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Page No 236:

Question A:

Solve the following crossword puzzle:
Figure

Across
1. It has five faces
3. This type of image is always inverted
4. A doctor uses this spherical mirror

Down
1. It is the midpoint of a spherical mirror
2. Red, green and ..................... are primary colours

Answer:

Page No 236:

Question B.1:

Formation of shadows suggests that light travels in
(a) vacuum
(b) straight lines
(c) dark rooms
(d) wave form

Answer:

(b) Straight lines

The formation of shadows suggests that light travels in a straight line.

Page No 236:

Question B.2:

A smooth polished surface which can return the rays of light into the same medium is called
(a) mirror
(b) lens
(c) prism
(d) table top

Answer:

(a) Mirror

A smooth, polished surface that can return the rays of light into the same medium is called a mirror.

Page No 236:

Question B.3:

The phenomenon of bouncing back of the rays of light into the same medium is called
(a) reflection
(b) refraction
(c) scattering
(d) descattering

Answer:

(a) reflection

The phenomenon of bouncing back of light rays into the same medium is called reflection.

Page No 236:

Question B.4:

If the image cannot be taken on the screen, it must be
(a) real
(b) virtual
(c) real or virtual
(d) none of these

Answer:

(b) Virtual

Virtual images cannot be taken on a screen.

Page No 236:

Question B.5:

If the speed of rotation of Newton's colour disc is fast enough, the various colours would blend and produce
(a) white light
(b) red light
(c) yellow light
(d) no light at all

Answer:

(a) White light

If the speed of rotation of Newton's colour disc is fast enough, the various colours would blend and produce white light.

Page No 236:

Question B.6:

Primary colours are
(a) red, blue, yellow
(b) red, yellow, magenta
(c) red, blue, green
(d) black and white

Answer:

(c) Red, blue, green
Red, blue and green are the primary colours.

Page No 236:

Question B.7:

A virtual image larger than the object can be produced by a
(a) concave lens
(b) concave mirror
(c) convex mirror
(d) plant mirror

Answer:

(b) concave mirror
A concave mirror can produce a​ virtual image that is larger than the object.

Page No 236:

Question C:

Fill in the blanks:
1. The phenomenon of turning back of light into the same medium on striking a polished surface is called ........................ of light.
2. A ........................ image is always inverted.
3. In a spherical mirror, if the reflecting surface is on the outside, it is called a ........................ mirror.
4. ........................ lens is thicker in the middle and thinner at the edges.

Answer:

1. The return of light into the same medium after striking a polished surface is called reflection of light.
2. A real image is always inverted.
3. In a spherical mirror, if the reflecting surface is on the outside, it is called a convex mirror.
4. Convex lens is thicker in the middle and thinner at the edges.



Page No 237:

Question 1:

Rear view mirrors of cars carry a warning message "objects in the rear view mirror are closer than they appear. Why?

Answer:

Rear view mirrors of cars are convex in nature. When a ray of light falls on a convex mirror, it gets converged and meets at a point. The resultant image is real and diminished. Though the images appear to be far away in the mirror, they will actually be nearer. Hence, rear view mirrors of cars carry a warning message stating that "objects in the rear view mirror are closer than they appear".

Page No 237:

Question 2:

An image of an object is observed on a plane mirror. The distance between the mirror and the image is 15 m. If the pencil is moved 3 m towards the mirror, then what would be the distance between image and object?

Answer:

In plane mirrors, the distance between the object and the mirror is the same as the distance between its image and the mirror.
Let the distance between the object and the mirror be d1 and the distance between the image and the mirror be d2.
Given:
Distance between the mirror and the image = 15 ​m, i.e., d1 d2 15 m
The pencil is moved 3 m towards the mirror, which means that:
Distance between the mirror and the image is 15 - 3 = 12 m, i.e., d1 = d2 12 m
Distance between image and object = d1 + d2
                                                                         = 12 + 12
                                                       = 24 m​

Page No 237:

Question 3:

The rear view mirror of a car is a plane mirror. A driver reverses his car at a speed of 3 m/s. The driver sees in his rear view mirror, the image of a truck approaching behind his car. Find the speed at which the image of truck appears to approach the driver.

Answer:

A driver reverses his car at a speed of 3 m/s , which means that the car is approaching the truck with a speed of 3 m/s.
The distance between the car and the truck decreases at double the rate.
This is because the image of the truck travels at twice the distance travelled by the car in equal time.
Hence, the image of the truck appears to approach the driver at a speed of 3 × 3 = 9 m/s.

Page No 237:

Question D:

Match the items in Column A with the items in Column B:

Column A Column B
1. A plane mirror (a) Used as a magnifying glass.
2. A convex mirror (b) The image is virtual, upright and diminished
3. A convex lens (c) Used by dentists to see enlarged image of teeth
4. A convex mirror (d) The image is always inverted and magnified
5. A concave lens (e) The image is erect of the same size as the object.
  (f) The image is erect and smaller in size than the object

Answer:

 Column A   Column B
1. A plane mirror (e) The image is erect and of the same size as the object
2. A convex mirror (b) The image is virtual, upright and diminished
3. A convex lens (a) Used as a magnifying glass
4. A concave mirror (c) Used by dentists to see an enlarged image of teeth
5. A concave lens (f) The image is erect and smaller in size than the object

Page No 237:

Question E:

Write True (T) of False (F) against the following statements in the given brackets:
1. Light is not a form of energy. ( )
2. A virtual image cannot be obtained on a screen. ( )
3. Yellow is a primary colour. ( )
4. A lens which is thicker in the middle and thinner at edges is called a convex lens. ( )
5. The behind of rays of light on passing through a medium is called dispersion. ( )

Answer:

1. False
Light is a form of energy.

2. True

3. False
Red, green and blue are the primary colours.

4. True

5. False
Dispersion is the splitting of white light into its seven constituent colours. The bending of light rays on passing through a medium is called refraction.

Page No 237:

Question F:

Match the terms in Column I with the corresponding terms appropriate in Column II and Column III:

Column I Column II Column III
1. Looking glass (a) Reflector (A) Dispersion
2. Headlight of car (b) Parallel beam (B) Virtual image
3. VIBGYOR (c) Mirror (C) Focus
4. Light (d) Spectrum (D) Convex mirror
5. Street lamps (e) Rectilinear Propagation (E) Shadows and eclipses

Answer:

 Column I  Column II  Column III
1. Looking glass (c) Mirror (B) Virtual image
2. Headlight of car (b) Parallel beam (C) Focus
3. VIBGYOR (d) Spectrum (A) Dispersion
4. Light (e) Rectilinear propagation (E) Shadows and eclipses
5. Street lamps (a) Reflector (D) Convex mirror



Page No 238:

Question 4:

A concave mirror is said to have a real focus whereas a convex mirror is said to have a virtual focus. Why do you think this is so? Also mention which of the two - concave of convex lenses has a real focus?

Answer:

When parallel light rays fall on a concave mirror, they meet at a point called the real focus. The image so formed can be taken on a screen. In convex mirrors, the rays diverge but appear to come from a point behind the mirror. The image cannot be taken on a screen. Hence, a concave mirror is said to have a real focus, whereas a convex mirror is said to have a virtual one. The convex lens, also known as a converging lens, has a real focus.

Page No 238:

Question 5:

Two convex lenses of same thickness are used to make two different materials - glass and diamond. Which lens will have greater focal length?

Answer:

The focal length of an object depends on the bending of light rays by the object. The higher the bending of light, the lower will be the focal length. Comparing glass and diamond, the rate of bending of light rays is higher in diamond. Hence, diamond will have lesser focal length.
Glass has greater focal length.

Page No 238:

Question 6:

Circle the alphabets that will not show lateral inversion:

A C X Z Y R T
V H S I P M B

Answer:

The alphabets that will not show lateral inversion are A, X, Y, T, V, H, I and M.



Page No 239:

Question A.1:

What is meant by light?

Answer:

Light is a form of energy that is invisible but causes a sensation of vision in us.

Page No 239:

Question A.2:

hat is meant by reflection of light?W

Answer:

When light falls on a surface, the direction of the rays is changed. This change in direction is known as reflection of light. The reflected light travels to the same medium it came from.

Page No 239:

Question A.3:

Is a real image always erect?

Answer:

No, a real image is always inverted.

Page No 239:

Question A.4:

What is a spherical mirror?

Answer:

A spherical mirror is one that has a curved surface. The curvature is either on the inside or the outside of the mirror, forming a part of a sphere.



Page No 240:

Question A.5:

What are primary colours?

Answer:

Primary colours are those which when mixed in the right proportions can generate all other colours. The three primary colours are red, green and blue.

Page No 240:

Question A.6:

What  is meant by rectilinear propagation of light?

Answer:

Light travels in a straight line. This is called the rectilinear propagation of light.

Page No 240:

Question A.7:

Name the phenomenon responsible for the formation of rainbow.

Answer:

Formation of a rainbow is due to the dispersion of white light.

Page No 240:

Question A.8:

State one use of plane mirror.

Answer:

A plane mirror is commonly used as a looking glass.

Page No 240:

Question A.9:

What is a mirror?

Answer:

A mirror is a highly polished, smooth surface that reflects the maximum amount of light falling on it into the same medium.

Page No 240:

Question A.10:

Is a virtual image always inverted?

Answer:

No, virtual images are always erect but laterally inverted.

Page No 240:

Question B.1:

State three important characteristics of an image formed in a plane mirror.

Answer:

Following are the important characteristics of the image formed by a plane mirror:
(i) It is always of the same size as the object.
(ii) It's virtual and upright.
(iii) It's laterally inverted.

Page No 240:

Question B.2:

Distinguish between real and virtual images.

Answer:

 Real images  Virtual images
A real image can be obtained on a screen. A virtual image cannot be obtained on a screen.
The rays of light after reflection or refraction meet at a point. The rays of light after reflection or refraction appear to meet at a point.
Real image is always inverted. Virtual image is always erect but laterally inverted.

Page No 240:

Question B.3:

Why does a coloured disc appear white when rotated?

Answer:

When a coloured disc is rotated, it appears white: as the disc spins, all the colours pass through a spot rapidly, one after another. This rapid movement has the effect of putting all the colours in one place, making them appear white.

Page No 240:

Question B.4:

Give one use each of a concave and a convex mirror.

Answer:

Concave mirrors form highly magnified images and are therefore, used as shaving and make-up mirrors.
Convex mirrors are used as reflectors in street lights to diverge the light rays over a wide area.

Page No 240:

Question B.5:

Which type of mirror can form a real image?

Answer:

Concave mirrors can form real and inverted images that are either enlarged or diminished.

Page No 240:

Question B.6:

What are lenses? Name the two kinds of lenses.

Answer:

Lenses are optical devices that are made of a transparent material having a polished, curved surface. The curved surfaces are either spherical or cylindrical.
The two types of lenses are as follows:
(i) Concave lens
(ii) Convex lens

Page No 240:

Question B.7:

State two differences between a convex and a concave lens.

Answer:

 Convex lens  Concave lens
A convex lens is thicker at the centre and thinner around the edges  A concave lens is thicker around the edges and thinner at the centre
It is also called a converging lens as it converges the light rays at a point It is also called a diverging lens as the light rays appear to diverge from a point

Page No 240:

Question C.1:

With the help of an experiment, prove that light always travels in a straight line.

Answer:

Light travels in a straight line. This is called the rectilinear propagation of light. This can be observed by a simple experiment: light a candle and place it on a table. Take a pipe that is straight and observe the flame of the candle through it. The flame can be clearly seen through the pipe. Now, try to see the flame of the candle through a bent pipe. The flame cannot be seen through it.
This experiment demonstrates the rectilinear propagation of light.

Page No 240:

Question C.2:

You are provided with (a) plane mirror (b) concave mirror (c) convex mirror. How will you distinguish them without touching?

Answer:

The classification can be done based on the images formed by different types of mirrors:
(i) Plane mirror : The image formed by a plane mirror is virtual and upright. The size of the image will be the same as that of the object.

(ii) Concave mirror : The image formed by a concave mirror will always be real. Its size will be either enlarged or diminished. Only when the object is very close to the mirror will the image be virtual.

(iii) Convex mirror : The image formed by a convex mirror will be virtual and upright, but diminished.
 
By observing the image formed by each mirror, the type of the mirror can be determined.

Page No 240:

Question C.3:

How will you prove that light consists of seven colours using Newton's colour disc?

Answer:

White light is composed of seven colours: violet, indigo, blue, green, yellow, orange and red.
This can be observed with the help of Newton's colour disc. It consists of the seven colours of white light. It is attached to a pulley, which helps the disc rotate. 
On being rotated, it appears white: as the disc spins, all the colours pass through a spot rapidly, one after another. This rapid movement has the effect of putting all the colours to one place, making them appear white.

Page No 240:

Question C.4:

Distinguish between reflection and refraction.

Answer:

 Reflection Refraction
This is a phenomenon in which light rebounds and travels back to the same medium, after hitting the reflecting surface. The angles of incidence and reflection are the same. This is a phenomenon in which light changes its direction when it strikes the interface that exists between two different media. The angles of incidence and refraction are not the same.
Reflection is found in mirrors. Refraction is found in lenses.

Page No 240:

Question C.5:

What is a virtual image? Give one situation where a virtual image is formed.

Answer:

An image that cannot be obtained on a screen is called a virtual image. The image formed by a plane mirror is virtual.



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