirror and as a concave mirror is used to concentrate the light rays, we can conclude that a convex mirror will have the widest field of view.

An object is placed 25 cm from a convex lens whose focal length is 10 cm. The image distance is . (50 cm, 16.66 cm, 6.66 cm, 10 cm)

Explanation:

Given: u= -25cm

f= +10cm

v=? (To be found)

According to the lens formula,

Hence, the answer is 16.33cm

C

A commutator is a device used in a motor to rectify the direction of current for a constant and unidirectional flow of current. Galvanometer and transformers don’t use a commutator.

The direction of the magnetic field is given by the right hand thumb rule, as the direction of the current is from east to

west, the direction of the magnetic field 5cm below will be from north to south.

b

Explanation: When the key is inserted, an electric current flows through coil 1 which produces a magnetic field. This change in the magnetic field (from zero) induces an emf in coil 2 due to electromagnetic induction.But as the key is removed no magnetic field is produced and the emf in coil 2 turns zero.

The crystalline lens attached to the ciliary muscles that help in changing the focal length of the lens.

Fig. Contraction and relaxation of eye lens due to ciliary muscles

This phenomenon is known as refraction of light.

When light passes from one medium to another medium the light rays tend to bend at the interface between two medium, this phenomenon is known as refraction.

According to the law of reflection,

The angle of incidence (i) = the angle of reflection (r)

Hence, the angle at which Chitra see the reflected image of the cat (i) is 40^o.

The magnetic field line represents the direction of force at a point (along the tangent). If two magnetic field lines were to cut each other then there would be two direction of force at a point which is not practical or impossible.

If two magnetic sources are present then the magnetic field lines will repel each other but never intersect.

The direction of magnetic field is given by right hand thumb rule and the direction of the magnetic field mid way between two parallel conductors carrying same amount of current in the same direction and in the opposite direction is shown in the figure.

When the direction of current is upwards the magnetic fields are anti clockwise and when the current direction is downwards the magnetic fields are clockwise. But in both the cases the direction of magnetic field midway is in opposite direction.

An AC generator can be converted into a DC generator by using a commutator. Commutator is an attachment, connected with the armature of a motor or dynamo, through which electrical connection is made and which ensures the current flows as direct current. What a commutator does is change the conductor after a half cycle so a single terminal always experiences same EMF.

Given,

Focal length= -f

(Because it is a concave mirror)

The magnification is given by

And the magnification is also given by

As the length of image (h_i) =length of object (h_o)

Hence, the magnification (m) =1

Therefore,

According to the mirror formula,

Hence, the object must be placed at double the focal length before the concave mirror.

(i) Electromagnet

Explanation: Electromagnets are much stronger than permanent magnets; moreover permanent magnets lose their magnetivity over time due to moisture and heat.

ii) Dioptres

Explanation:1 dioptre is the power of a lens whose focal length is 1 metre. The power of a convex lens is positive and that of aconcave lens is negative.

(i) The magnetic field is a quantity that has magnitude and direction.

Explanation: Magnetic field is represented by the magnetic field lines give both the direction and magnitude (the denser the magnetic field lines, the more are its magnitude), so the magnetic field have both magnitude and direction.

This type of quantity is known as vector quantity.

ii) Outside the bar magnet, the magnetic field lines emerge from the North Pole and merge at the South Pole.

Explanation:

i) As the ray of light falls through the focus on the concave mirror, the ray of light passes parallel to the axis.

This the correct ray diagram.

ii)

Upon incidence of the light ray the angle of incidence (i) must be equal to the angle of refraction(r),as shown in the figure above.

In traffic signals ___Red______ colour light is used to stop vehicles because it has __longest____wavelength. (Hint: scattering of light is inversely proportional to the fourth power of its wavelength)

Explanation: The primary reason for using red colour in traffic signals or danger signals is that it is scattered least by the air molecules. Scattering of light is inversely proportional to the fourth power of its wavelength; this is known as Rayleigh scattering.

Concave mirror, convex mirror.

Explanation: Concave mirrors are used by dentist’s to get the upright and magnified (at a short range i.e. the object length must be smaller than the focal length of the mirror) image of the tooth or the soft tissues of the mouth.

Convex mirrors are used as the rear view mirror in cars as it gives an upright image and a wider field of view.

(i) Iris is a structure behind the cornea. It is a dark muscular diaphragm that controls the size of the pupil.

(ii) The screenon which the image is formed in the eye is called retina.

Myopia or near sightedness is an eye defect in which a person can see clearly only when the objects are near. This happens when the lens becomes too thick and the light rays undergo greater refraction which makes the light rays to converge before the retina, making the eyesight blurry.

Using a concave lens can be helpful as it concentrates the light rays on the retina. The below image explains the process

The two other types of defects are:

• Hypermetropia

• Astigmatism

Hypermetropia or far sightedness is an eye defect there is difficulty with near vision but far objects can be seen easily. The image is focused behind the retina rather than upon it. This occurs when the eyeball is too short or the refractive power of the lens is too weak. Hypermetropia can be corrected by wearing glasses/contacts that contain convex lenses.

Astigmatism is an eye defect in which the light rays do not all come to a single focal point on the retina, instead some focus on the retina and some focus in front of or behind it. This is usually caused by a non-uniform curvature of the cornea. Astigmatism can usually be corrected by using a special spherical cylindrical lens.

(a)

Explanation: The magnetic field originates from the North Pole and terminates at the South Pole, so the magnetic lines of force are directed from the north to south. Among the compass needles only (a) follows the magnetic lines.

(b) Here the needle must point towards the right side

(c) Here the needle must point towards the left side

(d) Here the needle must point towards the right side

Magnetic monopoles do not exist because the magnetic field lines forms closed loops i.e. the field lines must start from one pole and terminate at the other. But if a monopole exits then the magnetic field lines will not terminate which is impossible. Hence, the magnetic field lines do not exist.

This is relevant from the fact that even after breaking a magnet into several pieces the small magnets will have a north pole and a south pole.

The diverging lens is a concave lens.

Given,

Object distance (u) =-20cm

The focal length (f) =-10.5cm

According to the lens formula,

Hence, the image formed is a virtual image on the object’s side

As the image is formed on the other side of the lens so the lens is a converging lens or convex lens.

Given,

Object length (u) =-30cm

Image length (v) =+60cm

Focal length (f) =?

According to the lens formula,

Hence, the focal length of the convex lens is 20cm

Given,

The angle of incidence (i) =30^o

The angle of refraction (r) = 45^o

According to Snell’s law or the law of refraction,

Hence, the refractive index of medium 1 ( is times the refractive index of medium 2 (.

The magnification is given by

And the magnification is also given by

Now,

A real image is formed only for a concave mirror. Therefore,

The image length (v) =-18cm

According to the mirror formula,

Hence, the focal length of the concave mirror is 22.5cm

The person is suffering from myopia, it is a defect in which the light rays from distant objects merge before the retina, and this can be corrected by using concave lens.

The figure below explains the use of corrective concave lens

Concave mirrors can concentrate parallel light rays (from distant objects e.g. Sun) at the focus.

This property of the concave mirror is used in solar concentrators (for e.g. solar cookers) as high concentration of the sun rays generate high amount of heat which farther can be used as a heat source. The diagram below shows the concentration of sun rays

This solar concentrator can be used as a solar cooker.

The refractive index of a medium is given by,

Hence, the speed of light in kerosene is 2.04*10⁸ m/s.

i) Given,

Focal length (f) =-18cm

Object length (u) =-12cm

Image length (v) =?

According to mirror formula,

Thus, the image formed is a virtual image at 36 cm behind the mirror.

ii) The position of the object (Murugan’s face) is of great importance as a concave mirror unlike a convex mirror forms different type of images depending upon the position of the object from the mirror.

Table: Summary of images formed by a concave mirror

The refractive index of a medium is given by,

Given,

Speed of light in the crystal = 1.90 × 10⁸m/s

Therefore,

Hence, the refractive index is 1.57.

Given,

Angle of incidence (i) =45^o

Angle of refraction (r) =30^o

According to Snell’s law or the law of refraction,

As the medium is air

Hence, the refractive index of gelatine is 1.414.

Given,

Far point of the defective eye (v) =-75cm

The object length (u) ==-infinity

Focal length (f) =?

According to the lens formula,

Now, the power of the corrective lens is

i) Given,

Object length (u) =-25 cm

Image length (v) =+20 cm

Focal length (f) =?

According to the lens formula,

ii) As the focal length of the lens is positive and the image formed is inverted, so we can say that the lens is a convex lens or a converging lens.

i) The angle of incidence (i) is (90^o-15^o) = 75^o

ii) From the law of reflection,

The angle of incidence (i) = the angle of reflection (r)

So, the angle of reflection (r) =75^o

iii) The angle of deviation (A) is the angle by which the light ray has been deflected due to reflection.

The angle of deviation (A) = 180^o- (i+r)

= 180^o – (75^o + 75^o)

= 180^o – 150^o

= 30^o

The three types of mirror can either be identified by their shape or the type of image it makes of a person standing in front of it.

We will look into the mirror by going close to it:
(i) If image is of same size and erect-plane mirror.
(ii) If image is of bigger size and erect – concave mirror.
(iii) If image is of smaller size and erect – convex mirror.

When the frequency of rotation in an AC dynamo is increased, the emf induced in the coil of the dynamo also increases by the same amount. So, when the frequency is doubled the emf induced is also doubled.

The time period of rotation of the coil will decrease with the increase in frequency. So, the time period in this case will be halved.

The figure below is the labelled diagram

Incident ray: The ray of light that is incident on a surface

Refracted ray: When a light ray passes through a second medium and changes its direction, it is called a refracted ray.

Emergent ray: It is the light ray that is leaving a surface after reflection or refraction.

Angle of refraction: It is the angle (w.r.t to the normal at the surface) by which the incident ray is deviated at the second medium.

Angle of deviation: It is the angle between the incident ray and the emergent ray.

Angle of emergence: It is the angle of the light coming out of the medium

The refractive index of a medium is given by,

Given,

Refractive index of diamond ( = 2.42.

The speed of light in diamond

=

=

Tagging|||Physics||The Human Eye And The Colourful World||Refraction And Dispersion Of Light Through A Prism

Difficulty ||| Easy

i) This diagram represents ___AC generator______

ii)

This is the labelled diagram of an AC generator

iii) AC generator follows the principle of electromagnetic induction which states that the change in the magnetic field due to the rotation of the rectangular coil generates emf or electromotive force. The direction of the current induced in the coil is given by Fleming’s Right hand rule.

a)

i) a)

Given,

Focal length (f) =+10 cm

Object length (u) = -15 cm

According to the lens formula,

Hence, the image formed is virtual and 30 cm behind the mirror.

Magnification of the object (m) =

So, the image is inverted and magnified.

b) Object length (u) = -8 cm

According to the lens formula,

Hence, the image is real and in front of the mirror.

Magnification of the object (m) =

So, the image is magnified and erect.

ii) In a film projector the image of the film is projected on a screen; to achieve this we have to get a real image of the film. So, this can be achieved by the second case of the above problem.

Given,

Height of object (h_o) = 5cm

Object length (u) = -10cm

Radius of curvature (R) =-30 cm

The focal length of the mirror is given by,

According to mirror formula,

Hence, the image formed is virtual and behind the mirror.

The magnification is given by

And the magnification is also given by

So, the image formed is magnified and erect.

ii) The figure below is the ray diagram of the above case

• C=Centre of curvature

• F=Focus

• f=Focal length

• u=Object length

• v=Image length

• AB=Object height

• A’B’=Image height

i) The person is suffering from myopia i.e. he has a difficulty in seeing distant objects.

ii) The corrective lens used in treating myopia is concave lens. These lenses are thicker at the edge and thinner at the middle.

iii) The power of the corrective lens is

a) For the right eye:

Power = -3.5D

b)For the left eye:

Power = -4.00 D

This are the required focal length of the corrective concave lenses.