Light

1. Activity 13.1
   Question (as described in the text):
   • You fix a white sheet of paper on a board. Take a comb and cover all its openings except one in the middle. Shine a torch through the open space so that a narrow beam of light falls on the sheet of paper. Place a strip of plane mirror in the path of the light ray.
   • “What do you observe after the ray of light strikes the mirror?”
   • Then measure the angle of incidence (∠i) and angle of reflection (∠r). “Do you see any relation between ∠i and ∠r? Are they equal?”

   Answer:

   • Observation: After the light ray hits the mirror, it bounces back in another direction.
   • Relation between angles: When we carefully measure the angle of incidence and the angle of reflection, we find that they are always equal (∠i = ∠r).
2. Activity 13.2
   Question (as described in the text):
   • Repeat Activity 13.1 using a stiff sheet that projects beyond the table. Let the reflected ray fall on the part of the sheet that can be bent. Bend that portion of the sheet and see if the reflected ray is still visible. “Can you see the reflected ray after bending the paper? What do you infer?”

   Answer:

   • When you bend the sheet, you change the plane where the incident ray and normal lie. You will not see the reflected ray in the bent part.
   • Inference: The incident ray, the normal, and the reflected ray all lie in the same plane. If you bend the sheet, you create a new plane and the reflected ray is no longer in that plane.
3. Activity 13.3
   Question (as described in the text):
   • A point source of light O is placed in front of a plane mirror PQ. Two rays OA and OC fall on the mirror at points A and C. Draw normals at A and C, then draw the reflected rays AB and CD. Extend them backward. Mark the point I where they appear to meet behind the mirror. “Do the reflected rays actually meet behind the mirror? What kind of image is formed?”

   Answer:

   • The reflected rays do not really meet behind the mirror. They only appear to come from the point I behind the mirror.
   • Such an image is called a virtual image, because it cannot be captured on a screen.
4. Activity 13.4
   Question (as described in the text):
   • Imagine several parallel rays of light falling on an irregular surface. Construct (draw) the reflected rays at different points of that rough surface. “Are these reflected rays parallel? What kind of reflection is it?”

   Answer:

   • The reflected rays go in many different directions and are not parallel.
   • This type of reflection is called diffused (irregular) reflection. It happens because the surface is rough, not because the law of reflection fails.
5. Activity 13.5
   Question (as described in the text):
   • Take two plane mirrors hinged at different angles (for example 90°, 60°, 45°, 120°, and finally parallel). Place a small object or candle between them. “How many images do you see for each angle? What happens when the mirrors are parallel to each other?”

   Answer:

   • When the two mirrors are at different angles, the number of images changes. For example, at 90°, you see 3 images. At 60°, you see 5 images, and so on.
   • When the two mirrors are placed parallel to each other, an infinite number of images are formed because the reflections keep bouncing between the two mirrors.
6. Activity 13.6 (Kaleidoscope)
   Question (as described in the text):
   • You make a kaleidoscope using 3 rectangular mirror strips forming a prism inside a cardboard tube. Put colored glass pieces at one end (between a plane glass plate and a ground glass plate). Look through the small hole on the other end. “What do you observe?”

   Answer:

   • You see beautiful, colorful patterns that keep changing each time the glass pieces move. Because of multiple reflections inside the mirrors, you never see the same pattern again. Kaleidoscopes are used by designers and artists for new pattern ideas.
7. Activity 13.7
   Question (as described in the text):
   • Place a mirror in a bowl of water and keep it in sunlight so that the mirror reflects the sunlight onto a white wall. “What do you observe in the reflected light on the wall?”

   Answer:

   • The reflected light spreads into different colors. This shows that sunlight consists of many colors (the seven colors of the rainbow). This spreading of white light into its different colors is called dispersion of light.
8. Activity 13.8
   Question (as described in the text):
   • Look at your friend’s eye in normal light. Then shine a torch gently and see the pupil (the black circular opening in the iris). “Is the pupil bigger in dim light or bright light? In which case do you need more light to enter the eye?”

   Answer:

   • The pupil is larger in dim light because the eye needs to allow more light to enter.
   • The pupil becomes smaller in bright light to reduce how much light enters.
9. Activity 13.9 (Blind Spot)
   Question (as described in the text):
   • Make a cross and a round dot on a paper. Close one eye and keep looking at the cross while slowly bringing the paper closer. “Does the round dot disappear at a certain point? What does it show?”

   Answer:

   • Yes, the round dot disappears when it falls on the blind spot in the eye (the spot on the retina where the optic nerve leaves the eye and there are no light-sensitive cells).
10. Activity 13.10 (Persistence of Vision)
    Question (as described in the text):
    • Draw a bird on one side of a cardboard disc and a cage on the other side. Twist the disc quickly using a string. “What do you see?”

    Answer:

    • You see the bird inside the cage because of the persistence of vision. The image stays in our eyes for a short time (about 1/16 of a second), so the two pictures (bird and cage) seem to merge into one.

B) EXERCISE QUESTIONS

1) Question: “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 completely dark room, there is no light. Our eyes need light reflected from objects to see them. So inside the dark room, we cannot see the objects.
• However, if there is light outside the room, we can see objects outside through a window or doorway because light is present there and reflects from those objects into our eyes.

2) Question: “Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?”
Answer:

• Regular Reflection: Occurs on a smooth, shiny surface (like a mirror). All parallel incident rays remain parallel after reflection. This produces a clear image.
• Diffused (Irregular) Reflection: Happens on a rough surface (like cardboard). Parallel incident rays get reflected in different directions. This does not produce a clear image.
• No, diffused reflection does not mean the laws of reflection have failed. Even on a rough surface, each light ray still follows the law “angle of incidence = angle of reflection.” The scattered reflection occurs only because the surface is uneven.

3) Question: “Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer.”
(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 (if very well polished) or it could be partially diffused if not perfectly smooth. Usually, if it is highly polished, it is closer to regular reflection.
• (b) Chalk powder: Diffused reflection, because chalk powder is rough and not shiny.
• (c) Cardboard surface: Diffused reflection, because cardboard is not smooth.
• (d) Marble floor with water spread over it: This becomes smoother and acts more like a regular reflecting surface if the water forms a smooth layer. So, it shows more regular reflection if the layer of water is smooth and still.
• (e) Mirror: Regular reflection, because it has a very smooth, shiny surface.
• (f) Piece of paper: Diffused reflection. Even though paper may look smooth, its surface has many tiny irregularities causing light to scatter.

4) Question: “State the laws of reflection.”
Answer:
The two laws of reflection are:

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

5) Question: “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:

• Activity: Place a mirror on a table and shine a beam of light on it, marking the incident ray and reflected ray on a piece of paper. Draw a line (the normal) perpendicular to the mirror at the point of incidence.
• Now gently lift or fold part of the paper upward. You will notice that the reflected ray is no longer on that folded part. This shows that the incident ray, the reflected ray, and the normal lie on the same flat surface (plane).

6) Question: “Fill in the blanks:
(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 the 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) 2 m (because the image is 1 m behind the mirror, so total distance is 2 m).
(b) Left ear, seen as left (mirror swaps left and right sides). (When you use your right hand to touch your left ear, the mirror shows you touching the “left ear” with the “left hand,” due to lateral inversion.)
(c) Larger in dim light.
(d) Fewer cones than rods (they have more rods for seeing in dim light).

7) Question (Multiple Choice): “Angle of incidence is equal to angle of reflection:
(a) Always
(b) Sometimes
(c) Under special conditions
(d) Never”

Answer:

• (a) Always

8) Question (Multiple Choice): “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.

9) Question: “Describe the construction of a kaleidoscope.”
Answer:

• Take three rectangular mirror strips of the same size (about 15 cm long and 4 cm wide).
• Join them to form a triangular tube (like a long prism) using adhesive tape.
• Place this arrangement inside a cylindrical cardboard tube slightly longer than the mirror strips.
• At one end of the tube, fix a plane glass plate and place small, colorful pieces (like broken bangle pieces) on it.
• Close that end with a translucent ground glass plate, leaving a little space for the colored pieces to move.
• At the other end of the tube, fix a circular disc (cardboard) with a small hole in the center to see through.
• When you look through the hole, multiple reflections of the colored pieces create beautiful patterns.

10) Question: “Draw a labeled sketch of the human eye.”
Answer (in words):
A simple labeled diagram would show the following main parts:

• Cornea: The clear front part.
• Iris: The colored part of the eye.
• Pupil: The opening in the iris that lets light in.
• Lens: A transparent structure behind the pupil that focuses light on the retina.
• Retina: The back inner surface of the eye where the image is formed.
• Optic Nerve: The nerve that carries the signals from the retina to the brain.

(You can mention or draw it on paper with these labels.)

11) Question: “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 advice?”
Answer:

• A laser torch produces a very intense and narrow beam of light that can be harmful to the eyes. It can damage the retina if shone directly into someone’s eyes. That is why the teacher advised not to use a laser torch.

12) Question: “Explain how you can take care of your eyes.”
Answer:

• Have regular check-ups with an eye doctor.
• Do not read in dim light or in very bright light.
• Do not stare at very bright sources like the Sun or laser light.
• Keep a proper distance when watching TV or reading books (about 25 cm from a book).
• Wash your eyes with clean water if there is irritation.
• Eat food rich in Vitamin A (carrots, green vegetables, milk, fruits) to keep eyes healthy.

13) Question: “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 90° from the incident ray, then the sum of angle of incidence (i) and angle of reflection (r) is 90°. But we know i = r.
  So i + r = 90° ⇒ i + i = 90° ⇒ 2i = 90° ⇒ i = 45°.
• Angle of incidence = 45°.

14) Question: “How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?”
Answer:

• For two parallel plane mirrors, the number of images formed is infinite (the reflections go back and forth forever).

15) Question: “Two mirrors meet at right angles (90°). A ray of light is incident on one at an angle of 30° (from the normal) as shown in the figure. Draw the reflected ray from the second mirror.”
Answer (in words):

• When a ray hits the first mirror at 30° from its normal, it reflects out at 30° on the other side of that normal.
• This reflected ray then meets the second mirror (which is at 90° to the first).
• The angle of incidence on the second mirror will be 60° from that mirror’s normal (because the mirrors are at 90°, so the directions add up). Hence, the reflected ray from the second mirror will come off at 60° from that normal.
• In a simple drawing, you would see the final reflected ray leaving at an angle that makes it symmetrical to the path it had when it arrived at the second mirror.

16) Question: “Boojho stands at A just on the side of a plane mirror as shown in Fig. 13.20. Can he see himself in the mirror? Also, can he see the image of objects situated at P, Q, and R?”
Answer (general explanation):

• If Boojho is completely to the side of the mirror (where no light from him can reflect back to his eyes), he cannot see his own image in that mirror.
• Whether he can see the images of P, Q, and R depends on whether the light rays from those objects can reflect off the mirror and reach his eyes.
• If P, Q, or R are in front of the mirror, and the angles are such that light reflects from them to the mirror and then to Boojho’s eyes, he will see them. Otherwise, he will not.

17) Question: “(a) Find the position of the image of an object A in the plane mirror (Fig. 13.21). (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 (general explanation):

• (a) The image of A will be behind the mirror, at the same distance from the mirror as A is in front.
• (b) Whether Paheli at B can see the image depends on whether the reflected rays from A’s image can reach B. If she is positioned so that the mirror can reflect the light from A to her eyes, then yes, she can see it. Otherwise, no.
• (c) The same logic applies to Boojho at C. If the mirror can reflect the image’s rays to him, he will see it.
• (d) The image behind the mirror does not really move when Paheli moves. The image remains in the same place behind the mirror. But from Paheli’s new position (at C), her viewing angle changes. To her, the image might appear to shift, but it is actually still the same distance behind the mirror.

• You usually see your image more clearly when there is a black paper behind the glass. The black background cuts down extra light passing through the glass and improves the reflection from the back surface, making your image clearer.

• Visually impaired students may use Braille to read and write. They often recognize objects by touch, hearing, or smell. For currency notes, they may feel the texture, size, or any special marks. They rely on sharper senses of touch and hearing to navigate.

• An eye specialist may check if you need spectacles or have any eye issues.
• To take care of your eyes, the doctor might advise:
  • Read in good light,
  • Avoid looking at very bright lights (like the Sun),
  • Maintain a healthy diet with Vitamin A,
  • And keep safe distance from screens.

• Possible reasons may include:
  • Reading in dim light,
  • Excessive use of mobile phones/computers,
  • Not having enough eye-friendly nutrients (like Vitamin A),
  • Or genetic (inherited) factors.