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#### Page No 151:

The colour red has the longest wavelength; hence, it travels at the highest speed in a glass prism among all the colours. Therefore, it appears at the top of the spectrum. Violet light has the shortest wavelength, and it travels at the lowest speed in a glass prism among all the colours. Therefore, it appears at the bottom of the spectrum after the light has passed through the prism.

#### Question 7:

We are not able to see the complete rainbow at all the places. Explain.

A rainbow is formed by total internal reflection and refraction of light within the water drops in the air. A very small amount of light passes through the water drops without total internal reflection. Because of the very low intensity of these rays, they do not form a rainbow. When the sun is in the west, a rainbow is seen in the eastern sky.
Thus, we are not able to see a complete rainbow at all places.

#### Question 8:

Lateral inversion takes place both in plane and spherical mirrors. Illustrate your answer by taking an example.

Yes, lateral inversion takes place in both plane and spherical mirrors.
In the case of a plane mirror, the size of the image is the same as the size of the object, and the image is laterally inverted. But in the case of a spherical mirror, the size of the image is not the same as the size of the object.
For example, when we raise our right hand in front of a plane mirror, our image seems to raise the left hand. The size of the image is the same as the size of the object. The same happens in the case of a spherical mirror, but the size of the image is not the same as the size of the object.

#### Page No 151:

We cannot see our image on a newspaper because its surface is not smooth and shiny. Hence, irregular reflection occurs when light rays fall on a newspaper, because of which our image is not formed on it.

#### Question 10:

All the objects reflect light falling on them, but brightness of different objects is different. Justify your answer.

Light is reflected from all objects, but they reflect light in different amounts. Some objects reflect all the light incident on them. Hence, they appear to shine. Other objects reflect only some of the light incident on them, and they do not shine.

#### Question 11:

If a mirror gets turned through an angle the reflected ray also gets turned through certain angles suppose the plane mirror is turned through 30° prove by using laws of reflection that the reflected ray turns by 60°.

Consider a plane mirror and a fixed incident ray of light reflecting from the surface at an angle θ. Before the mirror has rotated, the angle of incidence is θ as is the angle of reflection.
If the mirror is rotated by an angle of 30, the normal is rotated by an angle of 30; thus, the angle of incidence increases to θ + 30. Therefore, the angle of reflection must also increase by θ  to θ + 30 .

Difference between the final angle of reflection and the initial angle of reflection = $2×30={60}^{\mathrm{o}}$

#### Page No 151:

(a) Red light is used in a traffic signal because it has the maximum wavelength and scatters the least. Therefore, it can be seen from a distance even in fog.

(b) A kaleidoscope consists of three plane mirrors that are inclined at an angle of 60 with one another. When this arrangement of mirrors is kept in a cylinder made of cardboard with a few pieces of coloured glass pieces at one end, the mirrors form multiple images of the glass pieces. This results in beautiful patterns in the kaleidoscope.

(c) A convex lens increases the converging power; therefore, it is used in spectacles to correct long-sightedness.

(d) White light is a mixture of seven colours. When it is allowed to pass through a glass prism, it gets deviated at different angles and comes out of the prism as different colours.

(e) Chicks have only cones in their eyes, not rods. Therefore, they can see only in daytime. Hence, they sleep just after sunset and wake up at sunrise.

#### Page No 249:

(b) 60

The angle of incidence is 60⁰. Hence, according to the first law of reflection, the angle of reflection is also 60⁰.

#### Question B.2:

The device used for seeing over the heads of crowds is
(a) periscope
(b) kaleidoscope
(c) prism
(d) telescope

(a) periscope

A periscope is used for seeing over the heads of crowds.

#### Page No 249:

(a) Regular reflection
Regular reflection takes place from the wall of a building.

#### Page No 249:

(a) Regular reflection

In a regular reflection, the reflected rays are parallel to one another.

#### Page No 249:

(c) reflected ray

A ray that bounces off the surface of a mirror is called a reflected ray.

#### Question B.6:

Temporary blindness is caused in the eye when crystalline lens becomes
(a) transparent
(b) translucent
(c) opaque
(d) porous

(c) opaque
Temporary blindness is caused when the crystalline lens becomes opaque.

#### Page No 250:

When we are in a dark room, we cannot see the objects in the room.
We can see the objects outside the room, but this takes some time, because, in a dark room, the size of our pupil is small, and it takes some time for it to grow bigger.

#### Page No 250:

No, diffused reflection does not mean the failure of the law of reflection. In a diffused reflection, the law of reflection is followed for each particular ray. The only reason why the rays get diffused is, each point on an irregular surface has a different normal i.e., these points are at different angles with respect to each other and at each point the angle of incidence for each ray is always equal to angle of reflection.

#### Page No 250:

Given:
Angle between the mirrors = 40
Number of images =

Number of images = $\frac{360}{40}-1$
= 8
Number of images formed = 8

#### Question 4:

Complete tee following ray diagram:
Figure

#### Question B.7:

A device used for seeing hexagonal image of an object is called
(a) prism
(b) periscope
(c) kaleidoscope
(d) microscope

(c) kaleidoscope

A kaleidoscope is used for seeing a hexagonal image of an object.

#### Page No 250:

(a) vitamin A

A lack of vitamin A causes night blindness.

#### Page No 250:

1. The angle of incidence is equal to the angle of reflection.
2. The plane containing the incident ray and the reflected ray is called the plane of the mirror
3. If you stand one metre away from a plane mirror, the image is one metre behind the mirror.
4. The phenomenon of left appearing right and right appearing left on reflection in a plane mirror is called lateral inversion.
5. Periscope is based on the principle of successive reflections from two plane mirrors.
6. Braille  script is used by visually challenged people.
7. The eyes act as a window to the world.

#### Page No 250:

 Column A Column B 1. Image in the plane mirror (e) Virtual and erect 2. Left appearing right in a plane mirror (d) Lateral inversion 3. Angle of incidence equals (a) Angle of reflection 4. Kaleidoscope (b) Formation of images by inclined mirrors 5. Plane mirror (c) Used as looking glass

#### Page No 250:

1. False
The angle of incidence is always equal to the angle of reflection.

2. True

3. True

4. True

5. True

6. False
An image formed by a plane mirror is laterally inverted.

7. False
Poor vision is caused by a lack of vitamin A.

8. False
A plane mirror is used by a barber to show the back of the head of the customer.

#### Question 5:

Boojho stands at A just on the side of plane mirror as shown in the figure. Can he see himself in the mirror? Also can he see the image of objects situated at P, Q, and R?
Figure

Yes, Boojho can see himself in the mirror.
He can see the images of objects at P and Q, but he cannot see the image of the object at R because R is placed in line with the surface of mirror and not in front of it.

#### Page No 251:

When light falls on a surface, the direction of the ray is changed. This change in direction is known as reflection.

#### Page No 251:

A normal is a perpendicular drawn on the surface at the point of incidence of light.

#### Page No 251:

Yes, a real image can be obtained on the screen.

#### Page No 251:

The following are two uses of a plane mirror.
1. As a looking glass
2. In a solar cooker, to reflect the rays of the sun into the interior of the cooker

#### Page No 251:

Lateral inversion is the effect produced by a plane mirror of reversing images from left to right. As we observe our image in a plane mirror, we can see that our left side is at the right side of the image, and that our right side is at the left side of the image.

#### Page No 251:

The laws of reflection are as follows.
1. The first law of reflection states that the angle of incidence is equal to the angle of reflection.
2. The second law of reflection states that the angle of incidence, the normal and the angle of reflection all lie on the same plane.

#### Page No 251:

Dispersion of light through a prism is the splitting of white light into its constituent colours on passing through a prism.

#### Page No 251:

 REAL IMAGE VIRTUAL IMAGE A real image can be obtained on a screen. A virtual image cannot be obtained on a screen. A real image is formed when light rays meet after reflection or refraction. A virtual image is formed when light rays do not intersect each other after reflection or refraction but appear to meet at a point. A real image is always inverted A virtual image is always erect but laterally inverted.

#### Page No 252:

 REGULAR REFLECTION IRREGULAR REFLECTION Regular reflection takes place when light rays fall on a smooth surface. Irregular reflection takes place when light rays fall on a rough surface. In regular reflection, the reflected rays are parallel to one another. In irregular reflection, the reflected rays are not parallel to one another.

#### Page No 252:

(i) The retina is the part of eye that act as a screen for the eyes. The images of different objects that we see form on the retina.

(ii) Ciliary muscles are the part of the eye that changes the focal length of the eye lens.

(iii) The vitreous humour is a transparent fluid that fills the eyeball between the retina and the lens.

(iv) The cornea is a transparent tissue that covers the iris and the pupil. The cornea protects the eye from germs, dust and other harmful matter. It also controls the amount of light entering the eyes.

#### Page No 252:

Today, it is possible to translate words written on a page into Braille with the help of a computer. Also available are audio devices that can read out written material on a page, which help visually impaired students. Dogs are also trained to act as guides to visually impaired persons.

#### Page No 252:

To maintain good vision, it is necessary to take 600 micrograms of vitamin A daily. We should include milk, carrot, tomatoes, cod liver oil and yellow fruits in our diet because they all contain vitamin A.
Lack of vitamin A in our food can cause night blindness.

#### Page No 252:

When light rays fall on a smooth surface, all the rays are reflected in one direction. This is known as regular reflection.

 REGULAR REFLECTION IRREGULAR REFLECTION Regular reflection takes place when light rays fall on a smooth surface. Irregular reflection takes place when light rays fall on a rough surface. In regular reflection, the reflected rays are parallel to one another. In irregular reflection, the reflected rays are not parallel to one another.

#### Page No 252:

Many blind people read specially printed books using the Braille system, developed by a French investor named Louis Braille in 1824 when he was still only a 15-year-old boy.

Braille is a coded language based on a logical system. The Braille system has 63 dot patterns, or characters. These dot patterns, when embossed on a sheet, help visually challenged people to recognise words by touching the patterns. The dots are raised slightly to make it easier for the visually impaired to read the Braille code.

#### Page No 252:

A rainbow is formed owing to the phenomenon of dispersion of light. Tiny drops of water in the sky act as tiny prisms. When sunlight is passed through these tiny prisms, it splits into seven colours, which appear in the form of a rainbow in the sky.

#### Page No 252:

To explain the formation of an image of a pencil, consider two rays coming from points C and A of the pencil.
When rays of light from points C and A of the pencil are incident on a mirror, they are reflected from the mirror and enter into the eye as shown in the diagram. These rays, after reflection from the mirror, appear to come from A, and C, . Hence, a virtual image A,C, of AC is formed.

#### Page No 252:

A periscope is based on the principle of successive reflections from plane mirrors. It consists of two plane mirrors N1 and N2 facing each other at an angle of 45°to the framework of a tube, which is bent twice at right angles. Mirror N1 turns a beam of light through one right angle; similarly, mirror N2 turns the light coming from mirror N1 through another right angle. This enables the observer to see an object without being in direct sight of the object.

#### Page No 252:

The following are the properties of an image formed by a plane mirror.

1. It is a virtual image.
2. It is of the same size as the object .
3. It is at the same distance behind the mirror as the object is in front of the mirror.
4. It is laterally inverted.

Lateral inversion is the effect produced by a plane mirror of reversing images from left to right. As we observe our image in a plane mirror, we can see that our left side is at the right side of the image, and that our right side is at the left side of the image.
The following figures show lateral inversion.

#### Question C.4:

With the help of a diagram show the image formation in a plane mirror. Prove that the image is as far behind the mirror as the object is in front of it.

Take a white sheet and place a mirror perpendicular to it as shown in the diagram. Now, place an object 'O', in front of the plane mirror. The rays of light OA and OC from the object fall on the mirror and reach the eye after reflection from the mirror as shown in the figure. The reflected rays AB and CD from the object appear to come from point I in the mirror. This is a virtual image; hence, it cannot be obtained on the screen.
If we measure the distances OP and IP with a scale, we find that
OP = IP
This shows that the image is as far behind the mirror as the object is in front of it.

#### Page No 252:

 Regular reflection Irregular reflection Regular reflection takes place when light rays fall on a smooth surface. Irregular reflection takes place when light rays fall on a rough surface. In regular reflection, the reflected rays are parallel to one another. In irregular reflection, the reflected rays are not parallel to one another.

#### Page No 252:

When a beam of white light is passed through a prism, it is split into a band of seven colours. This band of seven colours is known as a spectrum. It is also commonly known as VIBGYOR as it consists of the colours violet, indigo, blue, green, yellow, orange and red.

The light that falls on one surface of the prism undergoes refraction and passes through the glass of the prism, and is refracted again when it emerges. This causes the dispersion of the light and splits the light into its constituent colours. The degree of bending depends on the angle at which the ray of light enters and leaves the faces of the prism. The faces of the prism are not parallel to each other. In the case of a glass slab, the opposite faces are parallel. So, only a slight deviation occurs.

#### Page No 252:

A kaleidoscope is used for making symmetrical patterns. Fashion designers use it for creating different patterns. It is also used as a toy for children.

#### Page No 252:

When light falls on an opaque surface, the direction of the ray is changed. This change in direction is known as reflection.

The laws of reflection are as follows.
1. The first law of reflection states that the angle of incidence is always equal to the angle of reflection.
2. The second law of reflection states that the angle of incidence, the normal and the angle of reflection all lie on the same plane.

An experiment to prove the law of reflection is as follows:
Take a sheet of black paper and make a small hole on it. Place it in front of a torch so that a narrow beam of light becomes incident on a mirror. Trace the incident ray and the reflected ray on the paper with a scale and a pencil.
Draw a perpendicular line at the point of incidence on the paper. You will find that the incident ray, the reflected ray and the normal lie on the same plane i.e., on the plane of the paper
If we  measure the angle of incidence and angle of reflection and then repeat the measurements for different angles of incidence, we will find that the angle of reflection is same as the angle of incidence for all the cases. Thus, we can conclude that the angle of incidence is always equal to angle of reflection.

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