Reflection of Light: Questions and Answers

Question 1: Vision in Darkness and Light

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

A: In a dark room, you cannot see objects because there is no light to reflect off the surfaces and into your eyes. However, if there is light outside the room, you can see objects outside because light from those objects enters your eyes, making them visible.

Question 2: Regular vs. Diffused Reflection

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

A: Regular reflection occurs on smooth surfaces where parallel incident rays reflect in parallel directions, producing a clear image. Diffused reflection happens on rough surfaces where incident rays scatter in different directions, causing no clear image. Diffused reflection does not mean the failure of the laws of reflection; the laws still apply but the reflected rays scatter.

Question 3: Types of Reflection on Different Surfaces

Q: 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

Polished wooden table: Regular reflection, because the surface is smooth.
Chalk powder: Diffused reflection, because the surface is rough.
Cardboard surface: Diffused reflection, due to its uneven surface.
Marble floor with water spread over it: Regular reflection, as the water creates a smooth surface.
Mirror: Regular reflection, because of its smooth, polished surface.
Piece of paper: Diffused reflection, because the surface is uneven at a microscopic level.

Question 4: Laws of Reflection

Q: State the laws of reflection.

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

Question 5: Activity Demonstrating the Laws of Reflection

Q: 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.

A: To demonstrate this, place a plane mirror vertically on a table and shine a laser beam at it. Place a sheet of paper at the base, perpendicular to the mirror. The beam of light (incident ray) will reflect off the mirror, and you can trace its path on the paper to show that the incident ray, reflected ray, and normal all lie in the same plane.

Question 6: Fill in the Blanks

Q: Complete the following sentences.

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

2 meters
left, left hand
larger
fewer

Question 7: Angle of Incidence and Reflection

Q: Angle of incidence is equal to the angle of reflection.

Always
Sometimes
Under special conditions
Never

A: Always

Question 8: Image Formed by a Plane Mirror

Q: The image formed by a plane mirror is:

virtual, behind the mirror, and enlarged.
virtual, behind the mirror, and of the same size as the object.
real at the surface of the mirror and enlarged.
real, behind the mirror, and of the same size as the object.

A: virtual, behind the mirror, and of the same size as the object.

Question 9: Construction of a Kaleidoscope

Q: Describe the construction of a kaleidoscope.

A: A kaleidoscope is constructed using three rectangular mirrors placed at an angle to each other inside a cylindrical tube. One end of the tube is closed with a translucent plate, while the other end is open to view through. Colored beads or glass pieces are placed in a chamber near the translucent end, and as you rotate the tube, the reflections create symmetrical patterns.

Question 10: Sketch of the Human Eye

Q: Draw a labeled sketch of the human eye.

The human eye is a complex organ with several key parts, each playing a crucial role in the process of vision. Here are the main parts of the human eye along with their functions:

Cornea: The transparent, dome-shaped surface covering the front of the eye. It helps to focus light entering the eye.
Pupil: The dark circular opening in the center of the iris that controls the amount of light entering the eye.
Iris: The colored part of the eye surrounding the pupil. It adjusts the size of the pupil to control the amount of light that enters.
Lens: Located behind the iris, the lens focuses light onto the retina. It changes shape to adjust focus, allowing us to see objects at different distances (accommodation).
Retina: A layer of tissue at the back of the eye that contains photoreceptor cells (rods and cones). It detects light and converts it into electrical impulses which are sent to the brain via the optic nerve.
Optic Nerve: Transmits electrical impulses from the retina to the brain, where visual information is processed and interpreted.
Macula: A small area in the retina responsible for central vision and detailed sight. It contains a high concentration of cones for color vision and visual acuity.
Fovea: Located at the center of the macula, the fovea is responsible for sharp central vision. It contains a high density of cones and is crucial for activities like reading and driving.
Sclera: The tough, white outer layer of the eye that helps maintain the shape of the eye and protects the delicate inner structures.
Choroid: A layer of blood vessels between the retina and the sclera that provides oxygen and nutrients to the retina.
Ciliary Body: Produces aqueous humor (fluid) and contains muscles that control the shape of the lens for near and far vision (accommodation).
Aqueous Humor: A clear fluid that helps maintain the shape of the eye and provides nutrients to the lens and cornea.

Detailed Answers to Questions 11-17

Question 11: Safety Concerns with Laser Torch

Q: 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?

A: The teacher advised against using a laser torch because laser light can be hazardous. Lasers emit concentrated light that can cause severe damage to the eyes, potentially leading to blindness if the laser beam is directed into the eyes. The intense, focused light can also cause burns or start fires. Therefore, using lasers requires strict safety precautions to avoid accidental exposure or injury.

Question 12: Eye Care

Q: Explain how you can take care of your eyes.

A: To take care of your eyes:

Maintain Proper Lighting: Ensure adequate lighting when reading or working to avoid straining your eyes.
Follow the 20-20-20 Rule: Every 20 minutes, look at something 20 feet away for at least 20 seconds to reduce eye strain.
Avoid Excessive Screen Time: Limit the time spent on screens and take regular breaks to reduce eye fatigue.
Wear Protective Eyewear: Use sunglasses to protect your eyes from UV rays and safety glasses when working with hazardous materials.
Maintain Hygiene: Avoid touching your eyes with dirty hands and keep contact lenses clean to prevent infections.
Get Regular Check-ups: Visit an eye specialist regularly for comprehensive eye exams to detect and address issues early.
Eat a Balanced Diet: Consume foods rich in vitamins A, C, and E, as well as omega-3 fatty acids, which are beneficial for eye health.

Question 13: Angle of Incidence and Reflected Ray

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

A: If the reflected ray is at an angle of 90° to the incident ray, then the angle of incidence is 45°. According to the law of reflection, the angle of incidence (i) equals the angle of reflection (r). Since the incident and reflected rays form a right angle (90°), the angles are split equally:
[ i + r = 90 degrees ]
[ i = r ]
[ i + i = 90 degrees ]
[ 2i = 90 degrees ]
[ i = 45 degrees ]

Question 14: Images Formed by Parallel Mirrors

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

A: When a candle is placed between two parallel plane mirrors, an infinite number of images are formed. This occurs because each mirror reflects the image created by the other mirror repeatedly, creating a series of images that appear to recede into the distance. The images diminish in brightness and clarity with each reflection, but the process theoretically continues infinitely.

Question 15: Reflected Ray in Right-Angle Mirrors

Q: 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.

A: When a ray of light strikes the first mirror at a 30° angle, it reflects off the mirror at the same angle, making a 60° angle with the surface of the second mirror. The reflected ray then strikes the second mirror at an angle of 60° (relative to the surface), and it reflects at the same angle:

Incident angle on the second mirror: (60 degrees) (relative to the surface).
Reflection angle from the second mirror: (60 degrees ) (relative to the surface).

Diagram:

Question 16: Visibility in Plane Mirror

Q: 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?

Seeing Himself: Boojho cannot see himself in the mirror because he is positioned at an extreme angle to the mirror’s surface, outside the line of sight needed for his reflection to be visible.
Seeing Objects at P, Q, and R: Boojho can see the images of objects located at P, Q, and R in the mirror. The reflections of these objects will be visible in the mirror if their reflected light reaches the mirror and then to Boojho’s eyes.

Diagram Reference:

Objects at points P, Q, and R will have their images formed at positions that Boojho can view depending on their orientation and position relative to the mirror.

Question 17: Position of Image in Plane Mirror

Q: (a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.21).
Q: (b) Can Paheli at B see this image?
Q: (c) Can Boojho at C see this image?

A:
(a) The image of an object placed at point A will be located directly opposite to point A on the other side of the mirror at the same distance as the object is from the mirror. If the distance from point A to the mirror is (d), then the image will be located at a distance (d) behind the mirror along the normal line extending from point A.

(b) Paheli at position B can see the image if she is within the angle of reflection from the point A image. This depends on her line of sight and the angle at which she views the mirror. If the mirror’s reflective surface allows her to see the image formed at point A, she will be able to see it.

(c) Boojho at position C can also see the image if he is positioned within the reflective view from the point A image. Similar to Paheli, this is contingent on whether his line of sight intersects with the reflected image of point A in the mirror. If the geometry of their positions aligns, both can see the image.