Question 1

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

Accepted Answer

In a dark room, you cannot see objects inside the room because there is no light to reflect from those objects to your eyes. However, if there is light outside the room and you look through a window or door, you may see objects outside because light from those objects can enter the room and reach your eyes. — Seeing objects requires light to reflect from the objects to your eyes. In darkness, no light is available inside the room, so objects are not visible. Outside the room, if there is light, objects can be seen as their reflected light reaches your eyes.

Question 2

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

Accepted Answer

Regular reflection occurs when light rays fall on a smooth and polished surface and reflect in a definite direction, producing a clear image. Diffused reflection occurs when light rays fall on a rough surface and reflect in many directions, so no clear image is formed.

No, diffused reflection does not mean the failure of the laws of reflection. The laws of reflection hold for each individual ray even in diffused reflection; it is just that the surface irregularities cause the reflected rays to scatter in different directions. — Regular reflection: smooth surface, parallel reflected rays, clear image.
Diffused reflection: rough surface, scattered reflected rays, no clear image.
Laws of reflection (angle of incidence = angle of reflection) apply to each ray regardless of surface type.

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

Accepted Answer

(a) Polished wooden table - Regular reflection because the surface is smooth and polished.
(b) Chalk powder - Diffused reflection because the surface is rough and powdery.
(c) Cardboard surface - Diffused reflection due to its rough texture.
(d) Marble floor with water spread over it - Regular reflection because water makes the surface smooth.
(e) Mirror - Regular reflection as it has a very smooth and polished surface.
(f) Piece of paper - Diffused reflection because paper surface is rough. — Smooth and polished surfaces cause regular reflection; rough surfaces cause diffused reflection. Water on marble smoothens the surface, enabling regular reflection.

Question 4

State the laws of reflection.

Accepted Answer

The laws of reflection are:
1. The incident ray, the reflected ray and the normal to the surface at the point of incidence all lie in the same plane.
2. The angle of incidence is equal to the angle of reflection. — These laws describe the behavior of light when it reflects from a surface and are fundamental to understanding 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.

Accepted Answer

Activity:
1. Take a plane mirror and place it on a table.
2. Draw a straight line on a paper placed under the mirror to represent the mirror surface.
3. Use a protractor to draw a normal (a line perpendicular) to the mirror surface at a point.
4. Shine a narrow beam of light (using a ray box or torch) towards the mirror at an angle to the normal.
5. Mark the path of the incident ray and the reflected ray on the paper.
6. Observe that the incident ray, reflected ray, and the normal all lie in the same plane (the plane of the paper).

This activity demonstrates the first law of reflection. — By marking the rays and normal on a flat surface, it is clear that all three lie in the same plane, confirming the law of reflection.

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 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.

Accepted Answer

(a) 2 m
(b) left, left hand
(c) larger
(d) fewer — (a) The image in a plane mirror appears as far behind the mirror as the object is in front, so 1 m in front means image is 1 m behind, total distance = 2 m.
(b) The mirror reverses left and right, so touching left ear with right hand appears as right ear touched with left hand.
(c) In dim light, pupils dilate (become larger) to allow more light in.
(d) Night birds have fewer cones (color receptors) and more rods (light-sensitive cells) to see better in low light.

Question 7

Angle of incidence is equal to the angle of reflection.
(a) Always
(b) Sometimes
(c) Under special conditions
(d) Never

Accepted Answer

The correct answer is (a) Always. According to the law of reflection, the angle of incidence is always equal to the angle of reflection for a light ray reflecting off a surface. — When a ray of light strikes a smooth surface, it reflects such that the angle it makes with the normal (angle of incidence) is equal to the angle it makes after reflection (angle of reflection). This is a fundamental law of reflection and holds true for all smooth reflecting surfaces.

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.

Accepted Answer

The correct answer is (b) virtual, behind the mirror and of the same size as the object. A plane mirror forms an image that is virtual (cannot be projected on a screen), laterally inverted, and of the same size as the object, located behind the mirror at the same distance as the object is in front. — When light rays reflect from a plane mirror, they appear to diverge from a point behind the mirror. This point is where the virtual image is formed. The image is upright, virtual, laterally inverted, and of the same size as the object.

Question 9

Describe the construction of a kaleidoscope.

Accepted Answer

A kaleidoscope is constructed using three long narrow strips of plane mirrors placed inside a tube. These mirrors are arranged such that their reflecting surfaces face each other and are inclined at an angle of 60° to each other. The tube has an object chamber at one end containing small colored beads or pieces of glass. When light enters the tube and reflects multiple times between the mirrors, it creates symmetrical patterns that can be seen through the viewing end. — The three mirrors inside the kaleidoscope reflect the objects multiple times, creating repeated symmetrical patterns. The angle between the mirrors determines the number of repetitions of the pattern. The colored beads or glass pieces act as the object whose image is reflected.

Question 10

Draw a labelled sketch of the human eye.

Accepted Answer

A labelled sketch of the human eye should include the following parts: cornea, iris, pupil, lens, retina, optic nerve, aqueous humor, vitreous humor, and ciliary muscles. The cornea is the transparent front layer; the iris controls the size of the pupil which regulates light entry; the lens focuses light on the retina; the retina contains light-sensitive cells; and the optic nerve transmits signals to the brain. — The human eye functions by allowing light to enter through the cornea and pupil, focusing it via the lens onto the retina. The retina converts light into electrical signals sent to the brain through the optic nerve, enabling vision. Labeling these parts correctly in the sketch demonstrates understanding of eye anatomy.

Question 11

Gurmit wanted to perform Activity 13.8 using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teacher's advise?

Accepted Answer

Laser torches emit highly concentrated and intense beams of light which can damage the eyes if looked at directly. The teacher advised Gurmit not to use a laser torch for Activity 13.8 to prevent any harm to her eyes or others' eyes. It is safer to use a normal torch or other light sources for such activities. — Laser light is coherent and intense, and direct exposure can cause eye injury. Hence, the teacher's advice is based on safety concerns.

Question 12

Explain how you can take care of your eyes.

Accepted Answer

To take care of your eyes, you should:
- Avoid looking directly at the sun or bright lights.
- Maintain proper distance while reading or watching TV.
- Use adequate lighting while reading or working.
- Eat nutritious food rich in vitamin A.
- Take regular breaks while working on computers or reading.
- Keep your eyes clean and avoid rubbing them with dirty hands.
- Get regular eye check-ups.
- Wear protective eyewear when necessary.
- Avoid using laser pointers or intense light sources directly.
These habits help maintain good eyesight and prevent eye strain or damage. — Good eye care habits prevent strain, infections, and damage, ensuring healthy vision.

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?

Accepted Answer

Let the angle of incidence be i.
According to the law of reflection, angle of incidence = angle of reflection = i.
The angle between the incident ray and reflected ray is given as 90°.
The angle between incident ray and reflected ray = i + i = 2i = 90°
Therefore, 2i = 90°
=> i = 90° / 2 = 45°
Hence, the angle of incidence is 45 degrees. — The angle between incident and reflected rays is twice the angle of incidence. Using this relation, we find the angle of incidence.

Question 14

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

Accepted Answer

When two plane mirrors are placed parallel to each other, an infinite number of images are formed due to multiple reflections between the mirrors.
Therefore, the number of images formed is infinite, regardless of the distance between the mirrors.
Hence, infinite images of the candle will be formed. — Parallel plane mirrors produce infinite images because light keeps reflecting back and forth between them.

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. 13.19. Draw the reflected ray from the second mirror.

Accepted Answer

Step 1: The ray strikes the first mirror at an angle of incidence of 30°. According to the law of reflection, the angle of reflection is also 30°.
Step 2: The reflected ray from the first mirror then strikes the second mirror.
Step 3: Since the mirrors are at 90°, the angle of incidence on the second mirror can be determined geometrically.
Step 4: The ray reflects from the second mirror at the same angle as the angle of incidence.
Step 5: The final reflected ray will be drawn accordingly.
Note: The reflected ray from the second mirror will be parallel to the incident ray but in the opposite direction.
Hence, the ray reflects back along a path parallel to the incident ray after two reflections. — Using the law of reflection and the geometry of two mirrors at 90°, the ray reflects twice and reverses direction.

LIGHT

LIGHT — Study Notes

13.1 What makes Things Visible

13.2 Laws of Reflection

Image Formation by a Plane Mirror

Practice Questions — LIGHT

All 13 Chapters in Science