Class - 8, Science, Chapter - 16 Light Notes, Ncert Solutions & Frequently Asked Questions

Science

Class - 8, Chapter - 16

Light 

Notes, Ncert Solutions & Frequently Asked Questions 

-- Notes --

⭐ Light:- Light is a form of energy which makes us able see around us. 

• When light falls on an object or is emitted by an object, our eyes detect that light and  the object becomes visible to us.

Figure 1.- Light makes things Visible

• Light can be called as a radiation that is emitted, reflected or absorbed by different objects.

• A mirror is a glass having a shiny surface. When light falls on a mirror it is reflected back. Hence a mirror changes the direction of light that falls upon it.

Figure 2.- Light falling on a mirror

⭐ Laws of reflection:- 

• When light strikes a surface and bounces back, it is called reflection.

Figure 3.- Reflection of Light

• Incident ray - The ray of light that falls on the surface or the incoming light is called incident ray.

• Reflected ray - The ray of the light that gets reflected back from the surface of the mirror is called reflected ray.

• Normal - It is an imaginary line that is drawn perpendicular to the reflecting surface at the point where the incident ray strikes the surface or at the point of incidence.

• The angle of incidence (∠i) - It is the angle that the incident ray makes with the normal.

• The angle of reflection (∠r) - It is the angle that the reflected ray makes with the normal.

• For an incident ray, there is always one reflected ray.

• First Law of Reflection:- The angle of incidence and the angle of reflection are always equal to each other, (∠i = ∠r).

• Second Law of Reflection:- The incident ray, the reflected ray and normal all lie in the same plane.

⭐ Characteristics of the image formed by a plane mirror:-

• It has the same height as that of the object.

• The distance between the image and the mirror and the object and the mirror is always equal.

• The image is erect or upright.

• The image is laterally inverted.

• It is a virtual image because it is formed by the apparent intersection of diverging rays.

⭐ Lateral inversion:- When an image is formed in a mirror the, the left side of the object appears at the right side of the image & the right side of object appears at the left side of the image. This is called a lateral inversion.

Figure 4.- Lateral Inversion

⭐ Regular and Diffused Reflection:- 

• Regular Reflection - When parallel rays of light fall on a smooth surface, they are reflected back as parallel rays only. This is called regular reflection.

- It is also called as Specular reflection. 

- It leads to the formation of an image.

- For example, the image formed by a plane mirror.

Figure 5.- Regular & Irregular Reflection 

• Diffused Reflection - When parallel rays of light fall upon an irregular surface, they are reflected back into all the directions. This is called diffused reflection.

- It is also called irregular reflection.

- It enables us to view different objects present in our environment.

- For example, the reflection of rays of light through a wall. 

⭐ Non - Luminous objects - Objects that do not produce light on their own but reflect the light that falls upon them are called non-luminous objects. 

- These are also called as illuminated objects. 

- For example - the moon, trees, plastic. 

⭐ Luminous objects - Objects which can produce light on their own are called luminous objects. 

- For example, tube light, bulb, the Sun and the Stars.

Figure 6.- Luminous and Illuminated Objects

⭐ Multiple Reflections:- Reflected light can be reflected again & again.

• Multiple reflections can be obtained by placing more than one mirrors at certain angles to each other which results in reflection of light for multiple times between the reflecting surfaces. 

• This phenomenon is used in certain scenarios such as:

- In a Periscope to see hidden objects. 

- In a kaleidoscope to see beautiful patterns. 

- In a barbershop to see the back of the head of a person getting a haircut. 

Figure 7.- Multiple Reflection of Light

⭐ Periscope:- A Periscope is an instrument that is used to look at objects that are not in the line of sight of the observer.

• It is used to see objects that are hidden.

• Periscope works on the phenomena of multiple reflections of light.

• Periscopes are generally used in submarines, tanks and bunkers by the soldiers.

Figure 8.- Structure of a Periscope

⭐ Kaleidoscope:- It is a device that works on the phenomena of multiple reflections with the help of multiple mirrors.

• It has a cylindrical structure with three plane mirrors inside it.

Figure 9.- Structure of a Kaleidoscope

⭐ Sunlight:- 

• The sun is the biggest source of light on the earth.

• The sunlight is white in colour however this white light often disperses into several colours which results in different colours of the sky at different times of the day.

• The white light consists of seven colours namely VIBGYOR (violet, indigo, blue, green, yellow, orange and red).

• This arrangement of seven colours of a white light is called as Spectrum.

• This phenomenon of the splitting of light into its constituents is called Dispersion of light.

Figure 10.-  Dispersion of Light

⭐ Examples of dispersion of light:-

• Formation of rainbow due to the dispersion of white light through the water droplets.

• Passing of Sunlight through a prism. 

• Dispersion of light through the soap Bubbles. 

• Dispersion of light on the CDs. 

Figure 11.- Examples of dispersion of light

⭐ Human Eye:- Eyes are sensory organs that allow us to see things present in the environment.

• The eyes absorb the light rays & form a visual image & transform this information of the image to the brain.

• The eye has a spherical structure which is called the eyeballs. 

• The eyeballs are attached in the eye socket & various muscles are connected to them called the eye muscles. These are muscles which allow the movement of the eyeballs.

• The eyes are protected with eyelids. 

• They prevent an excess of light & dust to enter into the eyes & therefore save the eyes from any outer objects.

Figure 12.- Human Eye

⭐ The parts of the human eye:- 

• Cornea - It is a transparent covering present on the outer side of the eyes.

- It consists of 6 layers. 

- Tear fluid covers cornea and protects the eye.

• Iris - It is a dark coloured muscular structure present in the centre of the cornea. 

- The Iris consists of a small opening called Pupil through which the light enters into the eye. 

- The Iris has colourful pigments like grey, blue, black, green, brown etc. 

- The amount of light that enters the eye is controlled by the Iris (Iris controls the Pupil).

• Lens - It is like a sphere of liquid present inside the eye. 

- The eye lens is located behind the Pupil which helps in the formation of an image in the eye.

• Retina- Retina is a layer located behind the lens at the back of the eye.

- The eye lens focuses the light that enters into the eye over the retina.

- The retina consists of different Nerve cells that perform different tasks.

- There are sensory cells that detect the light.

- They then transform that information into an electrical impulse.

- These electrical impulses are sent to the brain via the optic nerve. 

- Two types of vision cells in the retina are:

--- Rods or chopsticks - They get activated in darkness or dim-light & the responsible for light-dark vision.

--- Cones - they react to bright light and therefore sense colour in an object.

• Vitreous Chamber - The inner space of the eyeball called the transparent vitreous chamber consists of a gel-like substance.

- This substance maintains the right pressure in the eye and maintains the stability of the eyeball.

• Blind spot - There is a point in the eye located at the junction of the retina & the optic nerve where no sensory cells are present. 

- This spot is therefore called the blind spot as it does not support any vision.

⭐ Persistence of image on the retina:- The image that is formed on the retina persists for 1/16th of a second.

• Therefore, if one tries to move 16 still images per second of a moving object in front of our eye it appears as if the object is moving. This is how animation films and movies work. They are a collection of separate pictures which are moved in a sequence.

• However, this movement is so fast, around 24 pictures in a second, that it appears as if they are moving.

⭐ The vision may decrease with age:- Normally a person can see distant and nearby objects through the eyes.

• A person with normal eyes can read most comfortable at a distance of 25cm. However, this distance can decrease with age.

• Also sometimes people can see far objects clearly but not the objects that are nearby (farsightedness or hypermetropia).

- It can be corrected by using convex lens. 

• Some people can see the nearby objects clearly but not the far objects (nearsightedness or myopia). 

- It can be corrected by using a concave lens. 

• Sometimes with old age, the lens of the eye can become cloudy or foggy. This condition is called cataract with results in loss of vision.

- This defect can be rectified by removing the old lens from the eye and inserting an artificial lens at that place.

⭐ Causes of defects in Eye:-

• Presbyopia:- solidification of lens fluid

• Cataract:- protein structures of lens clubbed together 

• Myopia:- due to a strong refractive index of the eye

•  Hypermetropia:- due to a weak refractive index of the eye

⭐ How to take care of the eyes?:- One should go for a regular eye check up.

• Wear spectacles if the eyesight is found weak.

• Always study or watch TV in a sufficient light. 

• Excess or insufficient light both can lead to problems. 

• Insufficient light can cause strain and leads to a headache.

• Do not look at the sharp source of light like the sun directly. 

• Excess of light can damage the eyes.

• If dust enters into the eye one should not rub them and rather wash them straightway.

• Maintain a normal distance while reading or watching TV.

• Take a balanced diet to ensure that you get the right nutrients.

⭐ NIght blindness:- Night blindness also called nyctalopia is a defect in the vision in which people cannot view things properly in dim light or at night.

• It mainly occurs due to the lack of vitamin A in our body. 

• Hence we should always take foods that are rich in vitamin A such as broccoli, green vegetables, carrot, milk, eggs, curd, papaya, mango etc.

⭐ How can visually impaired people read and write?:- Visual impairment is also known as vision loss.

• A person with vision loss cannot see at all.

• This loss of vision is up to a degree that cannot be corrected using usual means like using glasses or lenses.

• The people who have no eyesight at all develop the other senses of listening and touching sharply which allows them to do their day to day activities.

⭐ The Braille system:- Braille is a tactual aid for visually challenged people that allow them to read and write.

• It was developed by a visually challenged person called Louis Braille.

• This system was published by him in 1821 however the present Braille System was adopted in 1932.

• For many common languages including Hindi, Sanskrit, Tamil, Telugu, Mathematics and scientific notations a Braille code is present.

• In the Braille code, there are 63 Dots or 63 Characters.

• These are arranged in a cell of two vertical rows having three dots each.

• The patterns are embossed on a Braille Sheet that allows a person to read by touching the pattern. Each character in the Braille system can represent:- 

            - a letter

            - a combination of letters or a word

            - a grammatical sign

Figure 14.- The Braille System for English Alphabets

⭐ Eyes of animals have different shapes:- 

• Crab:- Crabs have small eyes but they can look all around with them even behind them.

• Butterflies:- Butterflies have big eyes that consist of thousands of small eyes which allow them to see in the front and at the back.

• Night owls (Night Birds / Nocturnal animals):- Night owls are able to see things more clearly in the night rather than day time. 

- This is so because the cornea of their eyes is large with a large pupil which allows light to enter into the eye at night time. 

-However, their eyes have a large number of rod cells rather than cones that allow them to see the things at night easily.

• Kites and Eagles (Day Birds):- These birds can see very well during daytime but have decreased vision at night. 

- This is because they have a large number of cone cells and a few rod cells.

-- NCERT Solutions --

Question 1.- Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.

Answer:- When we are in a dark room then we cannot see objects in the room. We can see the objects outside the room, because out of the room the light is available and the rays of light can enter our eyes after reflection from the objects.

Question 2.- Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?

Answer:- 

No, diffuse reflection doesn’t mean the failure of laws of reflection.

Question 3.- Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case.

• Polished wooden table

• Chalk powder

• Cardboard surface

• Marble floor with water spread over it

• Mirror

• Piece of paper

Answer:- 

• Polished wooden table - reflection will take place because the surface is plane and polished.

• Chalk powder - Diffused reflection will take place because the surface is rough.

• Cardboard surface - Diffused reflection will take place because the surface is rough.

• Marble floor with water spread over it - Regular reflection will take place because the surface is smooth and plane.

• Mirror - Regular reflection will take place because the surface is plane and polished.

• Piece of paper - Diffused reflection will take place because the surface is rough.

Question 4.- State the laws of reflection.

Answer:- The laws of reflections are:-

• The incident ray, the normal and the reflected ray, all lie in the same plane.

• The angle of incidence is equal to the angle of reflection.

Question 5.- Describe an activity to show that the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.

Answer:- 

• Activity:- To show that the incident ray, reflected ray and the normal at the point of incidence lie in the same plane.

• Materials Required:-  Plane mirror, holder, ray box, etc.

• Procedure:- Fix sheet of white paper, a little beyond the edge of the board. Place a plane mirror strip vertically to the paper using a stand. Throw light from a ray box on the mirror. Look at the reflected ray. Mark the incident ray, normal ray and reflected ray. Fold the paper which is beyond the edge of the board. You will observe that the reflected ray is not seen in the folded portion of the chart paper. Now bring the folded portion back to its original position. The reflected ray of light is again seen on the page.

• Conclusion:- The sheet on the board can be considered as a plane. The incident ray, the reflected ray, the normal at the point of incidence lie in the same plane.

Question 6.- Fill in the blanks in the following.

a) A person 1 m in front of a plane mirror seems to be ______ m away from his image.

b) If you touch your ______ ear with a right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with _____

c) The size of the pupil becomes _______ when you see in dim light.

d) Night birds have _______ cones than rods in their eyes.

Answer:-

a) 2

b) left, left hand

c) larger

d) lesser

Choose the correct option in Questions 7-8.

Question 7.- The angle of incidence is equal to the angle of reflection

a) Always

b) Sometimes

c) Under special conditions

d) Never

Answer:- a) Always

Question 8.- Image formed by a plane mirror is:- 

a) virtual, behind the mirror and enlarged.

b) virtual, behind the mirror and of the same size as the object.

c) real at the surface of the mirror and enlarged.

d) real, behind the mirror and of the same size as the object.

Answer:- b) virtual, behind the mirror and of the same size as the object.

Question 9.- Describe the construction of a kaleidoscope.

Answer:- Kaleidoscope is a device based on the principle of multiple reflections. It consists of three long and narrow strips of plane mirrors inclined at an angle of 60° to one another forming prism. This is fitted in a tube. One end of this tube is closed by a cardboard disc having a hole at its centre. To the other end touching the mirrors plane glass plate is fixed on which broken pieces of coloured bangles are placed. This end of the tube is closed by a ground glass plate.

Question 10.- Draw a labelled sketch of the human eye.

Answer:-

Question 11.- Gurmit wanted to perform Activity 16.8 using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teacher’s advise?

Answer:- Teacher has advised Gurmit not to do so because laser light is very harmful for her eyes and can cause a permanent defect in the eye. Person can even lose his or her eyesight if laser torch is directed over the eyes.

Question 12.- Explain how you can take care of your eyes.

Answer:- Eyes are very precious. We must take proper care of them. We must :-

• always sit straight while reading or writing.

• if advised, use suitable spectacles.

• wash our eyes with clean water frequently.

• not look at the sun directly.

• always read or write in a proper light.

Question 13.- What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?

Answer:- Here, the angle of reflection is 90°. As we know, according to the laws of reflection that angle of incidence is equal to angle of reflection.

Here, the angle between the incident ray and reflected ray is 90°.

i.e., ∠i + ∠r = 90°

Since, ∠i = ∠r

We can write, ∠i + ∠i = 90°

⇒ 2∠i = 90°

⇒ ∠i = 45°

Angle of incidence = 45°.

Question 14.- How many images of a candle will be formed if it is placed between two parallel plane mir¬rors separated by 40 cm?

Answer:- Here, mirrors are placed parallel to each other 40 cm apart. Therefore, the infinite number of images will be formed.

Question 15.- Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig. 16.19. Draw the reflected ray from the second mirror.

Answer:- 

Question 16.- Boojho stands at A just on the side of a plane mirror as shown in Fig. 16.21. Can he see himself in the mirror? Also, can he see the image of objects situated at P, Q, and R?

Answer:- 

No, Boojho can’t see himself in the mirror. He can see the image of the object at P and Q but not of R.

Question 17.- 

a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.23).

b) Can Paheli at B see this image?

c) Can Boojho at C see this image?

d) When Paheli moves from B to C, where does the image of A move?

Answer:-

a) It is shown in the following figure.

b) Yes, Paheli can see the image of A.

c) Yes, Boojho can see the image of A.

d) Image of the object at A will not move as an object is not moving.

-- Frequently Asked Questions --

Question 1.- What are luminous objects? Give some examples.

Answer:- The objects which emit their own light are known as luminous objects. Example: sun, fire, flame of a candle and an electric lamp.

Question 2.- What are illuminated objects? Give one example.

Answer.- The objects which shine in the light of other objects are called illuminated objects. Example: moon.

Question 3.- Why cannot we see a book which is placed behind a wooden screen?

Answer:- Wooden screen are opaque object that does not allow light to pass through it. Thus, we are not able to see through it.

Question 4.- What is blind spot in the eye?

Answer:- At the junction of the optic nerve and the retina, there are no sensory cells, so no vision is possible at that spot. This is called the blind spot.

-- By Durgesh Pandey

(Eklavya Coaching Institute)