Kerala Syllabus Class 7 Basic Science: Chapter 04 When Light Reflects - Questions and Answers
Questions and Answers for Class 7 Basic Science (English Medium) പ്രകാശം പ്രതിപതിക്കുമ്പോൾ | Text Books Solution Basic Science (English Medium) Chapter 04 When Light Reflects - Teaching Manual | Teachers Handbook
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ഈ ബ്ലോഗ് ഉടമയുടെ രേഖാമൂലമുള്ള അനുമതിയില്ലാതെ ഈ ബ്ലോഗിൽ നൽകിയിരിക്കുന്ന ചോദ്യോത്തരങ്ങൾ, ഇതേരീതിയിലോ പി.ഡി.എഫ് രൂപത്തിലോ, മറ്റേതെങ്കിലും ഡിജിറ്റലോ, പ്രിന്റഡ് ഉൾപ്പെടെയുള്ള ഏതെങ്കിലും രൂപങ്ങളിലേക്കോ മാറ്റി മറ്റൊരു വെബ്സൈറ്റിലോ, ബ്ലോഗിലോ, യുട്യൂബ്, സോഷ്യൽ മീഡിയാ ഗ്രൂപ്പുകളിലോ ഉൾപ്പെടെ ഒരിടത്തും പ്രചരിപ്പിക്കാൻ പാടില്ലാത്തതാകുന്നു.
Std 7: Basic Science Chapter 04: When Light Reflects - Questions and Answers
♦ Point a lighted torch at different objects such as scratched glass sheet, glass filled with pure water, wooden block, a piece of cloth, white paper, black chart paper, butter paper, window glass, a coin, a mirror, a piece of reading glass, marble, polythene cover and a colourless plastic bottle.
What do you observe? Record your observations in the Science Diary.
| Objects that transmit light | Objects that do not transmit light |
| • Window glass • Scratched Glass Sheet • Glass Filled with Pure Water • Piece of Cloth • White paper, Butter paper • Reading glass • Polythene cover • Colourless plastic bottle | • Wooden block • Black chart paper • Coin • Mirror • Marble |
♦ Repeat the experiment using butter paper, window glass, scratched glass piece, oiled paper, glass piece, polythene cover, pure water taken in a glass tumbler and colourless plastic bottle.
On the basis of the experiment, classify them as those which transmit light completely and those which transmit light partially.
| Objects which transmit light completely | Objects which transmit light partially |
| • Window glass • Glass piece • Pure water taken in a glass tumbler • Colourless plastic bottle | • Butter paper • Scratched Glass Sheet • Oiled paper • Polythene cover |
♦ What are Transparent Objects?
Objects that transmit light very well are called transparent objects.
♦ What are Translucent Objects?
Objects which transmit light partially are called translucent objects.
♦ What are Opaque Objects?
Objects which do not transmit light are opaque objects.
♦ Can you change a transparent glass sheet into a translucent or opaque one? How?
• Yes
• By applying paint
• By metal coating
• By sooting Glass Sheet
• By applying frosted spray
♦ Can we convert pure water and air to translucent?
• Yes
• We can convert pure water into translucent:
by adding ink
by adding milk
by adding clay
by adding food colour
by adding chemicals
• We can convert air into translucent:
by using incense sticks
by fogging
♦ How do you know when the ink in a refill pen is completely used up?
The refill of the pen is transparent. So we can easily see the amount of ink left inside.
♦ Find out more situations and complete the table
| Situations in which transparent objects are used | • Eyeglasses, Contact lenses • Laboratory equipments, Aquariums • Microscope and Telescope lenses |
| Situations in which translucent objects are used | • Lampshades • Window glasses • Oiled paper • Plastic container |
| Situations in which opaque objects are used | • Walls • Mirrors • Doors • Clothes • Blackboard |
♦ Dim the light in your class room by closing the doors and windows. Hold a mirror facing the wall and let the light from a torch fall on it. What happens to the light? Haven’t you noticed that the light rays fall on the wall after hitting on the mirror? Repeat the experiment by holding the following objects against the wall and allowing light from the torch to fall on each of them.
Materials required:
smooth tile, new steel plate, bronze, hardboard, paper, wooden block.
Record your observations in the table given below.
| The object on which light fell | Difference in the returning of light after falling on the surface |
|---|---|
| Mirror | Light returns well |
| Paper | Very little light returns |
| Smooth tile | Light returns well |
| new steel plate | Light returns well |
| bronze | Light returns well |
| hardboard | Light did not return |
| wooden block | Light did not return |
♦ What is Reflection of Light?
Reflection of light refers to the returning of light when it strikes on an object.
♦ Touch and feel the surfaces of the objects which reflect light very well. What do you feel?
Smooth surfaces reflect light very well.
♦ What about the surfaces of objects that do not reflect light much?
Rough surfaces cannot reflect light well
♦ The reflection of light from a mirror and a sand paper are depicted below.
Analyse the figures and write the inferences.
• The light falling on the sandpaper undergoes irregular reflection.
♦ What is Regular Reflection?
Light falling on smooth surfaces reflects with regularity. This is regular reflection. Mirrors give a regular reflection.
♦ What is irregular reflection or diffused reflection?
When light falls on rough surfaces, it gets scattered in different directions. This is irregular reflection or diffused reflection.
♦ Let’s do an experiment using some materials in the Science Kit.
Materials required: A small piece of mirror, a protractor made as shown in the figure, a transparent plastic box, double-sided tape, laser torch, an incense stick and a match box.
Fix the small piece of mirror on one side of the transparent plastic box using the double sided tape. Fill up the box with smoke from the incense stick.
Arrange the protractor you have made under the plastic box as shown in the figure. Draw a perpendicular line from this protractor to the mirror at an angle of 90°. This is the normal. Allow the light from the laser torch to fall through different angles of the protractor on the point where the normal touches the mirror. Observe the reflected light. Measure the angle between the ray of light from the laser torch and the normal. Similarly measure the angle between the reflected ray of light and the normal. Record them in the table.
| Angle between the light ray from the torch and the normal | Angle between the reflected light ray and the normal |
| 40° | 40° |
| 55° | 55° |
| 70° | 70° |
| 60° | 60° |
♦ The diagram of the light ray falling on the mirror and the reflected ray.
• Incident ray: The ray of light falling on the mirror is the incident ray.
• Point of incidence: The point at which the incident ray falls on the mirror is the point of incidence.
• Normal: The line drawn perpendicular to the mirror at the point of incidence is the normal.
• Reflected ray: The light ray reflecting from the mirror is the reflected ray.
♦ What is the Angle of Incidence?
The angle between the incident ray and the normal is the angle of incidence.
♦ What is the Angle of Reflection?
The angle between the reflected ray and the normal is the angle of reflection.
♦ State the Laws of Reflection?
• The angle of incidence and the angle of reflection are equal.
• The incident ray, the reflected ray and the normal at the point of incidence are on the same plane.
♦ Observe the picture showing the path of the light ray from the bulb falling on the book and reflecting to our eyes. Complete the flow chart.
♦ How do we see things?
We see an object when light coming from any source of light falls on that object, gets reflected and reaches our eyes. But we see a source of light when the light from it reaches our eyes directly.
♦ Observe the figure and identify the path of light. Complete the flow chart
♦ On which surfaces other than the mirror, can you see your face?
• New steel plate
• Clear and still water surfaces
• Polished tile
♦ Plane mirror
A mirror with a flat surface is a plane mirror.
♦ Lateral Inversion
In a plane mirror, the left side of an object appears as the right side of the image and the right side of the object appears as the left side of the image. This phenomenon is lateral inversion.
♦ What would be the reason for writing the word “AMBULANCE” in reverse on the front of the vehicle?
The word “AMBULANCE” is written in reverse on the front of the vehicle, so that when viewed through the rearview mirror of another vehicle, it reads correctly due to lateral inversion.
♦ What are the characteristics of Images in a Plane Mirror?
• The image undergoes lateral inversion.
• In a plane mirror, the distance between the object and the mirror is equal to the distance between the image and the mirror.
• In a plane mirror, the size of the object will be equal to the size of the image.
♦ How many images will you see if a burning candle is placed in between two parallel plane mirrors?
If we place a burning candle between two parallel plane mirrors, we will see an infinite number of images.
♦ Multiple Reflection
A large number of images of the lighted candle placed in between the parallel mirrors are formed due to multiple reflections of light.
♦ Find more examples of situations involving multiple reflections in daily life.
• Barbershop
• Jewellery
• Beautyparlour
♦ Make a protractor and place it below the mirrors. Find out the angle between the mirrors and the number of images formed and complete the table. (Textbook Page No: 76)
| Angle | Number of images |
| 30° | 11 |
| 60° | 5 |
| 90° | 3 |
| 120° | 2 |
of images formed?
If the angle is x, then the number of images is
♦ What happens to the number of images when the angle between the mirrors increases?
the number of images decreases
♦ What if the angle decreases?
the number of images increases
♦ Write the method of construction of the Kaleidoscope.
• Materials required: Three plane mirror pieces of 6 inch x 2 inch, insulation tape, transparent plastic sheet.
• Method of construction: Using the insulation tape, fix the three plane mirrors
in a triangular pattern as shown in the figure. Cover one of the open ends with
the transparent plastic sheet using the insulation tape. Put some coloured
bangle pieces or beads inside this device and observe.
Tilt the device and enjoy the different patterns.
• Materials required: A cardboard piece of size 25 cm x 30 cm, two plane mirror pieces of size 3 inch x 2.5 inch (Sunpack sheet can also be used instead of
cardboard piece).
Stage 2: Draw lines on it with the same measure as shown in Figure 1.
Stage 3: Cut off the unshaded parts of the figure along the lines. Didn’t you get a shape as shown in Figure 2 now?
Stage 4: Fold this shape as shown in Figure 3 and glue it up.
Stage 5: Cut two pieces of plane mirrors of size 3 inch x 2.5 inch. While mfixing them on the slanting ends of the device you have made, make sure that the reflecting surface faces the inner side of the device.
♦ How a periscope is useful for the submarines in the Navy and for soldiers observing enemies from trenches in the battlefield.
• Submarines use periscopes to observe the surface of the water without exposing themselves. This is essential for detecting enemy ships, aircraft, or other threats.
• Soldiers in trenches use periscopes to observe enemy activity without exposing their heads above the parapet. This reduces the risk of being shot.
♦ How does the intense light at night affect Owls?
Owls can’t see anything due to the intense light at night. They can't go out for preying due to the intense light. This can also lead to habitat loss and reduced populations.
♦ Light Pollution
• Today, we use many sources of light that dispel darkness.
• In cities and parks neon bulbs and the likes that are kept lit up throughout
the night.
• This light is harmful to many organisms that hunt for prey in the
dark.
• Intense illumination at night also causes people to miss many of the sky views that can only be seen on clear nights.
• Too much light at night causes many difficulties for human beings as well as animals.
Let’s Assess
1. Examine the following table and find the odd one out.
Transparent Objects - the odd one
• Box filled with smoke
Translucent Objects - the odd one
• Turbid water
Opaque Object - the odd one
• Clearwater
2. Observe the pictures. Which type of reflections do you see here?
• Picture 1 - Regular Reflection
Light falling on smooth surfaces reflects with regularity. This is regular reflection.
• Picture 2 - Irregular reflection or diffused reflection.
When light falls on rough surfaces, it gets scattered in different directions. This is irregular reflection or diffused reflection.
3. Observe the following situations. Find out which type of reflection takes place in each.
| Situation | Reflection |
|---|---|
| • Ornaments shine | Regular Reflection |
| • We get light inside the home during day time | Irregular reflection |
| • A polished furniture shines | Regular Reflection |
| • See reflection of trees on stagnant water | Regular Reflection |
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ഈ ബ്ലോഗിലെ എല്ലാ പോസ്റ്റുകളും ഒരുമിച്ച് കാണാനും ആവശ്യമുള്ളവ എളുപ്പത്തിൽ തിരഞ്ഞെടുക്കാനും ഇവിടെ ക്ലിക്കുക.
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