Physics 7 Lesson 1: identifying light, Light sources and bright objects prepared by teachers of Cmm.edu.vn school, hopes to provide useful documents to help students master the lesson knowledge and achieve good results in the class. class tests and exams.
Summary of Physics Theory 7 Lesson 1
When do we perceive light?
⇨ We perceive light when light enters our eyes.
When do we see an object?
⇨ We see an object when light passes from that object into our eyes.
Light source and light object
A light source is an object that emits light on its own
For example: Candle is burning, light bulb is glowing, Sun,…
Light objects include the light source and the objects that reflect the light that falls on it.
For example: The book, the table and the chair, the trees during the day…
Note: A black body is an object that does not emit light on its own and does not reflect light on it. The reason we can see a black object is because it is placed next to other bright objects.
Black plastic bottles do not emit light on their own and do not reflect light. We see those black plastic bottles because they are placed next to other bright objects (chairs, potted plants, walls, etc.)
Solution method
Conditions of seeing objects
In order to see an object, two conditions must be met:
– There must be light coming from the object.
The light from the emitting object must be able to reach our eyes.
If one of these two conditions is missing, then we cannot see the object.
Identify and distinguish between light sources and bright objects
a) identification
* Light source
Based on the origin, we can classify light sources into two types:
– Natural light sources: Sun, active volcanoes, fireflies…
– Artificial light source: The gas stove is burning, the match is burning…
* Bright objects
We can divide luminous objects into two types:
– Light source.
Objects that reflect light: Objects that do not emit light on their own, but reflect light when light hits it.
b) Distinguish between a light source and a luminous object
* Same: Both light source and light object have light emitted from it.
* Different:
Light Sources: Objects that emit light on their own.
– Bright objects: Can be objects that do not emit light on their own, but only reflect light when it is hit by another light source.
Solve the exercises of the Physics textbook 7 Lesson 1
Lesson C1 (page 4 of Physics textbook 7)
In the cases where our eyes perceive light, what conditions are the same?
The answer:
The same condition is that there is light from the light source entering the eye.
Conclusion: Our eyes perceive light when light enters our eyes.
Lesson C2 (page 4 of Physics textbook 7)
Test setup as shown in Figure 1.2a. A piece of white paper glued on a black wall inside a sealed box. In which of the following cases do we see a white piece of paper?
a. The light is on (Figure 1.2a).
b. The light is off (Figure 1.2b).
Why see?
The answer:
The case we see the white piece of paper is the case a: the light is on. Because the light of the lamp hits the white piece of paper, the piece of paper reflects the light that hits the eye.
Conclude:
We see an object when light from that object enters our eyes.
Lesson C3 (page 5 of Physics textbook 7)
In the tests shown in Figures 1.2a and 1.3 we see that the white piece of paper and the filament of the light bulb glow because from both objects light reaches our eyes. Which object emits light on its own, and which object reflects light from another object?
The answer:
The self-luminous object is the filament of a light bulb.
The object that reflects light from another object is a piece of white paper.
Conclude:
The filament of a light bulb itself emits light called a light source.
The filament of a light bulb and a piece of white paper that reflects light from another object is called a luminous object.
Lesson C4 (page 5 of Physics textbook 7)
In the debate presented in the opening paragraph, which one is correct? Why?
The answer:
Thanh (D); Hai (S); Because the light from the bulb does not directly reach the eyes.
Lesson C5 (page 5 of Physics textbook 7)
In the experiment in Figure 1.1, if we light a handful of incense to let the smoke rise in front of the flashlight, we will see a trail of light from the lamp emitting through the smoke. explain why? Know that smoke consists of tiny particles floating in the air.
The answer:
Smoke particles are composed of tiny particles, so when illuminated by a lamp, they become bright objects, and small bright objects are arranged together to form a light streak that reaches our eyes. So we will see a trail of light from the lamp emitting through the smoke.
Physics 7 Lesson 1 multiple choice questions (with answers)
Lesson 1: Why do we see an object?
A. Because we awaken towards the object.
B. Because our eyes emit light rays that shine on objects.
C. Because light from an object enters our eyes.
D. Because the object is illuminated.
The answer
– if it is dark (no light), even if we wake up towards the object, our eyes cannot see the object ⇒ Answer A is wrong.
– The human eye does not emit light ⇒ Answer B is incorrect.
– The object is illuminated but if there is no light from the object entering the eye, the eye cannot see the object ⇒ Answer D is incorrect.
So the correct answer is C
Lesson 2: Which of the following is not a source of light?
A. The Sun
B. The volcano is burning
C. The light bulb is on
D. The Moon
The answer
– The sun, the burning volcano, the light bulb is the light source because they all emit light by themselves ⇒ Answers A, B, C are wrong.
The Moon is not a light source because it does not emit light on its own. The reason we see the Moon is because it reflects the light from the Sun shining on it ⇒ Answer D is correct.
Lesson 3: We cannot see an object because:
A. The object does not emit light on its own
B. The object emits light, but it is obstructed by an obstacle, making it impossible for the light from that object to reach our eyes.
C. Because our eyes do not receive light
D. the above sentences are correct
The answer
When an object receives light from another object, that object can also reflect light back into our eyes. Our eyes can also see that object, it doesn’t have to be a light source ⇒ Answer A is incorrect.
– We do not see an object not because our eyes do not receive light, but because it is not the light emitted by the object that we need to see ⇒ Answer C is wrong.
– When an object cannot transmit its light to our eyes, we cannot see the object ⇒ Answer B is correct.
Lesson 4: Which of the following is not a luminous object?
A. The candle is burning.
B. A piece of white paper placed in the sun.
C. A piece of black paper placed in the sun.
D. The Sun.
The answer
– The candle is burning and the Sun is both a light source and a bright object (because bright objects include both light sources and objects that reflect light when shining on it) ⇒ Answers A and D are incorrect.
– A white piece of paper placed in the sunlight is a bright object because the white piece of paper reflects the sunlight shining on it ⇒ Answer B is incorrect.
– Because a piece of black paper is an object that does not emit light on its own and does not reflect light on it, a piece of black paper placed in the sunlight is not a bright object ⇒ Answer C is correct.
Lesson 5: In which of the following cases, we do not recognize the black cover?
A. Stick the black cardboard on a piece of green paper and place it under the gas lamp.
B. Stick the black cardboard on a white piece of paper and place it in a dark room
C. Place the black cardboard in front of a burning candle
D. Put the black cardboard outside in the sun
The answer
A piece of black cardboard is an object that does not emit light on its own and does not reflect light that falls on it.
– When gluing the black cardboard on a green paper and then placing it under the gas light, we can identify the black piece of paper because the black cardboard is placed on top of the bright object (blue paper) ⇒ Answer A is incorrect.
– When placing a black card in front of a burning candle, we can identify the black card because the black card is placed in front of a bright object (a burning candle) ⇒ Answer C is incorrect.
– When placing a black card in the sun, we can recognize the black card because the black card is placed in front of a bright object (the sun is a light source and also a bright object) ⇒ Answer D is incorrect.
– In a dark room, there is no light, so we will not be able to recognize the black card. ⇒ Answer B is correct.
Lesson 6: I see a red book because
A. The book itself is red
B. The book is a bright object
C. The book is a source of light
D. There is red light from the book reaching my eyes
The answer
Conditions to see the red book:
+ There must be light from the book.
The light emitted by the book must reach our eyes.
⇒ Answers A, B, C are wrong. Answer D is correct.
Lesson 7: During the sunny day, use a flat mirror to catch sunlight, then rotate the mirror to shine sunlight through the window into the room, is that mirror a light source? Why?
A. As a light source because there is light from the mirror shining into the room
B. As a light source because the mirror reflects sunlight into the room
C. Not a light source because the mirror only shines light in one direction
D. Not a light source because mirrors do not emit light on their own
The answer
Mirrors are not a source of light because mirrors do not emit light on their own. A mirror is called a luminous object because it is an object that is illuminated and reflects light from the Sun shining on it.
⇒ Answers A, B, C are wrong. Answer D is correct.
Lesson 8: explain why in a room with closed wooden doors, without the lights on, we can’t see the white piece of paper placed on the table?
The answer
Because the white piece of paper is the object that reflects light, but at night without the light on, no light will shine on the piece of paper ⇒ No light from the piece of paper is reflected in the eyes ⇒ We cannot see the piece of paper.
Lesson 9: At night, in a dark room, we see a bright spot on the table. Arrange a test to see if the bright spot is the light source.
The answer
Find a way to make sure no light from anywhere in the room hits the bright spot on the table, if we can still see the bright spot, it’s the light source.
For example, use a sealed carton over the light spot and cut a small hole so that the light cannot pass in. If the point of light is still bright, it is a light source, whereas if the point is no longer bright, it is an object reflecting light.
Lesson 10: Why is it that in a dark room, when the light is on, even though we turn our back to the light bulb, we can still see the objects in front of us?
The answer
In a dark room when the light is on, even though we have our backs to the light bulb, there is still light transmitted from the bulb to the objects and reflected back to our eyes, so our eyes can still see the objects in front of us.
Lesson 11: If we light a handful of incense to let the smoke rise in front of the flashlight, we will see a trail of light from the lamp emanating through the smoke. explain why? Know that smoke contains tiny particles floating in the air.
The answer
Smoke consists of many tiny particles, the smoke particles are illuminated by the lamp to become bright objects. Tiny bright objects arranged together to form a visible light trail.
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