PHYSICS STD 8 CH 5 LIGHT ENERGY

PHYSICS STD 8 CH 5 LIGHT ENERGY

EXERCISE 5.1

OBJECTIVE TYPE QUESTIONS

A. Fill in the blank spaces by choosing the correct words from the list given below:

1) Incident 2) Normal 3) Same 4) Real 5) Regular

B. Statements given below are incorrect. Write the correct statements. 1) The ray of light after striking the mirror moves in another direction. 2) Real image can be obtained on screen. 3) Irregular reflection is caused by the irregularities in the reflecting surface. 4) Images in mirrors are formed by regular reflection. 5) In periscope, the mirror surfaces are arranged parallel to each other.

C. Write true or false for the following statements: 1) True 2) False (The reflected ray leaves the mirror at the same angle at which the incident ray strikes it) 3) True 4) False (Virtual image is always erect and laterally inverted) 5) True

D. Tick the most appropriate answers: 1) D (all of these) 2) D (trees) 3) B (plane mirror) 4) A (angle of reflection) 5) C (real)

E. Match the statements in Column A, with those in column B: 1) E 2) D 3) A 4) B 5) C

STUDY QUESTIONS

1. Define the following: a) Angle of incidence:

The angle which the incident ray makes with the normal is called angle of incidence.

b) Angle of reflection: The angle which the reflected ray makes with the normal is called angle of reflection.

c) Normal: The perpendicular drawn at the point of incidence to the surface of mirror is called normal.

d) Incident ray: A ray of light which travels from an optical medium towards a mirror is called incident ray.

e) Reflected ray: A ray of light which bounces off the mirror surface, into the same optical medium in which the incident ray was travelling is called reflected ray.

2. (a) What is meant by reflection?

The phenomenon due to which a ray of light, travelling from one optical medium to another optical medium, bounces off from its surface with the change of angle is called reflection of light. (b) State the laws of reflection. (i) The incident ray, the reflected ray and the normal lie in the same plane, at the point of incidence. (ii) The angle of incidence is always equal to the angle of reflection.

3. (a) Define real and virtual images.

Real Image: The image which can be obtained on a screen is called a real image. It is formed when the light rays after reflection or refraction actually intersect each other.

Virtual Image: The image which can be seen only in a mirror but cannot be obtained on a screen is called a virtual image .It is formed when the light rays after reflection or refraction do not actually intersect each other but they appear to diverge from it. Geometrically, they intersect when they are produced in the backward direction.

(b) State the differences between real and virtual images.

Sr No Real Image Virtual Image

1. It can be taken on the screen. It cannot be taken on the screen.

2. It is always inverted. It is always erect but laterally inverted.

3. The rays of light after reflection or refraction actually meet at a point.

The rays of light after reflection or refraction appear to meet at a point.

4. It is formed on the same side of the mirror as the object.

It is formed behind the mirror.

4. (a) Define plane mirror.

A smooth and polished surface which can turn back the rays of light into the same medium is called plane mirror. (b) State the characteristics of image formed by a plane mirror. (i) The image is as far behind the mirror as the object is in front of it. (ii) The image formed is erect. (iii) The image is of the same size as the object. (iv) The image formed is virtual, which means, such an image cannot be caught on a screen. (v) The image is laterally inverted that is the right side of the object appears as the left side of the image. (c) State uses of plane mirror. (i) Plane mirrors are used as looking glass. (ii) Plane mirrors are used for making periscope and kaleidoscope. (iii) Plane mirrors are used by barbers to show customers the backside during haircut. (iv) Plane mirrors are used for providing false dimensions in showcases, displaying

Jewellery, etc. 5. What do you mean by regular and diffused reflections?

Regular reflection: When a parallel beam of light on striking some smooth and polished surface is reflected as a parallel beam of light, such a reflection is called regular reflection.

Irregular reflection: When a parallel beam of light on striking rough surface is reflected in different directions then such a reflection is called diffused reflection.

6. Describe principle and working of the periscope. A periscope works on the principle of double reflection. A periscope is made up of a rectangular long tube with mirrors fixed at an angle of 45° within the tube. The mirror surfaces are arranged parallel to each other. Light from the object is incident on the mirror on the top part of the tube. This mirror then reflects the light to the mirror on the bottom part of the tube. The objects can be clearly seen by placing the eye in line with the bottom mirror. Take a long square cardboard tube. Cut two slits at both the ends of the tube at 45° angles. Insert mirrors into these slits in such a way that their reflecting surfaces face each other. Make a horizontal cut opposite the reflecting surface of each mirror. Glue the mirrors to fix at 45°. Close the upper and lower ends of tube with the help of paper. Periscope is ready to use. Things can be seen from the lower hole.

Fig: Periscope

EXERCISE 5.2 OBJECTIVE TYPE QUESTIONS A. Fill in the blank spaces by choosing the correct words from the list given below: 1) Spherical

2) Principal axis

3) Convex mirror

4) Concave

5) Principal focus

B. Statements given below are incorrect. Write the correct statements. 1) The hollow surface of a convex mirror is silvered, such that its outer surface acts as reflecting surface. 2) When the object is at infinity in front of a convex mirror, it forms a virtual, erect and highly diminished image. 3) A concave mirror can be used as a shaving glass as it forms virtual and enlarged image. 4) Any ray of light which initially passes through the principal focus of a spherical mirror, after reflection will travel parallel to principal axis. 5) Convex mirror are used as street light reflectors. C . Write true or false for the following statements: 1) False (Geometric centre of a spherical mirror is called pole)

2) True 3) True 4) False, (Convex reflectors are used in street lights) 5) True

D. Tick the most appropriate answer: 1) C (pole) 2) B (principal focus) 3) C (convex mirror) 4) C (convex mirror) 5) C (bigger than object)

E. Match the statements in Column A, with those in Column B.

1) C 2) A 3) E 4) B 5) D STUDY QUESTIONS:

1) Define the following: (a) Principal axis:

An imaginary line passing through the pole and centre of curvature of a spherical mirror is called principal axis.

(b) Focal Length: The distance between pole and principal focus of a spherical mirror is called focal length.

(c) Pole: Geometric centre of the spherical mirror is called pole.It is denoted by the letter P

(d) Centre of curvature: The centre of the hollow sphere of which the spherical mirror is a part is called center of curvature. It is denoted by the letter C.

2) Draw neat diagrams and state the characteristics of image formed when:

(a) An object is placed between the principal focus and centre of curvature of a concave mirror.

Following are the characteristics of image: (i) Image is real. (ii) Image is inverted. (iii) Image is enlarged. (iv) Image is formed beyond the centre of curvature but not at infinity, in front of

mirror.

(b) An object is placed between the principal focus and pole of a concave mirror.

Following are the characteristics of image: (i) Image is virtual. (ii) Image is erect. (iii) Image is enlarged. (iv) Image is formed behind the concave mirror.

(c) An object is anywhere between the pole and infinity of a convex mirror.

Fo1Iowing are the characteristics of image: (i) Image is virtual. (ii) Image is erect. (iii) Image is diminished. (iv) Image is formed between the principal focus and the pole, behind the mirror.

3) State two uses of:

(a) Concave mirror: (i) It is used in a solar cooker to converge the sunrays. It reflects the sunrays so

as to converge them on cooking material placed at the focus of concave mirror.

(ii) It is used as a reflector in flood lights. The source of light (bulb) is placed between the pole and focus of a concave mirror in order to obtain a diverging beam of light in flood lights.

(b) Convex mirror: (i) It is used as a street light reflector as it diverges the rays over a wide area.

(ii) It is used by drivers as a rear view mirror because it forms a small, erect and

diminished image close to the eye of driver, and hence, helps to have him a clear view of the traffic behind him due to its wide field of view.

4) State two difference between a real image and a virtual image.

Sr No Real Image Virtual Image

1. It can be taken on the screen. It cannot be taken on the screen.

2. It is always inverted. It is always erect but laterally inverted.

5) You are provided with (a) a plane mirror (b) a convex mirror and (c) a concave

mirror. How will you distinguish between them, without touching them? By observing the images formed by the three mirrors when the object placed close to the mirror, we can distinguish between the mirrors as: (i) If the image is erect, of the same size and does not change in size on moving

the mirror, the given mirror is a plane mirror.

(ii) If the image is erect, diminished and remains erect on moving the mirror away from the object, the given mirror is a convex mirror.

(iii) If the image is erect and magnified and it gets inverted on moving the mirror away from the object, the given mirror is a concave mirror.

EXERCISE: 5.3 OBJECTIVE TYPE QUESTIONS

A. Fill in the blank spaces by choosing the correct words from the list given below: 1) Deviates 2) Refracted 3) Normally 4) Refractive index 5) Less

B. Statements given below are incorrect. Write the correct statements. 1) When a ray of light travels from optically denser medium to optically rarer medium, it

always bends away from the normal.

2) A ray of light is incident perpendicularly on a glass slab. The angle of incidence is 0°.

3) The ratio between the velocity of light in vacuum to the velocity of light in a given

optical medium is called refractive index.

4) When a ray of light travels from an optically less dense medium to an optically more denser medium, the angle of incidence is always greater than angle of refraction.

5) During refraction through a glass block, with the increase in angle of incidence, the angle of refraction increases.

C. Write true or false for the following statements: 1) True

2) False (The angle of incidence is always greater than angle of refraction when a ray

travels from air to glass)

3) False ( The ratio of the velocity of light in air or vacuum to the velocity of light in a given optical medium is called refractive index)

4) True

5) False (A pencil held obliquely in water appears bent and shorter)

1. Tick the most appropriate answer: 1) A (less than 45°) 2) C (emergent ray)

3) B (bends away from the normal) 4) A (raised from its position) 5) C (both a and b)

2. Match the following in Column A, with those in Column B. 1) B 2) D 3) A 4) E 5) C STUDY QUESTIONS

1) What do you understand by the term ‘refraction of light’? The phenomenon due to which a ray of light deviates from its path, at the surface of separation of two optical media, when the ray of light is travelling from one optical medium to another optical medium, is called refraction of light.

2) Why is a coin placed in a bowl of water appears raised? The rays of light diverging from the coin, on emerging out of water, suffer refraction and hence, bends away from the normal. When these refracted rays reach the eye, then to the eye, they appear to come from the apparent position of coin, which is higher than real position of coin. Thus, a coin placed in a bowl of water appears raised.

3) Define the following with reference to refraction of light: (a) Incident ray:

A ray which strikes the surface of separation of two optical media is known as incident ray.

(b) Refracted ray: A ray which travels in the second optical medium, with the change of angle of the incident ray is known as refracted ray.

(c) Normal: A perpendicular drawn at the point of incidence on the surface of separation of two optical media is known as normal.

(d) Emergent ray: The refracted ray which is emerged out of a medium after refraction is called emergent ray.

(e) Angle of incidence: The angle which an incident ray makes with the normal, at the point of incidence, is known as angle of incidence.

(f) Angle of refraction: The angle which a refracted ray makes with the normal, at the point of incidence, is known as angle of refraction.

4) State the laws of refraction of light. State at least three examples of refraction of light. There are two laws of refraction:

(i) The incident ray, the refracted ray and the normal lie in the same plane at the point of incidence.

(ii) If a circle is drawn at the point of incidence with any radius, cutting the incident ray

and the refracted ray and if the perpendiculars are drawn from those points to the normal, then the ratio of perpendicular in air to the ratio of perpendicular in denser medium is a constant quantity and is commonly called refractive index of the denser medium.

Examples of refraction of light:

1. A stamp placed under a thick glass block appears raised. 2. The stars appear to twinkle. 3. A swimming pool, when filled with water appears shallow.

5) How are the angle of incidence and angle of refraction related to each other

when: (a) A ray of light travels from rare to dense medium,

When a ray of light travels from an optically rarer medium (air) to an optically denser medium (water), the angle of incidence is always greater than angle of refraction.

(b) A ray of light travels from dense to rare medium? When a ray of light travels from an optically denser medium (water) to an optically rarer medium (air), the angle of incidence is always less than angle of refraction.

6) (a) What do you understand by the term refractive index? The ratio between the velocity of light in vacuum or air to the velocity of light in a given optical medium is called refractive index. (b) If the velocity of light in vacuum is 3 x 108 ms-1 and in another medium M is 0.75 x 108 ms-1, what is the refractive index of M?

Exercise 5.4 Pg. 93

Objective – type Questions

A. Fill-ups

1. Base 4. deviation

2. dispersion 5. prism

3. two

B. Write the correct statements.

1. A prism is a piece transparent material having two triangular surfaces and three

rectangular surfaces.

2. For an equilateral prism, ∠i + ∠e = ∠A + ∠D

3. The angle between the incident ray and the emergent ray of a prism is called angle of

deviation.

4. The phenomenon due to which white light splits into seven colours on passing through a

prism is called dispersion.

5. During dispersion of white light by a prism, the violet colour deviates the most.

C. True/False

1. True

2. True

3. False (a ray bends towards the base)

4. True

5. False (angle of deviation)

D. Tick the most appropriate answer.

1. ( c ) dispersion 3. ( c) ∠A + ∠D = ∠i +∠e

2. (d) red 4. (a) angle of prism 5.( c) spectrum

E. Match the following

1 – (d) 2 - ( e ) 3 - (a) 4 - (b) 5 - ( c)

STUDY QUESTIONS

Q1.Draw a neat diagram for the passage of a ray of light through an equilateral glass

prism showing clearly (i) angle of incidence (ii) angle of emergence (iii) refracting angle

of prism (iv) angle of emergence (v) angle of deviation

The figure above shows an equilateral prism. An incident ray of light EF is incident on the face

AB of the prism. The ray FG is called the refracted ray. ∠EFL is called the angle of incidence

on the refracting face AB and ∠GFO is the angle of refraction. The ray GH is called emergent

ray and the ∠MGH is the angle of emergence.

On the whole, the incident ray which was travelling along EF has been turned along GH by

the prism such that, ∠NPH is the angle through which incident ray gets deviated. The angle

between the incident ray and the emergent ray is called angle of deviation. ∠NPH is the angle

of deviation.

Q2. (a) Define the following when refraction takes place through an equilateral glass

prism

(i) Angle of incidence: The angle which the incident ray makes with the normal on the

refracting face of a prism is called as the angle of incidence.

(ii) Prism: A prism is a piece of glass or any other transparent material which is bounded by

two triangular surfaces and three rectangular surfaces. Two opposite surfaces of a prism are

identical and parallel triangles. But the other three surfaces are rectangular and inclined on

each other

The rectangular surfaces are refracting faces. The angle between two refracting faces is

called angle of prism.

(iii) Refracting angle of prism: The rectangular surfaces of an equilateral prism are refracting

faces. The angle between two refracting faces is called refracting angle of prism.

(iv) Angle of emergence:. The ray which emerges out from the second refracting face of the

prism is the emergent ray and the angle which the emergent ray makes with normal on the

second refracting face of the prism is called as the angle of emergence.

(v ) Angle of deviation: The angle between the incident ray and the emergent ray is called

angle of deviation

(b) How are angle of incidence, angle of prism, angle of emergence and angle of

deviation related to each other?

If we measure (i) angle of incidence (ii) angle of emergence (iii) angle of the prism (iv) angle

of deviation, it has been found that,

angle of incidence + angle of emergence = angle of prism + angle of

. deviation

Or, ∠i + ∠e = ∠A + ∠D

Q3. Draw a neat and labeled diagram, when a ray of white light passes through an

equilateral glass prism.

Q4. (a) Define the following

(i) Spectrum: The band of seven colours obtained on the screen when white light splits

into seven colours is called spectrum.

(ii) Dispersion: The phenomenon due to which white light splits into its constituent seven

colours (VIBGYOR) when passed through an equilateral prism is called dispersion

(b) Why does the white light split on the first refracting face of prism?

The angle of deviation produced by coloured light depends upon its wavelength i.e. the

less the wavelength the more is the angle of deviation and vice versa. Thus when white

light strikes at the first face of equilateral prism, the angle of deviation for red colour is

least and that for violet colour is maximum. In other words, different colours in white light

deviate at different angles such that the deviation produced in the red colour is the least

and the violet colour is the maximum. It can be said that, light on the first face splits up into

colours and hence gets dispersed, while passing out from the opposite face of the prism;

the various colours suffer further refraction and hence separate out further.

Q5.Write all the colours of spectrum in proper order starting from red colour.

R - Red

O – Orange

Y- Yellow

G = Green

B – Blue

I – Indigo

V- Violet

Q6. During dispersion of white light, which colour,

(a) which colour deviates least

(b) which colour deviates most

The angle of deviation for red colour is least and that for violet colour is maximum. So

during dispersion of white light,

(a) red colour deviates the least

(b) violet colour deviates the maximum