Goa Board Class 8 Solutions for Science – Light (English Medium)
Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.
We can see objects only if light from any source enters our eyes. If we are in a dark room, then it is not possible for us to see objects in the room. However, if there is light outside the room, then objects outside the room are visible to us.
Concept insight: Light enables us to see things from which it comes or from which it is reflected.
Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?
Regular reflection takes place from a smooth or a regular surface. In regular reflection, all reflected rays are parallel to each other for parallel incident rays.
Irregular or diffused reflection takes place from a rough or an irregular surface. In diffused reflection, the reflected rays are not parallel to each other for parallel incident rays. This happens because of the presence of microscopic irregularities of the surface on which light rays are incident. Hence, parallel incident rays reflect in different directions. However, each ray obeys the laws of reflection. Therefore, laws of reflections are not violated in diffused or irregular reflection.
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
(f) Piece of paper
(a) Polished wooden table – Regular reflection
Concept insight: A polished surface is an example of a smooth surface. A polished wooden table has a smooth surface. Hence, reflections from the polished table will be regular.
(b) Chalk powder – Diffused reflection
Concept insight: Chalk power spread on a surface is an example of an irregular surface. It is not smooth. Therefore, diffused reflection will take place from chalk powder.
(c) Cardboard surface – Diffused reflection
Concept insight: Cardboard surface is also an example of an irregular surface. Hence, diffused reflection will take place from a cardboard surface.
(d) Marble floor with water spread over it – Regular reflection
Concept insight: Marble floor with water spread over it is an example of a regular surface. This is because water makes the marble surface smooth. Hence, regular reflection will take place from this surface.
(e) Mirror – Regular reflection
Concept insight: Mirror has a smooth surface. Therefore, it will give a regular reflection.
(f) Piece of paper – Diffused reflection
Concept insight: Although a piece of paper may look smooth, but it has many irregularities on its surface. Due to this reason, it will give a diffused reflection.
State the laws of reflection.
Laws of reflection:
- The angle of reflection is always equal to the angle of incidence.
- The incident ray, the reflected ray and the normal to the reflective surface at the point of incidence, all lie in the same plane.
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.
Fix a white sheet of stiff paper on a drawing board or a table. Let the sheet project a little beyond the edge of the table. Take a comb and close all its openings except one in the middle with the help of a black paper. Hold the comb perpendicular to the sheet of paper. Throw light from a torch through the opening of the comb from one side so that a ray of light can be seen along the paper on the other side of the comb. Place a strip of plane mirror in the path of the light ray. This ray of light striking the mirror is called the incident ray. After striking the mirror, the ray of light is reflected in another direction. This is called the reflected ray.
Now, cut the projecting portion of the sheet in the middle. Look at the reflected ray. Make sure that the reflected ray extends to the projected portion of the paper. Bend that part of the projected portion on which the reflected ray falls. The reflected ray that fell along this part can no longer be seen.
This shows that the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.
Fill in the blanks in the following.
(a) A person 1 m in front of a plane mirror seems to be ____ __ m 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.
Concept insight: Distance between object and mirror is equal to the distance between mirror and image.
(b) left, left hand
Concept insight: Image formed by a plane mirror is laterally inverted.
Concept insight: Size of pupil increases in dim light so that more amount of light can enter in the eye.
(d) lesser (fewer)
Concept insight: Rods are sensitive to dim light.
Angle of incidence is equal to the angle of reflection.
(c) Under special conditions
Concept Insight: The angle of incidence is always equal to the angle of reflection. This is the first law of reflection.
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.
(b) Image formed by a plane mirror is virtual, behind the mirror and of the same size as the object.
Describe the construction of a kaleidoscope.
Construction of a kaleidoscope:
- Three rectangular mirror strips of dimensions 15 cm4 cm (l×b) are joined together to form a prism (fig a).
- This prism is fixed into a circular cardboard tube. The circular cardboard tube should be slightly longer that the prism (fig b).
- This circular tube is now closed at one end with a cardboard disc. This disc has a hole in it through which we can see (fig c).
- At the other end of the circular tube, a plane glass plate is fixed. It is important that this glass plate touches the prism mirrors. On this glass plate, several small and broken pieces of coloured glass are placed.
- This end is now closed by a round glass plate allowing enough space for the coloured glass pieces to move (fig d).
Draw a labelled sketch of the human eye.
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?
Laser light is harmful for the human eye, because its intensity is very high. It can cause damage to the retina and lead to blindness. Hence, it is advisable not to look at a laser beam directly.
Explain how you can take care of your eyes.
We can take care of our eyes in the following ways:
- We should wash our eyes at least twice a day with clean water.
- We should avoid reading in dim light and very bright light.
- We should avoid direct exposure of eyes to the sunlight.
- We should clean our eyes with cold water quickly if dust particles or small insects enter your eye. We should not rub the eyes.
- We should maintain a distance of at least 25 cm between the book and our eyes while reading.
What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?
How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?
Infinite images of the candle will be formed because of multiple reflections between the two parallel mirrors.
Concept Insight: When two mirrors are placed parallel to each other, then infinite number of images are formed.
Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30o as shown in figure. Draw the reflected ray from the second mirror.
The laws of reflection are used to obtain the path of reflected light.
It can be observed that the given ray of light will reflect from the second mirror at an angle 60o.
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?
A plane mirror forms a virtual image behind the mirror. The image is as far behind the mirror as the object is in front of it. A cannot see his image because the length of the mirror is too short on his side. However, he can see the objects placed at points P and Q, but cannot see the object placed at point R (as shown in the given figure).
(a) Find out the position of the image of an object situated at A in the plane mirror (Figure).
(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) Image of the object placed at A is formed behind the mirror. The distance of the image from the mirror is equal to the distance of A from the mirror. Image of A is shown in the given figure.
(b) Yes. Paheli at B can see this image.
(c) Yes. Boojho at C can see this image.
(d) Image of the object at A will not move. It will remain at the same position when Paheli moves from B to C.