Light

Light is a form of energy that enables us to perceive the world around us. It travels in straight lines, and understanding its properties helps us comprehend various scientific concepts and everyday phenomena. Light is crucial in numerous technologies, including cameras, telescopes, and fiber optics. Additionally, it plays an important role in processes like photosynthesis in plants, making it essential for life on Earth.

Reflection of Light

The process of light bouncing back when it encounters a boundary between different media that it cannot penetrate is known as reflection.

Laws of Reflection

1. Ray of Light:: A ray of light is a hypothetical concept representing a single, thin line of light. In reality, many rays combine to form a beam of light.
2. Incident Ray: The light ray that hits any surface is known as the incident ray.
3. Reflected Ray: The ray that returns from the surface after reflection is referred to as the reflected ray.
4. Normal: A line drawn perpendicular to the surface of the mirror at the point where the incident ray hits it.
5. Angle of incidence: It is the angle created between the normal and the incident ray, commonly represented as i
6. Angle of reflection: The angle of reflection is defined as the angle between the normal and the reflected ray, commonly represented as r.

Laws of Reflection

1. Law 1 : The incident ray, the normal and the reflected ray all lie in the same plane.
2. Law 2 : Angle of incidence is equal to the angle of reflection
3. Law 3 or Law of Lateral Inversion: In a mirror, the left side of an object appears on the right, and the right side appears on the left. This effect is called lateral inversion.

Regular And Diffused Reflection

Types of reflection

1. Regular Reflection: When light drops on a smooth uniform surface (like a mirror), all the parallel incident rays are reflected perfectly parallel. This is called Regular Reflection.
2. Diffused Reflection: When light strikes a rough surface, such as stone, skin, or clothing, the parallel incident rays scatter in various directions instead of reflecting parallel to each other. This phenomenon is known as diffused reflection.

Note: Both Regular and Diffused Reflection obey the Laws of Reflection. Diffused reflection is caused by irregularities in the reflecting surface and not due to the failure of Laws of Reflection.

Luminous and Non-Luminous objects

Luminous : Objects which emit light of their own. Eg: Sun, Bulb, Candle, Fire.

Illuminated or Non-luminous :Objects that do not emit their own light but instead reflect light from luminous sources, making them visible to our eyes. Examples include the moon, blue sky, clouds, trees, cars, and tables.

Components of White Light

Sunlight or white light is made of seven colours as in VIBGYOR(Violet, Indigo, Blue, Green, Yellow, Orange, Red). The process of separating white light into its constituent colors as it passes through a medium, like a prism, is known as dispersion.

Human Eye

Light mainly tumbles on the cornea - the white clear front part of the eye. It is tough and guards the interior of the eye from accidents.The iris (a muscular structure behind the cornea) of the eye controls the amount of light reaching the interior of the eye by automatically adjusting the size of the pupil. (The iris is that part of the eye which gives it the colour such as black, brown, green eyes commonly called cat-eyes etc)The eye's crystalline biconvex shaped lens is located behind the pupil. It further focuses the light on the retina. The retina contains several nerve cells which convert the optical images to signals to the brain via the optic nerve.

There are 2 types of nerve cells. 

(i) Cones : which are responsive to bright light and help us recognize color 

(ii) Rods: which are responsive to dim light.

The brain then decodes this signal as the image we see

Note : The light is focused by the cornea, then the iris by controlling the opening of the pupil and then the lens. In dim light, your pupil expands to allow more light to enter your eye. In bright light, it contracts.

Care of Eyes:

Eyes are sensitive and essential, so it’s important to take good care of them. If you experience pain, irritation, or poor visibility, consult a doctor promptly. Use appropriate spectacles or lenses if recommended. To avoid eye strain, ensure you have sufficient lighting for daily activities. Reading should be done at a distance of about 25 cm, and the same applies when watching TV or using devices like laptops and tablets. Avoid rubbing your eyes too hard and wash them regularly with clean water. If irritation continues, seek medical advice. A healthy diet contains Vitamin A—present in foods like carrots, leafy greens, eggs, milk, and dairy products—can aid in preventing eye problems.

Braille Script

Some individuals may experience visual impairment due to congenital conditions or loss of sight from accidents or diseases. They rely on their other senses—such as touch, hearing, and speech—to communicate and engage with the world around them.They use aids like non-optical devices (hearing aids, Braille kits, audio materials) and optical devices (lenses, bifocals). Braille is a widely used method for reading among the visually impaired. The Braille script is an alphabet system designed for the visually impaired to read and write. Each Braille character consists of a combination of six dots arranged in a matrix. These dots can be raised or flat, and individuals who are blind can touch and feel the patterns of raised dots to recognize the letters.

Textbook Solutions

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

Answer: If a person is inside a room where there is no light, it is then impossible to see the object inside the room, but the object outside the room can be seen easily.

When light falls on the eyes after reflecting from the object, it becomes visible. If the room is dark, then the object which is in the room reflects no light. Hence, the person is not able to see the objects in the room where there is no light.

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

Answer:

Sl.No	Regular Reflection	Diffused Reflection
1.	It occurs when the surface is smooth.	It occurs when the surface is rough.
2.	Reflected rays move in a particular direction.	Reflected rays scatter in random directions.
	Example: Reflection by the plane mirror	Example: Reflection by the road surface.

The laws of reflection have not failed because each ray obeys the law of reflection. All the reflected rays are parallel to each other, whereas in diffused reflection, the rays aren’t parallel to the incident rays.

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.
   1. Polished wooden table
   2. Chalk powder
   3. Cardboard surface
   4. Marble floor with water spread over it
   5. Mirror
   6. Piece of paper

Answer:

a) The wooden table that has a polished surface – Regular .The surface that has been recently polished can be a good example of a smooth surface. The polished wooden table has a smooth surface.

b) White chalk powder that is used in school – Diffused reflection. Chalk powder spread on a surface is an example of an irregular surface because it is rough. Therefore, the diffused reflection will appear from chalk powder.

c) Cardboard surface – Diffused reflection the surface of the cardboard is a kind of irregular surface. Hence, the diffused reflection will take place from a cardboard surface.

d) Marble floor – Regular reflection. A marble floor can be a good example of a surface that is regular. Since water makes the ceramic glossy, the reflections that are regular occur on this surface.

e) Mirror – Regular reflection. A mirror has a very smooth surface; hence it gives a regular reflection.

f) Piece of paper – Diffused reflection. Although a piece of paper may look smooth, it has many irregularities on its surface. Due to this reason, it will give a diffused reflection.

4. State the laws of reflection.

Answer:

The law of reflection states that:

1. a) The angle of reflection and the angle of incidence are always equal to one another.
2. b) The reflected ray, the incident ray, and the normal to the reflective surface at the point of incidence all come on the same plane.

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: On a table, place a plane mirror perpendicular to the plane of the table. Make a small hole in a piece of paper and hold it perpendicular to the plane of the table. Try to do this experiment in a dark room. Take one more piece of paper and place it on the table so that it makes contact with the mirror. Draw a line perpendicular to the mirror on the piece of paper which is on the table. Now, beam light rays with the help of a torch through the small hole, such that the beam of light hits the normal at the bottom of the mirror. The ray of light will be reflected in the light rays from the hole incident on the mirror. Looking at the piece of paper on the table, we can easily show that the incident ray, the normal line and the reflected ray at the point of incidence lie in the same plane.

 Fill in the blanks in the following.

1. A person 1 m in front of a plane mirror seems to be _______________ m away from his image.
2. If you touch your ____________ ear with your right hand in front of a plane mirror, it will be seen in the mirror that your right ear is touched with your ____________.
3. The size of the pupil becomes ____________ when you see in dim light.
4. Night birds have ____________ cones than rods in their eyes.

Answer:

1. A person 1 m in front of a plane mirror seems to be 2 maway from his image.
2. If you touch your leftear with your right hand in front of a plane mirror, it will be seen in the mirror that your right ear is touched with your left hand.
3. The size of the pupil becomes largewhen you see in dim light.
4. Night birds have fewercones than rods in their eyes.

Choose the correct option in Questions 7 – 8

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

1. Always
2. Sometimes
3. Under special conditions
4. Never

Answer: The answer is (a) Always

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: The answer is (b) virtual, behind the mirror and of the same size as the object.

9. Describe the construction of a kaleidoscope.

Answer: The construction of a kaleidoscope is detailed below:

Take three rectangular mirror strips of dimensions 15 cm x 4 cm (l x b) and join them together to form a prism. A prism is fixed into a circular cardboard tube. The circular cardboard tube should be slightly longer than the prism. This circular tube is now closed at one end with a cardboard disc. This disc has a hole through which we can see. At the other end of the circular tube, a plane glass plate is fixed. It is important that this glass plate touches the prism mirrors. On this glass plate, several small and broken pieces of coloured glass are placed. This end is now closed by a round glass plate, allowing enough space for the coloured glass pieces to move.

10. Draw a 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 advice?

Answer: Gurmit’s teacher advised her not to do so because the intensity of the laser light is very high, and it is harmful to the human eyes. It can cause damage to the retina and leads to blindness. Hence, it is advisable not to look at a laser beam directly.

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

Answer: The following points help a person to take care of his eyes:

1. Reading should not be done in bright light as well as in dim light.
2. We should visit an eye specialist at regular intervals of time.
3. If any small insects or dust particles enter our eyes, do not rub them but clean them immediately with cold water.
4. We should avoid direct exposure of our eyes to sunlight.
5. While reading, there should be a distance of at least 25 cm between the eyes and the book.

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: If the reflected ray is at the angle of 90^o to the incident ray, then the angle of incidence is 45^o. According to the law of reflection, the angle of incidence and the angle of reflection are equal. Therefore, both the angle of incidence and the angle of reflection are 90/2 = 45^o.

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

Answer: If a candle is placed between two parallel plane mirrors separated by 40 cm, then multiple and infinite images will be formed due to the multiple reflections between the mirrors. Infinite numbers of images are formed when two mirrors are placed parallel to each other.

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 first law of reflection is used to obtain the path of reflected light.

It can be observed that the given ray of light will reflect from the second mirror at an angle of 60°.

• The incident ray OA reflects at point O. Since Angle of Reflection = Angle of Incidence
  OO’ makes an angle of 30⁰with the normal of the first mirror. Now, drawing normal at O’
• The two normals intersect at 90⁰angle and applying angle sum property in ? OXO’ We get the angle of Incidence in 2^nd mirror = 60⁰
• Applying the law of reflection in 2^ndmirror, We get the below figure.

1,268

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: Boojho cannot see his image because the reflected ray won’t reach his eyes. He can see the image of objects situated at P and Q because the rays coming from P and Q get reflected by the mirror and reach his eyes. Boojho can’t see the image of objects situated at R because the ray from object R does not get reflected.

17. (a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.21). Can Paheli at B see this image? Can Boojho at C see this image? When Paheli moves from B to C, where does the image of A move?

Answer:

1. Image of the object placed at A is formed behind the mirror. The distance of the image from the mirror is equal to the distance of A from the mirror.
2. Yes, Paheli at B can see this image.
3. Yes, Boojho at C can see this image.
4. Image of the object at A will not move. It will remain in the same position when Paheli moves from B to C.