Class-7, Science, Chapter:- 15 Light Notes, NCERT Solutions & Frequently Asked Questions

Science

Class:-7, Chapter:- 15

 Light

Notes, NCERT Solutions & Frequently Asked Questions

-- Notes --

⭐Light:- Light is a form of energy which enables us to see objects from where it comes or reflected. 

• Light always travels along a straight line.

Figure 1.- Light always travels in straight line 

⭐ Types of Objects:-

a) Transparent objects:- Objects which allow light to pass through them completely.

                 • Eg.- glass

b) Translucent objects:- Objects which allow light to pass through them partially. 

                • Eg.- Butter paper

c) Opaque objects:- Objects which do not allow any light to pass through them. 

                • Eg.- table, book, etc.

⭐ Reflection of Light:- The change in direction of light by a mirror or mirror like objects is called a reflection of light. 

Figure 2.- Reflection of light by plane mirror 

• The surface of the water can also act as a mirror and can also change the path of light and that is why we see the reflection of trees or buildings in the water.

⭐ Laws of reflection:- 

• The ray of light that strikes a reflecting surface is called incident ray & the ray that gets reflected back is called reflected ray. 

• The imaginary line that is perpendicular to the reflecting surface at the point of incidence is called the normal. 

• Angle of incidence ∠i is the angle between the incident ray and the normal. Angle of reflection ∠r is the angle between the reflected ray and the normal.

• Law of Reflection states that the angle of incidence ∠i is always equal to the Angle of reflection ∠r (∠i=∠r).

• The angle of incidence, angle of reflection and the normal all lie on the same plane. 

⭐ Image:- Image is an impression / photocopy of any object created by light on the mirror. 

• The distance between image & mirror and  object & mirror is always same. 

⭐ Types of images:-

a) Real Image:-  When the light rays coming from an object actually meet at a point after reflection from the mirror, then it results in the formation of a real image.

• It is an image which can be obtained on a screen, eg.- the image formed on a cinema screen.

b) Virtual Image:- When the light rays coming from an object appear to meet after reflection from the mirror, then it results in the formation of virtual image.

• It is an image which cannot be obtained on a screen because light rays actually do not pass the screen or cannot be received on a screen, eg.- image formed by a plane mirror. 

c) Erect Image:- When the image is formed of the same size & same side as that of the object.

d) Inverted Image:- When the image is formed upside-down compared to the object. 

Note :- 

If we increase or decrease the distance between the object and the mirror then the distance between the image and the object also increase or decrease. 

⭐ Difference between real & virtual image:-

⭐ Lateral Inversion:- 

• If we stand in front of a plane mirror and lift our right hand, then we see our image lift its left hand. And if we lift our left hand, then the image appears to lift its right hand.

• The right side of a body becomes the left side in the image while the left side of the body becomes the right side of the image.  

• The effect of reversing the sides of an object and its image is called lateral inversion.

• The word AMBULANCE is written from backward on vans because when a driver of a vehicle ahead of an ambulance look in his/her rear view mirror, then he/she can read AMBULANCE written on it and give way to it. So, it is the duty of everyone of us to allow an ambulance to pass without blocking its way.

⭐ Mirror:- A mirror is generally any shiny surface that can reflect back light.

• A mirror that has a plane surface is called a Plane Mirror.

(Formation of real image by concave mirror)

(Formation of virtual image by concave mirror)

• A mirror that is curved, it either bulges in or out, is called a Curved Mirror.

⭐ Spherical Mirrors:- A mirror whose reflecting surface is the part of a hollow sphere of glass is known as a spherical mirror.

• The spherical mirrors are of two types:-

a) Concave mirror:- The outer surface of the curved mirror is painted and its inner surface is a reflecting surface. 

• A concave mirror converges a beam of parallel light rays therefore, it is also known as a converging mirror. 

• Concave mirrors form a real, inverted and magnified image. 

• Concave mirrors are used by doctors and dentists for enlarged images of our ears, eyes, teeth etc.

b) Convex mirror:- The inner surface of the curved mirror is painted and its outer surface is a reflecting surface.

• A convex mirror diverges a beam of parallel light rays therefore, it is also known as a diverging mirror. 

• Convex mirrors form erect and virtual images that are diminished (smaller than the object size). 

• Convex mirrors are used in vehicles as it gives the driver images spread over a large area.

⭐Range of visibility (plane mirrors):- 

• An observer can only see a reflection on a plane mirror if the observer is within its range of visibility

• If an observer wants to see his entire image on a plane mirror, the mirror must be at least half the height of the observer.

⭐ Lenses:- It is a part of reflecting materials like glass or plastic but curved from both sides.

• Lenses are unlike mirrors that have a reflecting surface on only one side. 

⭐ Types of Lenses:- 

a) Convex Lens:- It is curved outwards, thicker at  the center & thinner at the edges.

• It merges the light rays passing through it.

• Convex lens forms a real, inverted and magnified image. 

• They are also called as converging lens. 

• This type of lens is also used as a magnifying glass.

b) Concave Lens:- It is curved inwards,  thicker at edges & thinner at centre.

• It reflects the light rays back in different directions which travels through it. 

• Concave lens always forms a virtual, erect and diminished image. 

• They are also called as diverging lens. 

• They are used as camera lenses and binoculars.

(Images formed by convex & concave lenses)

⭐ Refraction:- The bending of the path of light when it moves from one medium to another medium is known as refraction.

• The object we see after refraction is called the apparent object.

• There exists some displacement between the apparent and actual object.

⭐ Dispersion:- The splitting up of white light into its component colours is called dispersion.

• Dispersion of white light can be seen with the help of a prism. 

⭐ Rainbow:- It is a natural phenomenon in which light rays of the sun are reflected and refracted by the water droplets present in the atmosphere.

• It contains seven colours - VIBGYOR - Violet, Indigo, Blue, Green, Yellow, Orange & Red. 

-- NCERT Solutions -- 

Exercise Questions

Question 1.- Fill in the blanks.

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

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.

Solution:-

a) virtual image

b) mirror 

c) plane 

d) real 

e) lens

Question 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.

b) A concave lens always form a virtual image.

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

d) A real image cannot be obtained on a screen.

e) A concave mirror always forms a real image.

Solution:-

a) False

b) True

c) True

d) False

e) False

Question 3.- Match the items given in Column I with one or more items in Column II.

Solution:- 

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

Solution:- Characteristics of the image formed by a plane mirror are as follows:-

• Image distance and object distance are equal.

• The size of the object and image are equal.

• The image formed is erect and virtual.

• Images are laterally inverted.

Question 5.- Find out the letters of the 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.

Solution:- A, H, I, M, O, T, U, V, W, X, and Y alphabets form images in a plane mirror exactly like the letter itself because these alphabets are laterally symmetric.

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

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

Question 7.- State two differences between a convex and a concave lens.

Solution:- The difference between a Convex & a concave lens are as follows:- 

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

Solution:-

• Concave mirrors are used in the headlights of cars and scooters.

• Convex mirrors are used as side-view mirrors in vehicles.

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

Solution:- The concave mirror can form a real image.

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

Solution:- A concave lens forms a virtual image.

Choose the correct option in questions 11 – 13.

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

      i) concave lens           ii) concave mirror

     iii) convex mirror      iv) plane mirror

Solution:- ii) concave mirror.

Question 12.- 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

        i) 3 m                                  ii) 5 m

       iii) 6 m                                iv) 8 m

Solution:- iii) 6 m

Question 13.- The rearview 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 rearview 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

         i) 1 m/s                          ii) 2 m/s

        iii) 4 m/s                         iv) 8 m/s

Solution:- iii) 4 m/s.

-- Frequently Asked Questions --

Question 1.- What is definition of ‘Light’?

Solution:- The energy derived from the sun, a lamp, etc. that allows you to see things is called as light.

Question 2.- What is ‘Range of visibility’?

Solution:- The horizontal distance (in kilometers or miles) at which a large dark object can just be seen against the horizon sky in daylight.

Question 3.- What are the uses of ‘Spherical mirrors’?

Solution:- 

• Used as a rear mirror 

• Security mirrors 

• Survelliance mirror

-- By Durgesh Pandey

(Eklavya Coaching Institute)