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MULTIPLE CHOICE QUESTIONS

CBSE TERM 1 PRACTICE QUESTIONS

CLASS X

TERM 1

Chapter : Light – Reflection and Refraction

(To download the MCQ pdf - click the link MCQ - Light: reflection and refraction)

To check your score in MCQs, attempt the online test  - Online Test -Light reflection and refraction

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'.

Question of the Day 25 Nov 2017

Neha's father was reading the manual of a camera which was in very small print. He was not able to read comfortably. Neha had two convex lens A and B of focal length 25 cm and 40 cm respectively. She gave the better of the two lenses to her father. He then read the manual comfortably without any strain in his eyes.

Now answer the following question.
(a) Which one of the two convex lenses did she gave to her father and why?
(b) What is the power of the lens A and B?
(c) What is the range of the distance that his father can keep the manual from the lens for comfortable reading?
(d) Unfortunately while handling the better of the two lenses, it got slipped from her father hand and broke into two piece. Will her father still be able to use the broken pieces for magnifying the small prints? 

SPHERICAL MIRRORS

A mirror whose reflecting surface is a part of an imaginary hollow sphere is known as a spherical mirror.

Spherical mirrors are of two types:

(1) Concave mirrors

(2) Convex mirrors

Concave mirror: It is a spherical mirror whose reflecting surface is towards the centre of the imaginary sphere of which the mirror is a part.

Convex mirror: It is a spherical mirror whose reflecting surface is away from the centre of the imaginary sphere of which the mirror is a part.

Example: The inner shining surface of a steel spoon serves as a concave mirror and the outer shining surface of the steel spoon serves as a convex mirror.

ACTIVITY 1

(1)    Take a large shining spoon. Try to view your face in the inner shining surface of the spoon.

(2)    Bring the spoon close to your face. Observe the image.

(3)    Now move the spoon away from you. Observe the image.

(4)    Now try to view your face in the outer shining surface of the spoon.

(5)    Bring the spoon close to your face. Observe the image.

(6)    Now move the spoon away from you. Observe the image.

Terms related to spherical mirror

(1) Centre of curvature (C)

The centre of curvature of a spherical mirror is the centre of the imaginary hollow sphere of which the spherical mirror is a part.  It is denoted by C.

The centre of curvature is not a part of the spherical mirror. The centre of curvature of a concave mirror lies in front of the spherical mirror and centre of curvature of a convex mirror lies at the back of the mirror.

(2) Pole (P)

The pole of a spherical mirror is the centre of reflecting surface of the mirror. It is denoted by the point P

(3) Radius of curvature (R)

The radius of curvature of a spherical mirror is the radius of the imaginary hollow sphere of which the spherical mirror is a part. It is denoted by R

(4) Principal axis

The principal axis of a spherical mirror is the straight line passing through the centre of curvature C and pole P of the spherical mirror, produced on both sides.

(5) Aperture

The aperture of a spherical mirror is the diameter of the reflecting surface of the mirror

(6) Principal focus of a concave mirror (F)

The principal focus of a concave mirror is a point on the principal axis at which the incident rays parallel to the principal axis after reflection from the concave mirror actually meet at a point on the principal axis.  It is denoted by the letter F

(7) Principal focus of a convex mirror (F)

The principal focus of a convex mirror is a point on the principal axis at which the incident rays parallel to the principal axis after reflection from the convex mirror appears to meet at a point (or appears to diverge from a point)  on the principal axis.  It is denoted by the letter F

(8) Focal Length (f)

It is the distance between the principal focus (F) and pole P of the mirror. It is denoted by letter f.

Relation between focal length (f) and radius of curvature

Provided the aperture of a spherical mirror is much smaller than the radius of curvature, the focal length f can be related to radius of curvature R as

REFLECTION OF LIGHT BY A PLANE MIRROR

Image formed by a point object

Image formed by a finite object

Following are the important characteristics of images formed by a plane mirrors:

(1)  The image formed is always virtual. Such a image cannot be taken on a screen.

(2)  The image formed is always erect.

(3) The size of the image is same as the size of the object.

(4)  The image formed in a plane mirror as far behind the mirror, as the object is in front of the mirror.

(5) The image formed in a plane mirror is laterally inverted i.e. the left side of the object becomes the right side of the image and vice-versa.

LAWS OF REFLECTION OF LIGHT

First Law: The incident ray, the reflected ray and the normal at the point of incidence, all lie in the same plane.

Second Law: The angle of reflection (r) is equal to the angle of incidence (i).

These laws of reflection are valid for all types of reflecting surfaces. The surface may be smooth or rough. It may be a plane mirror, curved mirror, cylindrical or spherical mirror. The surface may be a spoon or a wall or a book. These laws of reflection are valid for all types of surfaces.

When light falls on smooth surfaces, regular reflection will occur. This type of reflection gives rise to image formation.

When light falls on rough surfaces, irregular reflection will occur. This type of reflection gives rise to scattering of light. Such a surface can be seen from all possible directions.

REFLECTION OF LIGHT

When light travelling in a given medium strikes any surface, a part of the incident light bounces back into the same medium. This phenomenon is called reflection.

Light reflected from a surface

Thus in reflection, the path of light rays changes its direction without any change in the medium of light.

Reflection is of two types:

Regular reflection

When the reflecting surface is smooth and well polished, the parallel rays of light incident on it are reflected parallel in one particular direction. This is known as regular reflection.

The regular reflection gives rise to image formation.

Regular reflection

Irregular reflection

When is reflecting surface is rough, the parallel rays falling on it are reflected in different directions. Such a reflection is known as irregular reflection or diffused reflection. No clear image is formed in case of irregular reflection.

Diffused or irregular reflection

Nature of Light

NATURE OF LIGHT

When we switch on a bulb/tube light, everything in the room becomes visible. When we switch off the bulb/tube light nothing can be seen. So we may conclude that it is the light which makes things visible when it falls on objects.

During the day, it is sunlight which makes things visible to us. The sunlight falling on objects is reflected or scattered and this reflected or scattered light when enter our eyes enable us to see those objects.

Thus,

Light is a form of energy which produces in us the sensation of sight.

What actually is light? or What is it made up of?
It is really a mysterious topic. It is an old topic of debate among scientists and science students. The explanation given by scientists appears to be against our common sense or the way we perceives the world. The theories are mind boggling for a student at the secondary level.

In junior classes, our teachers taught that light is a form of a ray which travels in straight line. A combination of rays forms a beam. A ray falling on a mirror is reflected in such a way that angle of incidence and angle of reflection both are equal. The concept of reflection and refraction can be explained completely on the ray theory of light. 

Interference and diffraction are the characteristics of a wave. It is observed that light also undergoes interference and diffraction supporting that light is also a wave. Scientists also explained reflection and refraction on the basis of wave theory. 

Scientists also found some phenomenon which cannot be explained on the basis of wave theory such as photoelectric effect and compton effect. Several experiments conducted on light proves that light is composed of particles known as photon. 

Now this is very confusing whether to call a light a wave or particle.  It is safer to assume that light has a dual character. It behaves like a wave as well as particle.


Properties of light 
(1)    Light travels in a straight line. This property of light is known as rectilinear propagation of light. This straight line path is usually indicated as a ray of light.
(2)   Nothing can travel faster than light. In vacuum the speed of light is 3 x 10⁸ m/s.
(3)  The speed of light in different medium is different. For example, speed of light in glass is 2 x 10⁸ m/s.
(4)   On entering from one transparent medium (say air) to another (say water) light changes its direction. The extent and way of bending depends upon the optical density of two media.
(5)   According to the modern theory of light, Light has a dual character. It is emitted or absorbed as a particle (called photon), but it propagates as a wave.
(6)     The particle character of light is called PHOTON. When light propagates in the form of wave it consist of electric and magnetic field hence light is also known as an electromagnetic wave.

Phenomena related to light:

(1)    Reflection of light
(2)    Refraction of light
(3)    Dispersion
(4)    Scattering
(5)    Interference
(6)    Difraction
(7)    Polarisation
(8)    Doppler effect