Solution:

Can You See Objects in a Dark Room?

In a dark room, you cannot see objects clearly because light is necessary for vision. Our eyes can only detect objects when light from those objects enters the eye. In the absence of light, there is no way for the image of the object to form on the retina, so it becomes impossible to see anything.

Can You See Objects Outside the Room?

If there is light outside the room, you might be able to see objects outside. However, the amount of light entering the room will depend on factors such as the size of the opening (e.g., a window or door) and the brightness of the surroundings. If the light from outside is not enough to reach your eyes, you may still not be able to see objects outside clearly.

Solution:

Difference Between Regular and Diffused Reflection

Regular Reflection:

• Occurs when light reflects from a smooth surface, such as a mirror or a still water surface.
• The reflected rays are parallel to each other, and the image formed is clear and sharp.
• Follows the laws of reflection, i.e., the angle of incidence is equal to the angle of reflection.

Diffused Reflection:

• Occurs when light reflects from a rough or uneven surface, such as paper or wall.
• The reflected rays scatter in many different directions, and the image formed is not clear or sharp.
• The laws of reflection still apply locally at each point on the surface, but the overall scattering prevents the formation of a distinct image.

Does Diffused Reflection Mean the Failure of the Laws of Reflection?

No, diffused reflection does not mean the failure of the laws of reflection. The laws of reflection are still followed at each point where light strikes the surface, but because the surface is rough, the reflected rays scatter in different directions, resulting in a diffused light pattern.

Solution:

(a) Polished wooden table

Regular reflection will take place because the surface of the polished wooden table is smooth, allowing the light rays to reflect in a single direction.

(b) Chalk powder

Diffused reflection will take place because the surface of the chalk powder is rough, scattering the light rays in multiple directions.

(c) Cardboard surface

Diffused reflection will take place because the surface of the cardboard is uneven, leading to scattered reflection of light rays.

(d) Marble floor with water

Regular reflection will take place on the water surface because water is smooth, reflecting light in a single direction. The marble surface may cause diffused reflection depending on its texture.

(e) Mirror

Regular reflection will take place because the surface of the mirror is smooth and shiny, reflecting light rays in a specific direction.

(f) Piece of paper

Diffused reflection will take place because the surface of the paper is rough, causing light rays to scatter in multiple directions.

Solution:

Laws of Reflection

The laws of reflection are as follows:

1. The angle of incidence is equal to the angle of reflection.
2. The incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane.

Solution:

Activity to Show That the Incident Ray, the Reflected Ray, and the Normal at the Point of Incidence Lie in the Same Plane

Materials required:

• A plane mirror
• A ray box or a laser pointer
• A protractor
• A piece of white paper
• A ruler

Procedure:

1. Place the plane mirror vertically on the white paper.
2. Switch on the ray box or laser pointer and direct the beam of light towards the surface of the mirror at an angle.
3. Mark the point where the incident ray strikes the mirror. This point is called the point of incidence.
4. Measure the angle of incidence ($i$) using the protractor, which is the angle between the incident ray and the normal (a line perpendicular to the mirror at the point of incidence).
5. Observe the reflected ray that bounces off the mirror. Mark the direction of the reflected ray.
6. Measure the angle of reflection ($r$) using the protractor, which is the angle between the reflected ray and the normal.
7. Now, use the ruler to draw a line connecting the point of incidence, the incident ray, the reflected ray, and the normal. This will show that the incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.

Observation:

You will observe that the incident ray, the normal, and the reflected ray all lie in the same plane, which confirms the law of reflection.

Solution:

Fill in the blanks in the following:

(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.

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

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

Solution:

Angle of incidence is equal to the angle of reflection.

(a) Always CORRECT

(b) Sometimes

(c) Under special conditions

(d) Never

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. CORRECT

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

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

Solution:

Construction of a Kaleidoscope

A kaleidoscope is an optical instrument that produces beautiful patterns by reflecting light. It consists of the following components:

• Tube: The outer body of the kaleidoscope is a long, hollow tube, usually made of cardboard or plastic. This tube holds all the other components together.
• Mirrors: Three rectangular or triangular mirrors are arranged in a tube in such a way that they form a prism. The mirrors are placed at an angle of 60$^\circ$ to each other. The reflections in these mirrors create the colorful patterns seen in the kaleidoscope.
• Glass pieces or beads: Small colored glass pieces, beads, or other reflective objects are placed at the open end of the kaleidoscope. These objects are the ones that form the patterns when reflected in the mirrors.
• Viewing hole: There is a small hole at the other end of the tube, through which the viewer looks. This hole allows the person to see the reflections of the colored glass pieces, creating intricate patterns.
• End cap: The open end of the kaleidoscope is sealed with a cap or transparent material to keep the objects in place. This cap also allows the light to enter and reflect off the glass pieces.

Solution:

Labelled Sketch of the Human Eye

Below is the labelled sketch of the human eye:

         ________
        /        \
       /          \
      |    Iris    |     <--- Iris
      |     &      |     
      |  Pupil     |     <--- Pupil
      |            |      
      |    Lens    |     <--- Lens
      |            |     
       \__________/     
       /          \     <--- Cornea
      /            \
     /              \
    |   Retina       |     <--- Retina
    |                | 
    |    Optic Nerve |     <--- Optic Nerve
    |________________|

Note: The diagram is a basic representation of the human eye. The eye consists of several parts, including the cornea, iris, pupil, lens, retina, and optic nerve. Each part plays a critical role in vision.

Solution:

Explanation of Teacher's Advice Regarding Activity 13.8

Gurmit wanted to perform Activity 13.8 using a laser torch, but her teacher advised her not to do so. The basis of the teacher's advice is primarily safety concerns. Lasers are powerful beams of light that can cause harm if not used with caution. The intensity of the laser light can damage the eyes, especially if it is pointed directly at them. This can lead to permanent eye damage or blindness. Additionally, lasers are usually concentrated beams of light that can pose a hazard to the skin or other sensitive areas if exposed for a prolonged period.

In Activity 13.8, it is important to ensure the safety of the students, and using a laser torch without proper precautions can lead to accidents. Therefore, it is better to use other safer light sources, such as a torch or lamp, for this experiment.

Solution:

How to Take Care of Your Eyes

To maintain good eye health, you should follow these steps:

• Always wash your hands before touching your eyes to prevent infections.
• Rest your eyes regularly when working on screens or reading for long periods. Follow the 20-20-20 rule: take a 20-second break every 20 minutes by looking at something 20 feet away.
• Use proper lighting while reading or working to reduce strain on your eyes.
• Avoid staring at bright lights or looking directly at the sun, as it can damage your eyes.
• Wear sunglasses with UV protection to protect your eyes from harmful ultraviolet (UV) rays.
• Eat a balanced diet rich in vitamins and minerals like Vitamin A, C, and E to support eye health.
• Keep a safe distance from screens and ensure they are at eye level to reduce strain.
• Get regular eye check-ups to detect any vision problems early.
• Avoid rubbing your eyes, as it can cause irritation or introduce dirt and bacteria.
• Wear protective eyewear while playing sports or working with harmful materials.

Solution:

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

According to the laws of reflection, the angle of incidence ($i$) is always equal to the angle of reflection ($r$).

In this case, it is given that the angle between the incident ray and the reflected ray is 90°. This means:

$$ i + r = 90° $$

Since $i = r$, we can substitute and get:

$$ 2i = 90° $$

Solving for $i$:

$$ i = \frac{90°}{2} = 45° $$

Thus, the angle of incidence is 45°.

Solution:

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

When an object is placed between two parallel plane mirrors, multiple images are formed due to the repeated reflection between the mirrors. The number of images formed can be calculated using the formula:

Number of images = $2 \times \left( \frac{d}{l} \right) - 1$

Where:

• $d$ = Distance between the two mirrors (40 cm in this case)
• $l$ = Distance of the object from each mirror

In the case where the candle is placed between the mirrors, the number of images formed depends on the relative position of the candle. If the distance is such that the candle is not exactly at the midpoint, the images will continue to form as long as the reflections occur within the angle limits of the mirrors.

Solution:

Reflection of Light by Two Mirrors Meeting at Right Angles

When a ray of light strikes the first mirror at an angle of incidence of 30°, it gets reflected according to the laws of reflection. The angle of reflection will also be 30° with respect to the normal to the mirror's surface.

After the light ray is reflected from the first mirror, it strikes the second mirror, which meets the first mirror at a right angle (90°). The incident ray on the second mirror will form an angle of 60° (since the angle between the two mirrors is 90° and the angle of incidence on the first mirror is 30°). The light will then reflect off the second mirror, following the law of reflection. The angle of reflection on the second mirror will also be 60°.

The reflected ray from the second mirror will be at an angle of 60° with respect to the second mirror's normal.

Solution:

Can Boojho see himself in the mirror?

Boojho stands at point A, which is just on the side of a plane mirror. In a plane mirror, the image formed is always behind the mirror and is laterally inverted. For Boojho to see himself in the mirror, his eyes must be in the line of sight to the image formed by the mirror. Since he is standing just on the side of the mirror, he is not directly in front of it, so he cannot see himself in the mirror.

Can Boojho see the image of objects situated at P, Q, and R?

The ability to see the images of objects in the mirror depends on whether the line of sight from Boojho's eyes to the mirror can reflect the image formed. If the objects P, Q, and R are located in such a way that their images are reflected in the mirror and Boojho's line of sight intersects with these reflected images, then Boojho will be able to see their images. However, if the objects are outside the range of reflection, Boojho will not be able to see them.

Solution:

(a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 13.21).

The image of the object situated at point A in the plane mirror will be formed at a point behind the mirror, at the same distance as the object is in front of the mirror. The image will be virtual, upright, and of the same size as the object.

(b) Can Paheli at B see this image?

Yes, Paheli at point B can see the image formed in the plane mirror. The image appears to be behind the mirror, and Paheli's line of sight would intersect with the virtual image formed.

(c) Can Boojho at C see this image?

Yes, Boojho at point C can also see the image. The image is formed in the plane mirror and Boojho can view it by looking in the mirror from point C.

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

When Paheli moves from point B to C, the image of object A will appear to move correspondingly in the plane mirror. The image will always remain at the same distance behind the mirror as the object is in front of the mirror, so as Paheli moves, the perceived position of the image will also change, but it will always remain at the same relative distance and orientation.