Science STD 8 Chapter 13: Light - Exercises
1. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room. Explain.
No, you cannot see objects inside a dark room because there is no light to reflect from them into your eyes. You can, however, see objects outside the room if there is a light source outside, as light from those objects will reflect and enter your eyes.
2. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?
| Regular Reflection | Diffused Reflection |
|---|---|
| Occurs on smooth, polished, and regular surfaces. | Occurs on rough or irregular surfaces. |
| Parallel incident rays remain parallel after reflection. | Parallel incident rays are reflected in different directions. |
| Forms clear images. | Does not form a clear image. |
No, diffused reflection does not mean the failure of the laws of reflection. The laws of reflection are valid at each point on the surface. Diffused reflection is caused by the microscopic irregularities on the surface, which causes parallel light rays to be reflected in different directions.
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
Regular reflection will take place because a polished surface is smooth and reflects light in a regular pattern.
- (b) Chalk powder
Diffused reflection will take place because chalk powder consists of a collection of small particles with an irregular surface, scattering the light in different directions.
- (c) Cardboard surface
Diffused reflection will take place because the surface of the cardboard is rough and has irregularities that cause the light to scatter.
- (d) Marble floor with water spread over it
Regular reflection will take place because the thin layer of water makes the surface smooth and acts like a mirror, reflecting light in a regular pattern.
- (e) Mirror
Regular reflection will take place because a mirror is a highly polished and smooth surface designed for regular reflection.
- (f) Piece of paper
Diffused reflection will take place because the surface of a piece of paper, despite looking smooth, has microscopic irregularities that cause light to be scattered in all directions.
4. State the laws of reflection.
The two laws of reflection are:
- The angle of incidence is equal to the angle of reflection.
- The incident ray, the reflected ray, and the normal drawn at the point of incidence all lie in the same plane.
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.
To demonstrate this, you can fix a white sheet of stiff paper on a table, with a portion of it hanging over the edge. Place a plane mirror on the paper. Use a comb and a torch to create a single ray of light (incident ray) that hits the mirror. Mark the incident and reflected rays on the paper. Now, bend the projecting portion of the paper on which the reflected ray falls. You will observe that the reflected ray disappears. When you bring the paper back to its original position, the ray reappears. This shows that the incident ray, the reflected ray, and the normal all exist in the same plane.
6. 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 hand.
- (c) The size of the pupil becomes larger when you see in dim light.
- (d) Night birds have fewer cones than rods in their eyes.
7. Angle of incidence is equal to the angle of reflection.
- (a) Always
- (b) Sometimes
- (c) Under special conditions
- (d) Never
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.
9. Describe the construction of a kaleidoscope.
A kaleidoscope is constructed using three rectangular mirror strips, each about 15 cm long and 4 cm wide. These strips are joined together with adhesive tape to form a prism. This prism is then placed in a circular cardboard tube. One end of the tube is closed with a cardboard disc that has a small hole in the centre for viewing. The other end is fitted with a plane glass plate on which small pieces of coloured glass or broken bangles are placed. This end is then closed with a ground glass plate. When viewed through the hole, the multiple reflections from the mirrors create numerous beautiful and ever-changing patterns.
10. Draw a labelled sketch of the human eye.
A labelled sketch of the human eye would show the main parts, including the cornea, iris, pupil, lens, ciliary muscles, retina, and optic nerve. The diagram should illustrate the spherical shape of the eye and the path of light as it enters the eye and is focused on the retina.
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?
The teacher's advice was based on the fact that a laser torch is very powerful and its light can be extremely harmful to the eyes. A laser beam can damage the retina, which is a sensitive part of the eye. Therefore, it is very dangerous to look at a laser beam or its reflection, and it should not be used for activities that involve directing light into the eye.
12. Explain how you can take care of your eyes.
To take care of your eyes, you should follow these practices:
- Avoid reading in too little or too much light.
- Never look at the sun or a powerful light source directly.
- If any dust particles enter your eyes, wash them with clean water and consult a doctor if there is no improvement.
- Always read at a normal distance. Do not hold the book too close or too far from your eyes.
- Consume a balanced diet rich in Vitamin A, such as carrots, broccoli, and green vegetables, which is essential for eye health.
13. 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 first law of reflection, the angle of incidence is equal to the angle of reflection (∠i = ∠r). The total angle between the incident ray and the reflected ray is 90°. Therefore, the angle of incidence would be half of that, which is 90° / 2 = 45°
14. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?
If an object is placed between two parallel plane mirrors, an infinite number of images are formed. This is because the image formed by one mirror acts as an object for the other mirror, and this process continues indefinitely.
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.
The diagram would show the incident ray striking the first mirror at 30 degrees. This ray is reflected at an angle of 30 degrees. The reflected ray then strikes the second mirror, which is at a right angle to the first. Based on the geometry, the angle of incidence on the second mirror will be 60 degrees, and it will be reflected at an angle of 60 degrees.
16. 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?
Boojho cannot see himself in the mirror because he is not standing in front of it. He can see the image of objects situated at P and Q, as the reflected rays from these objects reach his eyes. He cannot see the image of object R because the reflected rays from R do not reach his eyes.
17. (a) Find out the position of the image of an object situated at 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?
(a) The image of object A will be formed at the same distance behind the mirror as the object is in front of it. The position of the image is fixed at point A' (A prime), as shown in the diagram.
(b) Yes, Paheli at B can see the image of object A.
(c) Yes, Boojho at C can also see the image of object A.
(d) When Paheli moves from B to C, the image of A does not move. The position of the image remains fixed at A' because the object (A) and the mirror are not moving.
Suggested Activities and Projects
1. Make your own mirror. Take a glass strip or glass slab. Clean it and put it on a white sheet of paper. See yourself in the glass. Next put the glass slab on a black sheet of paper. Again look into the glass. In which case do you see yourself better and why?
Outline: This activity demonstrates how reflection is affected by the background. When the glass slab is on the black sheet of paper, you see yourself better because the black surface absorbs the light passing through the glass, preventing it from interfering with the reflection. This makes the reflection clearer, as it more closely resembles a proper mirror.
2. Make friends with some visually impaired students. Enquire from them how they read and write. Also find out how they are able to recognise objects, hurdles and currency notes.
Outline: This activity promotes empathy and learning about different abilities. By talking to visually impaired students, you would learn about the Braille system, a tactile method of reading and writing. You would also learn that they use their other senses, such as touch and hearing, more acutely to navigate their environment, recognise objects, and identify currency notes by their texture and size.
3. Meet an eye specialist. Get your eye sight checked and discuss how to take care of your eyes.
Outline: This project is about personal health and eye care. You would visit an eye specialist to get your eyesight checked and receive advice on how to care for your eyes. This would include advice on a healthy diet, proper lighting for reading, avoiding excessive screen time, and wearing appropriate spectacles if needed.
4. Survey your neighbourhood. Find out how many children below the age of 12 years use spectacles. Find out from their parents what, in their view, could be the reason for the weak eyesight of their children.
Outline: This is a social survey to investigate the prevalence of weak eyesight in children. You would collect data from your neighbourhood and interview parents about the possible causes they believe in, such as excessive screen time, poor diet, or genetic factors. This activity helps in understanding common health concerns in your community and the importance of eye care from a young age.