Class 07 || Science || Ch. 15 Light

Light 

Class 7th

Q. what is light?

Ans. Light: Light is a form of energy Which enables us to see. We can see the world around us because of light.

Q. What do you mean by the beam of light? Also give some other examples for your support of answer.

Ans. Beam of light :–> The group of the the Ray's of the light is known as beam of light. 

Example, when sunlight enters into a room through a narrow opening or a hole is an example of beam of sunlight.

some other examples of beam of lights

Beam of search light, light of headlamps of scooters, cars and engines of trains, beam of a torch. 

(a) Rail engine

(b) Light house

Q. With an experiment proves that light always travels in a straight line. 

Ans. Propagation of Light:

Light always travels in a straight line. 

When the candle is viewed through a straight tube, its flame can be seen. In case of a bent tube, the flame cannot be seen.

This experiment proves that light always travels in a straight line. 

Q. what is reflection ?

Ans. Reflection: The bouncing back of light after falling on a shiny surface is called reflection. 

We are able to see images in mirrors or in water, because of reflection.

* The change of direction by a mirror is also a reflection of light. 

Q. What are law of reflection explain with a neat diagram?

Ans. Laws of Reflection: There are two laws of reflection.

1. The incident ray, the reflected ray and the normal and the point of incidence lie in the same plane.

2. Angle of incident and angle of reflection are equal.

Q. What is image ? What are its main types? Explain with examples.

Ans. Image: When a number of the rays starting from first point (one source) and after reflection meet at another second point, this second point is called the image of the first point. These are of two types (i) Real Image and (ii) Virtual Image

(i) Real Image: The image obtained on a screen, is called real image. Real image is formed in front of mirror. The real image are inverted (↓).

* Images formed on the retina, Images formed on the film of a camera are real images.

* Concave mirror and convex lens form it.

(ii) Virtual Image: The image which can not be obtained on a screen, is called virtual image. virtual image is formed behind the mirror. The virtual image are erect (↑).

* The image formed in a plane mirror is virtual image.

* The concave lens and the convex mirror form virtual image.

Q. How is the image formed in mirror? What are main properties of the image formed by plane mirror?

Ans. Mind it that whenever an image is formed by a mirror or by any reflecting surface; the laws of reflection are obeyed.

Image formation in Plane Mirror: Image is of the same size as object. The distance of image and object from the plane mirror is same. Image is erect and virtual.

The properties of image formed by a plane mirror are:

(i) It forms an erect image.

(ii) It forms a virtual image.

(iii) Size of the image is equal as the object.

(iv) Image is formed at the same distance behind the mirror as the object stands in front of it.

(v) Image shows laterally inversion.

So that in case of plane mirror, the image is erect, virtual and of same size as the object.

Q. What do you mean by the the lateral inversion?

Ans. Lateral Inversion :– In which phenomena the left side of the objects appears on the right side of image and vice versa is known as laterally inversion.

Due to the lateral inversion of plane mirror we looke our right hand in place of the left hand of our image. 

Q. What do you mean by the the lateral inversion? Write the letters which do not show the lateral inversion.

Ans. Lateral Inversion :– In which phenomena the left side of the objects appears on the right side of image and vice versa is known as laterally inversion.

Lateral Inversion in Plane Mirror: 

A plane mirror makes laterally inverted image. 

Letters A, H, I, M, O, T, U ,V, W and X appear exactly like the letter itself when seen through a plane mirror.

Q. Why did the ambulance is written in laterally inversion (or in mirror form) at the ambulance?

Ans. The word ‘AMBULANCE’ is written in laterally inverted form on the front of the ambulance. So that it can be read comfortably in rear view mirror of vehicle moving ahead of it.

Review Questions

Q. What is light? 

Ans. Light is a form of energy which enables us to see the objects.

Q. What is normal to the point of incidence? 

Ans. The perpendicular of the surface at which point the incidence ray falls is called normal.

Q. What do you mean by the spherical mirrors what are its main types?

Ans. Spherical Mirror:

When the mirror is formed from a part of a hollow sphere, then it is called spherical mirror. 

Spherical mirrors are of two types. 

(i) concave mirror and (ii) convex mirror

(i) concave mirror : If the reflecting surface of a spherical mirror is concave or inner, so it is called concave mirror.  

(ii) Convex mirror: If the reflecting surface of a spherical mirror is convex or outer, so it is called a convex mirror.  

Q. How is the image formed by the concave mirror what is its property?

Ans. Image formation in Concave Mirror: In concave mirror, the image is usually inverted, real and smaller than the object. 

Only When the object is kept too close to the concave mirror, the image is erect, virtual and larger than the object.

Q. What are main uses of concave mirror?

Ans. Uses of Concave Mirror:

Concave mirrors are used for many

purposes. 

* ENT specialists uses the concave mirrors for examining eyes, ears, nose and throat. 

* Concave mirrors are also used by dentists to see an enlarged image of the teeth. 

* The reflectors of torches, headlights of cars and scooters are concave in shape.

* Concave mirror is used in solar furnace, because rays of sunlight converge at a point once they reflect from the concave mirror. 

* Concave mirror is used as barber’s mirror, because it shows a larger image when object is too close.

Q. How is the image formed by the Convex mirror what is its property?

Ans. Image formation in Convex Mirror: In case of convex mirror, the image is erect, virtual and smaller than the object.

Q. What are main uses of  Convex  mirror?

Ans. Uses of Convex Mirror:

* Convex mirror is used as side mirrors in scooters, bike's and vehicles for rear view so these are also called the rear view mirrors.

* Rear view mirrors shows smaller images from a bigger field of view. 

* Convex mirror is used on hairpin bends, to see the vehicles coming from other side of the bend.

Review Questions:

Q. Which type of image is usually formed in case of concave mirror? 

Ans. Inverted, real and smaller

Q. Which type of image is formed in case of convex mirror? 

Ans. Erect, virtual and larger

Q. What is a lens ? What are its main types? 

Ans. Lens :– A lens is a piece of transparent glass bound by two spherical surfaces. these are of two types (i) convex lens and (ii) concave lens.

(i) Convex lens: Convex lens are of two types 

(a) Plano convex and 

(b) Biconvex 

(a) Plano convex: Plano convex has one side convex and other plane so that it is called plano-convex. 

(b) Convex lens : Convex lens has both side convex or bulging outside so this it is called biconvex lens or convex lens.

[Note –Lenses which feel thicker in the middle than at the edges are convex lenses.]

(ii) Concave lens: Concave lens are of two types 

(a) Plano Concave and 

(b) Biconcave 

(a) Plano Concave: Plano Concave lens has one side Concave and other plane so that it is called Plano Concave. 

(b) Concave lens : Concave lens has both side convex or bulging outside so this it is called biconvex lens or convex lens.

[Note –Lenses which feel thinner in the middle than at the edges are convex lenses.]

Q. How is the image formed by the Convex lens? What are its main properties?

Ans. Image formation in Convex Lens: In case of convex lens, the image is usually inverted (↓), real and smaller than the object. 

Only when the object is kept too close to the convex lens, the image is erect (↑), virtual and larger than the object.

Q. How is the image formed by the Concave lens? What are its main properties?

Ans. Image formation Concave Lens: In case of concave lens, the image is erect (↑), virtual and smaller than the object.

Q. What are main uses of lenses?

Ans. Uses of Lens: 

* Convex lens is used in microscope, telescope, camera and in reading glasses. 

*Concave lens is used in spectacles.

Q. Why is it dangerous to look through a lens at the sun or a bright light?  

Ans. It is dangerous to look through a lens at the sun or a bright light it can harm or damage our eyes. We should also be careful not to focus sunlight with a convex lens on any part of your body.

Components of White Light: 

Q. What is a rainbow ? How many colours it have? Write name of the colours of it?

Ans. Rainbow: –The large arc seen in the sky with many colours is called 🌈 rainbow. There are seven colours in a rainbow. These are red, orange, yellow, green, blue, indigo and violet.

Q. What is spectrum of light?

Ans. The band of seven colours of the light is called spectrum of light. These seven colours of the lights can be known by the name VIBGYOR means 'violet, Indigo, blue, green, yellow, orange and red.

The white light, or visible spectrum, is composed of seven colours. These seven colours are its component colours. 

When white light passes through a prism, it breaks down into its component colours. This is the reason rainbows show all the colours of the visible spectrum and drops of the the rain works as a prism.

These seven component colours of light are VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange and Red). 

Q. What do you mean by the Newton's disc?

Ans. Newton’s Disc: Since Newton was the first to make this; it is called Newton’s Disc. It is a circular disc on which seven colours of the rainbow are painted on seven different sectors. When the disc is rotated at certain speeds, it appears to be white. This shows, how the seven colours make the white light.

Review Questions

Q. How many colours are there in the visible spectrum? 

Ans. Seven 

Q. What are the main colours of the light?

And. The white light, or visible spectrum, is composed of seven colours. These seven colours are VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange and Red). 

Q. Which type of lens is used in a camera? 

Ans. Convex

Solution of NCERT Exercise

Q. 1 - Fill in the blanks:

(a) An image that cannot be obtained on a screen is called ____________. 

(b) Image formed by a convex __________ is always virtual and smaller in size. 

(c) An image formed by a __________ mirror is always of the same size as that of the object.

(d) An image which can be obtained on a screen is called a _________ image.

(e) An image formed by a concave ___________ cannot be obtained on a screen.

Ans.

(a) Virtual

(b) Mirror 

(c) Plane

(d) Real

(e) Lens

Q. 2 Mark ‘T’ if the statement is true and ‘F’ if it is false:

(a) We can obtain an enlarged and erect image by a convex mirror. (T/F)

(b) A concave lens always form a virtual image. (T/F)

(c) We can obtain a real, enlarged and inverted image by a concave mirror. (T/F)

(d) A real image cannot be obtained on a screen. (T/F)

(e) A concave mirror always form a real image. (T/F)

Ans. 

(a) (F)

(b) (T)

(c) (F)

(d) (F)

(e) (F)

Q. 3 Match the items given in Column I with one or more items of Column II.

Ans. 

(a).  (v)

(b). (ii)

(c).  (i)

(d).  (iii)

(e).  (vi)

Q. 4 State the characteristics of the image formed by a plane mirror.

Ans. In case of plane mirror

(i) It forms an erect image.
(ii) It forms a virtual image.
(iii) Size of the image is equal as the object.
(iv) Image is formed at the same distance behind the mirror as the object stands in front of it.
(v) Image shows laterally inversion.

So that in case of plane mirror, the image is erect, virtual and of same size as the object.

Q. 5 Find out the letters of English alphabet or any other language known to you in which the image formed in a plane mirror appears exactly like the letter itself. Discuss your findings.

Ans. Letters A, H, I, M, O, T, U ,V, W and X appear exactly like the letter itself when seen through a plane mirror

Q. 6 What is a virtual image? Give one situation where a virtual image is formed.

Ans. The image which cannot be obtained on a screen is called virtual image. 

For example, the image of the plane mirror and convex mirror always formed virtual.

Q. 7 State two differences between a convex and a concave lens.

Ans. 

1. A convex lens is thicker at centre, while a concave lens is thinner at centre. 

2. A convex lens is thinner at edge, while a concave lens is thicker at edge. 

3. A convex lens can make enlarged image, while a concave lens would always make smaller image. 

4. A convex lens usually makes real image, while a concave lens always makes a virtual image.

Q. 8 Give one use each of a concave and a convex mirror.

Ans.

(a) Concave mirror is used by dentists to examine the teeth.

(b) Concave mirror is used in solar furnace. 

(c) Convex mirror is used as side view mirror in vehicles.

Q. 9 Which type of mirror can form a real image?

Ans. Concave mirror can form a real image.

Q. 10 Which type of lens forms always a virtual image?

Ans. Concave lens

Choose the correct option in questions 11–13

Question: 11 - A virtual image larger than the object can be produced by 

(a) Concave lens

(b) Concave mirror

(c) Convex mirror

(d) Plane mirror

Ans. (c) Concave mirror

David is observing his image in a plane mirror. The distance between the mirror and his image is 4 m. If he moves 1 m towards the mirror, then the distance between David and his image will be

(a) 3 m

(b) 5 m

(c) 6 m

(d) 8 m

Ans. (a) 3 m

Q. 13 The rear view mirror of a car is a plane mirror. A driver is reversing his car at a speed of 2 m/s. The driver sees in his rear view mirror the image of a truck parked behind his car. The speed at which the image of the truck appears to approach the driver will be

(a) 1 m/s

(b) 2 m/s

(c) 4 m/s

(d) 8 m/s

Ans. (b) 2 m/s