GSEB Class 8 Science Solutions Chapter 16 Light

 

Question 1. suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.
Answer: We can see only those objects from which reflected light rays enter our eyes. When we are in a dark room, we cannot see objects within it. However, we can perceive objects outside the room because light is present there, and its rays enter our eyes after bouncing off those objects.
In simple words: We see things when light bounces off them and goes into our eyes. In a dark room, there's no light to bounce, so we can't see. Outside, there's light, so we can.

Exam Tip: Always remember that light reflection is essential for vision; without incident light, no reflection occurs, and thus no object can be seen.

 

Question 2. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the law of reflection?
Answer:Difference between regular and diffused reflection:

• Regular Reflection:
  • It happens on a smooth and shiny surface.
  • In this case, all rays remain parallel after bouncing off.
• Diffused Reflection:
  • It occurs on a rough surface.
  • Reflected rays move in many different directions.

Diffused reflection does not mean that the laws of reflection fail. It happens because of the unevenness on the reflecting surface.
In simple words: Regular reflection happens on smooth surfaces, making light rays stay parallel. Diffused reflection occurs on rough surfaces, scattering light rays in many directions. Neither type breaks the rules of reflection; diffused reflection is just due to rough surfaces.

Exam Tip: Clearly define the surface type and the behavior of reflected rays for both regular and diffused reflection. Emphasize that the laws of reflection are always followed.

 

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.
Answer:
(a) Regular reflection happens on a polished wooden table because its surface is smooth.
(b) A diffused reflection takes place on chalk powder because its surface is uneven.
(c) The cardboard surface has tiny irregularities on it, so it will show an irregular reflection.
(d) Marble floor with water spread over it acts as a smooth surface for regular reflection because it will act as a plane surface.
(e) A mirror produces regular reflection. Its surface is also shiny and smooth.
(f) At a piece of paper, irregular or diffused reflection will happen because it has an uneven surface.
In simple words: Smooth surfaces like a polished table or mirror cause regular reflection, where light bounces off neatly. Rough or uneven surfaces like chalk powder or paper cause diffused reflection, scattering light. A wet marble floor becomes smooth, so it causes regular reflection.

Exam Tip: When justifying, always link the type of reflection (regular/diffused) directly to the nature of the surface (smooth/rough/uneven).

 

Question 4. State the laws of reflection.
Answer: There are two laws of reflection:

• The angle of incidence is always equal to the angle of reflection, meaning \( \angle i = \angle r \).
• The incident ray, the normal at the point of incidence, and the reflected ray, all exist in the same plane.

In simple words: The first rule says the angle light hits a surface is the same as the angle it bounces off. The second rule says the incoming light, the outgoing light, and the imaginary line perpendicular to the surface at the point of contact all lie on one flat plane.

Exam Tip: State both laws clearly, including the mathematical representation for the first law. Ensure the definitions of incident ray, reflected ray, normal, and plane are implicitly understood.

 

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.
Answer: For this experiment, take a plane mirror and position it on a flat sheet of paper with a block. Now, draw an incidence line AB. Look into the mirror and mark the points on the paper where you see the light ray traveling after it gets reflected from the mirror. Then, remove the mirror and draw a perpendicular line on the mirror line. Join the marked points to create the reflected ray. You will observe that the incident ray, the reflected ray, and the normal all exist in the same plane, which is the sheet of paper.

In simple words: To show that all rays are in one plane, use a mirror on paper. Draw where light hits and where it bounces off. Add a perpendicular line. You'll see that all these lines – the incoming light, the outgoing light, and the perpendicular line – are all flat on the paper.

Exam Tip: When describing an activity, clearly list the materials, procedure, and observation. A simple diagram can enhance understanding, even if not explicitly requested.

 

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.
Answer:
(a) 2
(b) left, left hand
(c) large
(d) lesser
In simple words: For a plane mirror, your image is twice as far as you are from it. If you touch your right ear with your right hand, the mirror makes it look like you're touching your left ear with your left hand. In dim light, your pupil gets bigger. Night birds have fewer cones than rods, meaning they see better in low light but not as much color.

Exam Tip: Understand the properties of images formed by plane mirrors, the function of the pupil, and the roles of rods and cones in vision to accurately answer fill-in-the-blank questions.

 

Question 7. The angle of incidence is equal to the angle of reflection.
(a) Always
(b) Sometimes
(c) Under special conditions
(d) Never
Answer: (a) Always
In simple words: The angle at which light hits a surface is always the same as the angle at which it bounces off. This is a basic rule of reflection that always holds true.

Exam Tip: This is a fundamental law of reflection that applies universally, not just under specific conditions. Remember "always" for this concept.

 

Question 8. The 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.
In simple words: A plane mirror creates an image that appears to be behind the mirror, is the same size as the actual object, and cannot be projected onto a screen (it's virtual).

Exam Tip: Memorize the key characteristics of images formed by a plane mirror: virtual, erect, same size, same distance behind as object in front, and laterally inverted.

 

Question 9. Describe the construction of a kaleidoscope.
Answer: To construct a Kaleidoscope, take three rectangular mirror strips, each about 15 cm long and 4 cm wide. Join them together to form a prism. Secure this arrangement of mirrors in a circular cardboard tube, making sure the tube is slightly longer than the mirror strips. Next, close one end of the tube by using a cardboard disc that has a small hole in its center. To make the disc more durable, attach a piece of transparent plastic sheet under the cardboard disc. At the other end, touching the mirrors, fasten a circular plane glass plate. On this glass plate, place several small pieces of colored glass. Finally, close this end of the tube with a ground glass plate. Now, your kaleidoscope is ready.

In simple words: To make a kaleidoscope, join three mirrors to form a triangle inside a tube. Put a disc with a hole on one end and a clear glass plate on the other. Add small colored glass pieces on the clear plate, then cover that end with another glass plate.

Exam Tip: Focus on the arrangement of the mirrors (prism shape), the placement of the viewing hole and colored objects, and the types of glass used at each end.

 

Question 10. Draw a labeled sketch of the human eye.
Answer:
In simple words: The human eye is like a camera. It has a clear front part (cornea), a colored part (iris) that controls light, a lens that focuses light, a light-sensitive back part (retina), and a nerve (optic nerve) that sends pictures to the brain. Small muscles (ciliary muscles) help the lens change shape.

Exam Tip: Practice drawing and labeling all major parts of the human eye, including the cornea, iris, pupil, lens, ciliary muscles, retina, and optic nerve. Accuracy in relative size and position is important.

 

Question 11. using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teacher's advice?
Answer: Laser light is harmful to the eye and can cause a permanent defect. She might lose her eyesight if exposed to it.
In simple words: Laser light can seriously hurt your eyes and might cause lasting damage, even blindness. That's why the teacher warned against using it.

Exam Tip: Always emphasize the permanent and serious nature of damage caused by direct laser exposure to the eyes.

 

Question 12. Explain how you can take care of your eyes.
Answer: Do the following to care for your eyes:

1. Avoid reading in too little or too much light.
2. Wash your eyes with clean water at least three times each day.
3. Do not look directly at the sun or any powerful light source.
4. Always read at the normal distance for vision.
5. Do not rub your eyes with dirty hands.

In simple words: To protect your eyes, read in good light, wash them often, avoid looking at bright sun or strong lights, hold books at a proper distance, and never rub them with dirty hands.

Exam Tip: Provide clear, actionable advice for eye care. Numbered lists are effective for presenting multiple points concisely.

 

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?
Answer: The angle of incidence will be 45°.
In simple words: If the incoming light ray and the outgoing light ray form a 90° angle, then the angle at which the light hits the surface must be 45°, because the angle of incidence equals the angle of reflection.

Exam Tip: Remember that the angle of incidence (i) equals the angle of reflection (r), and the total angle between the incident and reflected ray is \( i + r \). So, if \( i + r = 90^\circ \), then \( i = r = 45^\circ \).

 

Question 14. How many images of a candle will be formed if it is placed between two parallel mirrors separated by 40 cm?
Answer: An infinite number of images will be formed.
In simple words: When two mirrors are placed facing each other, they create an endless series of reflections, so a candle placed between them would appear to have an infinite number of images.

Exam Tip: Parallel mirrors create an infinite series of images due to multiple reflections back and forth between the surfaces. This is a key property of parallel mirror systems.

 

Question 15. Two mirrors meet at right angles. A ray of light is incident at an angle of 30° as shown in Fig. 16.10. Draw the reflected ray from the second mirror.
Answer:
In simple words: When a light ray hits the first mirror at 30 degrees, it bounces off at 30 degrees, then hits the second mirror at 60 degrees (because the mirrors are at a right angle). It then bounces off the second mirror also at 60 degrees.

Exam Tip: For problems involving multiple mirrors, apply the laws of reflection sequentially for each reflection. Pay close attention to how angles are measured relative to the normal for each surface.

 

Question 16. Boojho stands at A just on the side of a plane mirror as shown in Fig. 16.11. 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 own image. He can only see the image of P but not of Q and R.

In simple words: Boojho is standing too close to the side of the mirror to see himself. He can only see the image of point P, as light from P reflects into his eyes. Points Q and R are positioned such that their reflections do not reach Boojho's eyes.

Exam Tip: To determine visibility in a mirror, draw ray diagrams from the object to the mirror and then to the observer's eye. Remember that the image formed by a plane mirror is virtual and located behind the mirror at the same distance as the object in front.

 

Question 17.
(a) Find out the position of the image of an object situated at A in the plane mirror
(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:

(a) The position of the image of an object situated at A in the plane mirror will be at the same perpendicular distance behind the mirror as A is in front, along the normal.
(b) Yes, Paheli can see this image.
(c) Yes, Boojho can see this image.
(d) There will be no change in the position of the image when Paheli moves from B to C.
In simple words: (a) The image of object A will appear behind the mirror, at the same distance as A is from the front. (b) Paheli (at B) can see it. (c) Boojho (at C) can also see it. (d) The image of A stays in the same spot, even if Paheli moves, because the object A and the mirror don't move.

Exam Tip: Remember that for a plane mirror, the image is formed at the same distance behind the mirror as the object is in front. The position of the observer only affects *if* they can see the image, not *where* the image is formed.