Chapter 16: Light

Extra Internal Question and answer:

Question: What is reflection of light?

Answer: Reflection of light is the bouncing back of light when it falls on a surface.

Question: What do you mean by incident ray and reflected ray?

Answer: The ray of light that falls on a reflecting surface is called the incident ray.
The ray of light that comes back from the surface after reflection is called the reflected ray.

Question: Define the normal in reflection of light.

Answer: The normal is an imaginary line drawn perpendicular to the reflecting surface at the point of incidence.

Question: What is the angle of incidence?

Answer: The angle of incidence is the angle between the incident ray and the normal at the point of incidence.

Question: What is the angle of reflection?

Answer: The angle between the reflected ray and the normal at the point of incidence is called the angle of reflection.

Question: What type of image is formed by a plane mirror?

Answer: A plane mirror forms a virtual, erect image of the same size as the object. The image is laterally inverted and appears as far behind the mirror as the object is in front of it.

Question: What is diffused (irregular) reflection?

Answer: When parallel rays of light fall on a rough or irregular surface and are reflected in different directions (not parallel), the reflection is called diffused or irregular reflection.

Question: What is regular reflection?

Answer: When parallel rays of light fall on a smooth surface and are reflected in the same direction, remaining parallel after reflection, it is called regular reflection.

Question: What is multiple reflection?

Answer: When a ray of light is reflected again and again from two or more reflecting surfaces, it is called multiple reflection.

Question: What happens when light falls on a smooth surface?

Answer: When light falls on a smooth surface, it is reflected in a regular manner. The parallel rays of light remain parallel after reflection.

Question: What happens when light falls on a rough surface?

Answer: When light falls on a rough surface, it is reflected in different directions.

Question: What is dispersion of light? How is a rainbow formed in nature due to dispersion?

Answer: Dispersion of light is the splitting of white light into its seven colours (VIBGYOR) when it passes through a prism.

A rainbow is formed when sunlight passes through tiny water droplets in the atmosphere. The light is refracted and dispersed into different colours, then internally reflected inside the droplets and refracted again as it comes out, forming a beautiful spectrum in the sky.

Question: What is lateral inversion? Give an example.

Answer: In an image formed by a mirror, the left side of the object appears on the right side and the right side appears on the left side. This is called lateral inversion. Example: The word “AMBULANCE” is written in reverse on the front of an ambulance so that it can be read correctly in the rear-view mirror.

Question: What is the Braille System?

Answer: The Louis Braille system is a method that enables visually impaired persons to read and write. It is based on patterns of raised dots arranged in cells. Each pattern represents a letter, number, or symbol, which can be felt and read using fingertips.

Question: Write the function of : (a) cornea (b) iris (c) pupil (d) retina

Answer: (a) Cornea: It is the transparent front part of the eye. It allows light to enter the eye and helps in bending (refraction) of light.

(b) Iris: It is the coloured part of the eye. It controls the size of the pupil and regulates the amount of light entering the eye.

(c) Pupil: It is a small opening in the centre of the iris. It allows light to enter the eye.

(d) Retina: It is the inner layer at the back of the eye. It forms the image of objects and sends signals to the brain through the optic nerve.

Exercises

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

Answer: In a dark room, you cannot see objects inside the room because there is no light to reflect from them into your eyes. You also cannot see objects outside the room because walls block the light from outside sources from entering the room. For vision, light is essential.
2. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?

Answer:  The difference between Regular Reflection and Diffused Reflection :

Regular Reflection	Diffused Reflection
It occurs on smooth and polished surfaces like a mirror.	It occurs on rough or uneven surfaces like a wall or paper.
The reflected rays remain parallel to each other.	The reflected rays scatter in different directions.
A clear and sharp image is formed.	No clear image is formed.
Example: Reflection from a plane mirror.	Example: Reflection from a rough wall.

No, diffused reflection does not mean the failure of the laws of reflection. The laws of reflection are still obeyed at each point of the rough surface. But since the surface is uneven, the reflected rays spread out in different directions, which is why no clear image is formed.

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.
(a) Polished wooden table     (b) Chalk powder       (c) Cardboard surface      (d) Marble floor with water spread over it       (e) Mirror     (f) Piece of paper

Answer: (a) Polished wooden table – Regular reflection
Because the surface is smooth and shiny, most of the reflected rays remain parallel, so regular reflection takes place.

(b) Chalk powder – Diffused reflection
Chalk powder has a very rough and uneven surface, so light is scattered in different directions, causing diffused reflection.

(c) Cardboard surface – Diffused reflection
Cardboard is a rough surface, so the reflected rays go in many directions. Hence, diffused reflection occurs.

(d) Marble floor with water spread over it – Regular reflection
Water fills the small irregularities of the marble floor, making the surface smooth. Therefore, regular reflection takes place.

(e) Mirror – Regular reflection
A mirror has a highly smooth and polished surface, so it produces regular reflection and forms a clear image.

(f) Piece of paper – Diffused reflection
A piece of paper has a rough surface at the microscopic level, so light is scattered, resulting in diffused reflection.
4. State the laws of reflection.

Answer:  The laws of reflection are:

(a) The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.

(b) The angle of incidence is equal to the angle of reflection. 
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: Fix a plane mirror on a table and place a sheet of chart paper slightly projecting out. Draw an incident ray and observe the reflected ray on the sheet. Now bend the projected part of the paper where the reflected ray falls. The reflected ray disappears. When the paper is brought back, it reappears.

         

This shows that the incident ray, reflected ray and the normal all lie in the same plane.
6. Fill in the blanks in the following.
(a) A person 1 m in front of a plane mirror seems to be _______________  away from his image.
(b) If you touch your ____________ ear with 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) A person 1 m in front of a plane mirror seems to be 2 m away from his image.

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

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

(d) Night birds have fewer cones than rods in their eyes.
Choose the correct option in Questions 7 – 8
7. Angle of incidence is equal to the angle of reflection.
(a) Always     (b) Sometimes      (c) Under special conditions     (d) Never

Answer: (a) Always

[ According to the laws of reflection, the angle of incidence is always equal to the angle of reflection for every case of reflection of light.]
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.

[ A plane mirror always forms a virtual image, which appears behind the mirror and is of the same size as the object.]

9. Describe the construction of a kaleidoscope.

Answer: A kaleidoscope is made using three rectangular strips of plane mirrors. These mirrors are arranged in a triangular shape with their reflecting surfaces facing each other and are fixed inside a tube. One end of the tube has two circular glass plates, and some small coloured glass pieces are placed between them. The outer plate is transparent to allow light to enter. When the tube is rotated, the coloured pieces form beautiful changing patterns due to multiple reflections inside the mirrors.
10. Draw a labelled sketch of the human eye.

Answer: The labelled sketch of the human eye :

    

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: The teacher advised Gurmit not to use a laser torch because laser light is very intense and can damage the eye. It may harm the retina. A normal torch is safe to observe changes in pupil size.

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

Answer: Our eyes can be protected by following some easy but essential habits.

(i) If there is any problem with the eyes, you should visit an eye specialist and have a regular eye check-up.

(ii) If advised by the doctor, use suitable spectacles regularly.

(iii) Too little or too much light is harmful for the eyes. Insufficient light causes eyestrain and headache, while very strong light such as the Sun, powerful lamps or laser torches can damage the retina.

(iv) Do not look directly at the Sun or any powerful light source.

(v) Never rub your eyes. If dust or some particle enters the eyes, wash them with clean water. If irritation continues, consult a doctor.

(vi) Always read at a normal distance. Do not bring the book too close to your eyes or keep it too far while reading.
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: Let the angle of incidence = i and the angle of reflection = r.

Law of reflection, we have,

Therefore, the angle of incidence is 45°.
14. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?

Answer: When a candle is placed between two parallel plane mirrors, multiple reflections take place. Hence, an infinite number of images are formed, because the mirrors keep reflecting the images of each other again and again.
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:  The incident ray strikes the first mirror at 30° and is reflected at 30°. Since the mirrors are at right angles, the ray strikes the second mirror at 60° (= 90° – 30°) and is reflected at 60°. The final ray travels parallel to the initial incident ray.

          

16. Boojho stands at A just on the side of a plane mirror as shown in Fig. 16.20. Can he see himself in the mirror? Also can he see the image of objects situated at P, Q and R ?

      

Answer: No, Boojho cannot see himself because he is outside the mirror’s range . However. He can see the images of P and Q because their reflected rays reach his eyes . He cannot see R as its reflected rays pass too far away from his position.

17. (a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.21).
(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) The image A’ is at the same distance behind the mirror as A is in front .

(b) Yes , the reflected ray from A can reach B .

(c) Yes, the reflected ray from A can reach C .

(d) The image remains stationary at A’ because the objects position has not changed.