# Samacheer Kalvi 8th Science Solutions Chapter 3 Pdf

## Samacheer Kalvi 8th Science Solutions Chapter 3 Pdf

Tamilnadu Board Samacheer Kalvi 8th Science Solutions Chapter 3: Tamilnadu State Board Solution Class 8 Science Chapter 3 – Light.

### Samacheer Kalvi 8th Science Solutions Chapter 3: Overview

 Board Samacheer Kalvi Class 8 Subject Science Chapter 3 Chapter Name Light

### Samacheer Kalvi 8th Science Solutions Chapter 3 Pdf

I.) Choose the best answer.

( 1 ) Which of the following has curved reflecting surface?

a ) plane mirrors

b) spherical mirrors

c) simple mirrors

d) None of the above

(2) The spherical mirror with a reflecting surface curved inward is called

a) convex mirror

b) concave mirror

c) curved mirror

d) None of the above

(3) The spherical mirror used as a rear view mirror in vehicle is

a) concave mirror

b) convex mirror

c) plane mirror

d) None of the above

(4)  The imaginary line passing through the centre of curvature and pole of a spherical mirror  is called

a) centre of curvature

b) pole

c) principal axis

(5) The distance from the pole to the focus is called

a) pole length

b) focal length

c) principal axis

d) None of the above

(6) If the image and object distance is same , then the object is placed at

a) infinity

b) at F

c) between f and P

d) at C

(7) If the focal length of a spherical mirror is 10 cm , what is the value of it’s radius of curvature?

a) 10 cm

b) 5cm

c) 20 cm

d) 15 cm

II.) Fill in the blanks.

1)The spherical mirror used in a beauty parlour as make -up mirror is concave mirror.

2) Geometric centre of the spherical mirror is pole .

3) Nature of the images formed by a convex mirror is smaller, virtual and erect.

4) The mirror used by the ophthalmologist to examine the eye is concave mirror.

5) If the angle of incidence is 45° , then the angle of reflection is 45°.

6) If an object is placed between two mirrors which are parallel to each other, the number of images formed is Infinite.

III.) Match the following.

 Convex mirror Radio telescopes Parobolic Rear – view mirror Snell’s law Kaleidoscope Dispersion of light sin i / sin r = u Refractive index Rainbow

 Convex mirror Rear – view mirror Parobolic Radio telescopes Snell’s law sin i / sin r= u Dispersion of light Rainbow Refractive index Kaleidoscope

1 ) Define focal length.

Focal length refers to the distance between the pole and the principal focus. A relationship exists between the focal length of a spherical mirror and its radius of curvature.  The focal length represents half of the radius of curvature.

2 ) Give any two applications of a concave and convex mirror.

Concave mirror

1 ) A concave mirror provides a magnified image , for this reason ,it is used for the purpose of applying make- up or shaving .

2 ) As they direct light to a far distance ,for that reason it is used in torches ,search lights and headlights.

3) They are used in solar cookers because they possess the capacity to collect light from a greater area and focus it into a small spot.

4) As they provide a shadow- free illumination of the organ ,for that reason it is used as head mirror by doctors for the purpose of examining the eye, ear, nose  and throat.

5) Reflecting telescopes also makes use of concave mirror .

Convex mirror

1 ) As convex mirror are curved outwards ,  for that reason,they  provide an upright image and gives a greater field of view .So ,they are used as rear view mirrors in vehicles.

2) Convex mirror are mostly mounted on a wall or ceiling of various buildings like  hospitals, hotels,  schools and stores, where hallways make sharp turns.

3) They are also used on roads, where there are sharp  turning points and curves.

3.) State the laws of reflection .

Law of reflection is represented as the association between the incident ray ,the reflected ray and the normal.

The law of reflection as follows:

• At the point of incidence, the incident ray ,the reflected ray and the normal ,all lie in the similar plane.
• The angle of incidence (i) and angle of reflection (r) are never  unequal ,they always potray an equivalency.

4) Define the refractive index of a medium.

Refractive index of the medium refers to the quantity of refraction of light in a medium .  Refractive index of the medium represents the ratio of the speed of light in the air to the speed of light in that specific medium. It is also called absolute refractive index and is indicated by ‘u’ which is the Greek letter.

5) State Snell’s law of refraction .

Refraction of light rays obey two laws ,  when they move from one medium to another medium which is termed as  Snell’s law of refraction.

They are as follows:

i ) At the point of intersection, the incident ray, the refracted ray and the normal ,all lie in the similar plane.

ii) The refractive index of the medium ,which is uniform is equivalent to the ratio of the sine of the angle of incidence (i) to the sign of the angle of refraction ( r) .

V.) Answer in detail.

1 ) Explain the images formed by a concave mirror.

 Position of the Object Position of the Image Image Size Nature of the I’m age At infinity At F Highly diminished Real and inverted Beyond C Between C and F Diminished Real and inverted At C At C Same size as the object Real and inverted Between C and F Beyond C Magnified Real and inverted At F At infinity Highly magnified Real and inverted Between F and P Behind the mirror Magnified Virtual and erect

2 ) What is reflection? Write a short note on regular and irregular reflection.

Reflection refers to a ray of light that falls on a smooth ,shiny and polished surface of a body and is reflected back . This reflecting back of the light rays  as they fall on the body having a shiny ,polished and smooth surface is known as reflection . Incident ray and reflected ray are the two rays of reflection.

Regular reflection and irregular reflection are mainly the two types of reflection.

Regular reflection

There is reflection of a ray of light that falls on a polished surface. The reflected rays will be collateral to each other after reflection.Here ,there will be an equivalence of the angle of incidence and angle of reflection of each ray. So in this case, there is an adherenceto the law of reflection and thus there is a formation of clear image . This reflection is termed as ‘ regular reflection ‘or ‘ specular reflection’.

Irregular reflection

The light rays are reflected at distant angles ,when they fall on a rough surface . So, in this situation the angle of incidence and angle of reflection of each ray will not be equivalent . So as a result of which, an equivalency will not be maintained. Therefore, there is no adherence to the law of reflection and thus, the image that are created will not be clear . This reflection is known as a ‘regular reflection ‘ or ‘diffusedreflection’.

3) Explain the working of a periscope.

Periscope

Periscope is a device that is mainly designed for the purpose of viewing bodies or ships,  which are above and around another body or submarine . It is dependent on the concept of the law of reflection of light . It contains a long exterior case and at each end, mirror and prisms are kept within this case ,having a 45° angle of inclination . The light that falls at the topmost end of the periscope comes from the different body and gets reflected downward in a vertical direction. The second mirror which is kept at the bottom ,once more reflectsthis light , with the intention to move in a horizontal direction and arrives the observer’s eye. Optic fibre are used in some complex periscopes rather than mirrors, for the purpose of obtaining a greater revolution. Depending on the purpose, there is a variation in the distance between the mirrors.

4.) What is dispersion ? Explain in detail.

Dispersion occurs because white light contains different colours of light which usually possess distinct wavelength and they move with varying speeds in a medium. As we all know that, the  refraction of a ray of light in a medium is dependent on its speed. As each coloured light possess varying speed ,there are refraction of the constituent coloured lights at  distant extents, within the prism .

Therefore, there  is an inverse proportion of the refraction of a light ray to its wavelength.

So, the deviation of the red coloured light is quite less, as it possess a large wavelengths whereas ,   the deviation of the violet coloured light is quite more ,which possess a short wavelength.

VI.) Numerical problems

1) The radius of curvature of a spherical mirror is 25 cm . Find its focal length.

Here, Radius of curvature is already given .

So, Radius of curvature= 25 cm

The formula of focal length is :

f = R/ 2

Here , f stands for focal length and R stands for Radius of curvature.

So, f = 25/2

= 12. 5 cm

Hence , the focal length will be 12. 5 cm .

(2) If two plane mirrors are inclined to each other at an angle of 45° , find the number of images formed.

The number of images formed (N)  = 360°/ -0– 1

Angle of inclined = 45°

N = 360 /45 -1

= 7

Therefore, the number of images formed are 7.

3 ) Speed of light in air is 3× 10 8m s – 1and the refractive index of a medium is 1.5 . Find the speed of light in the medium.

Here , the speed of light in air and Refractive index of a medium are already given .

So, speed of light in air ( c ) =   3× 10 8 m s – 1

Refractive index of a medium ( u ) = 1.5

Now, we have to find the speed of light in the medium ( V )

The formula is u = c/v

1.5 = 3 × 10 8 /v

V = 3 × 10 8/ 1.5

V = 2 × 10 8m s – 1

Therefore, the speed of light in the medium ( v ) is 2 × 10 8 m s – 1

Updated: November 18, 2021 — 11:05 am