7th Std Science Term 3 Solution in English | Lesson.1 Light

Lesson 1. Light

Lesson 1. Light

I . Choose the correct answer.

1.  Light travels only in a _____________ . It is because of this property that _____________ are formed

1. curved line, shadows
2. straight line, shadows
3. straight line, reflection
4. curved line and then straight line, shadows

Ans : straight line, reflection

2. Light that hits a mirror gets ____________ 

1. Transmitted
2. Reflected
3. Absorbed
4. Refracted

Ans : Reflected

3.  __________ Surface reflects the light well

1. water
2. compact disc
3. mirror
4. stone

Ans : mirror

4. Light is a form of __________

1. matter
2. energy
3. medium
4. particle

Ans : energy

5. You can see your image in polished floors, but not in wooden table because  _________

1. regular reflection takes place in wooden table and irregular reflection in polished floor
2. regular reflection takes place in polished floor and irregular reflection in wooden table
3. regular reflection takes place in both polished floor and wooden table
4. irregular reflection takes place in both polished floor and wooden table

Ans : regular reflection takes place in polished floor and irregular reflection in wooden table

6. Choose the translucent substance from the following

1. glass
2. wood
3. water
4. Clouds

Ans : r

7. Reflection occurs , when the light

1. about to reach a surface
2. approaches a surface
3. passes through a surface
4. None of these

Ans : passes through a surface

8. Which of the following is the best reflector of light?

1. plastic plate
2. plane mirror
3. wall
4. paper

Ans : plane mirror

9. Sivarajan placed a meter stick in the playground at 7.00 am in the morning. How will the shadow of the stick at noon look in comparison to the one in the morning

1. There will be no shadow
2. The shadow will be longer and on the opposite side as the sun
3. The shadow will be shorter and on the same side as the sun
4. The shadow will be shorter

Ans : The shadow will be shorter

10. The image formed by a pinhole camera is inverted because,

1. a. light travels in straight lines
2. light rays become laterally inverted as they pass through a pinhole camera
3. light rays pass through the pinhole d. light rays get reflected
4. light rays get reflected

Ans : light travels in straight lines

11. Which of the following facts explain how shadows are formed?

1. both A and B
2. both A and D
3. both B and C
4. only A

Ans : both A and B

II. Fill in the blanks:

1. A plane mirror produces a ____________ image

Ans : Virtual

2. A ____________ reflection helps us to see the objects

Ans : Light

3. The light ray gets ____________ when it falls on any polished surface.

Ans : reflected

4. Sunlight is a blend of ____________ colors

Ans : Seven 

5. The splitting of white light in to seven colors is called ____________

Ans : Dispersion

6. The moon ____________ sun light

Ans : reflects

7. The sunlight can be split into its constituent colors using ____________

Ans : Prism

8. Reflection of light from rough surface is called ____________ reflection

Ans : Diffuse

III. Match the following:

IV. True or False – If false give the correct answer:

1. The image of right hand in a plane mirror looks like a left hand

Ans : True

2. Rainbow is formed by dispersion of light by water drops

Ans : True

3. The image formed by the plane mirror is laterally inverted, hence the image seen through the periscope is also laterally inverted

Ans : False

Cu. An. : The image formed by the plane mirror is laterally inverted, but the image seen through the periscope is upright.

4. We see planets because they reflect light from the sun

Ans : True

5. We see a book because it reflects the light that falls on its surface

Ans : True

6. The image formed in a pinhole camera is always inverted

Ans : True

7. The image formed in a pinhole camera is always the same size as the object.

Ans : False

Cu. An.  : The image formed in a pinhole camera may not be equal to the size of the object.

8. The image formed in a plane mirror is upside down.

Ans : False

Cu. An. : The image formed in a plane mirror is laterally inverted.

9. A plane mirror is opaque.

Ans : True

10. A shadow is formed on the same side of the object as the source of light.

Ans : False

Cu. An. : A shadow is formed on the opposite side of the object as the source of light.

11. we are able to see things around us with the help of regular reflection.

Ans : False

Cu. An. : we are able to see things around us with the help of irregular reflection.

12. After passing through a prism, white light splits into a band of seven colours

Ans : True

V. Answer the following questions in short

1. With the help of a diagram, state the laws of reflection Laws of reflection:

• The angle of incidence is always equal to the angle of reflection.

i = r

• The incident ray, the reflected ray and the normal at the point of incidence lie on the same plane.

2. Figure shows a pencil placed above a mirror

a. Draw its image formed by the mirror

Image formed by the mirror

b. Show how light rays from the object are reflected at the mirror to form the image for the eye

• The image is caught by the intersection of extended reflected rays.
• The image is formed behind the mirror.

3. A person is looking at the image of a tree in a mirror placed 3.5 m in front of him. Given that the tree is at 0.5 m behind his eyes. Find the distance between the image of the tree and his eyes. What are needed to see an object?

Needed materials to see a object :

• Light
• Mirror
• Object

4. What are luminous objects?

Object which exhibit light of their own are known as luminous objects.

Eg : Sun.

5. Is the moon a luminous object?

• No, the moon is not the luminous objects because it cannot produce light of its own.
• The light emitted by the Moon is the light of the Sun reflected towards the Earth.

6. What are the three types of materials based on the absorption of light?

Transparent Material:

• Materials that allow light to pass through completely are known as transparent material.
• Example: Eye glasses, clear drinking glass, clear water, face glasses used in buses. Translucent

Material:

• Objects that allow light to pass through partially are called translucent material.
• For example, we cannot see the image of someone who stands behind a rough window glass, because it allows only a part of light from the person.

Opaque Material:

• Materials that are not able to allow light to pan through, are called opaque material.
• Example: Wall, thick card board, stone, etc.

7. What are the parts of shadow?

Parts Properties of shadow :

• When an opaque object is placed in the path of light from a point source, a uniform dark shadow will appear on the screen.
• This is shadow is called as umbra. When an opaque object is placed in the path of light coming from a broad source of light, a small umbra will appear on the screen and an illuminated shadow area appears around umbra.
• This illuminated shadow area is called as penumbra.
• The penumbra always surrounds the umbra.
• The umbra is the darkest part of a shadow.
• In this part, light rays are completely prevented by the opaque object. The lighter shade of shadow is the penumbra.

8. What are the properties of shadow?

Properties of shadow :

• All objects do not form shadows. Only opaque objects form shadows.
• Shadows will be formed in the opposite side of light source. It cannot be determined the characteristics of an object by its shadow.
• The shadow will be always darker, whatever may be the color of light rays.
• Light source, opaque object are shadow all are in a straight line.
• The size of shadow depends upon the distance between light source and object and the distance between object and the screen.

9. What is plane mirror?

• A polished (or) smooth surface (like glass) which forms image by reflection is known as mirror.
• A plane mirror is a mirror with a flat reflective surface.
• A plane mirror makes an Image of objects in front of it.

10. What is prism?

A prism is an object made up of a transparent material, like glass or plastic that has at least two flat surfaces that from an acute angle (less than 90^o degrees).

11. What do you mean by visible light?

• Visible light is a spectrum of a number of waves with different wavelength range from 400nm to 700nm. (1nm = 10^-9 meter)
• Each wave has a definite wavelength represents a particular colour.

12. Write the items given here in the correct column

(Stars, brick walls, plants, mirror, planets, electric light bulb,candle)

13. A boy of height 1m 45 cm is standing in front of a long mirror at a distance of 2 m. From this information, fill up the following sentences:

a. The distance between the boy and his image is ________

a) The distance between the boy and his image (u) is same as the distance between the mirror and the image (v).

ஃ u = v = 2m

Distance between object (boy) and the image is U + V = 2 + 2 = 4m

b. The height of the image is

The height of the image is 1m 45cm because the image and the object (boy) are of the same size.

c. When the boy moves 1m forward, the distance between her and her image is

If the boy moves 1m forward, then distance between the boy and the image = 4 – 1 = 3m

14. Draw a diagram of a pin hole camera showing the rays of light passing between the Object and its image

15. Explain with examples, why some capital letters look the same in a mirror but others are reversed.

• Writing on the front of an ambulance back to front, is due to lateral inversion. The phenomenon due to which the left side of an object appears to be right side of the object in its image in a reflecting medium (mirror).
• so that drivers see the word the right way around in their rear-view mirror

16. Two plane mirrors M1 and M2 are placed perpendicular with each other, as shown in figure. The ray AB makes an angle 39 ° with the plane mirror M1, then

1. The reflected rays are _____________, _____________

Ans : BC, CD

2. The incident rays are _____________, _____________

Ans : AB, BC

3. What is the angle of incident corresponding to the ray BC?

Angle of incidence = 90° – 39° = 51°

4. What is the angle of reflection corresponding to the ray CD

Angle of incidence = angle of reflection

ஃ Angle of reflection = 51

17. Rajan was playing with the mirror images of a clock. He looked at the clock in his room. It was showing 1:40. Draw the position of the hands on the real clock and on its mirror reflection. Write below the picture what time each picture is showing.

18. What is reflection of light?

The light rays bouncing off the surface of the mirror is known as reflection of light.

Types of reflection :

Diffuse reflection – Rough surface
Specular reflection – Smooth surface.

19. If a ray of light is falling on a plane mirror at an angle of 50 is formed, what will be the angle of reflection?

• Angle of incidence = 50°
• Angle of reflection = 50°
• because, i = r
• The angle of incidence is always equal to angle of reflection

20. What do you mean by lateral inversion?

Lateral inversion is the phenomenon due to which the left side of an object appears to the right side of the object in its image in a reflecting medium (mirror).

21. How do you obtain a spectrum of light?

• The spectrum of light can be obtained when white light is passed through a prism.
• The colours of the rainbow (VIBGYOR) emerge from the prism.

22. Why do we see white color in Newton’s disc, when we rotate it very fast?

When Newton’s disc is rotated fast, the retina receives the sensation of the spectrum simultaneously and the disc appears white.

Reason :

• When light ray of particular wavelength (colour) strikes the retina of our eye, our brain perceives that specific colour.
• When all colours of visible light strikes the retina of our eye at the same time, our brain perceives white.

23. What is a shadow? What things are necessary for the formation of a shadow?

A Shadow is a dark area where light from a light source is blocked by an opaque object.

Things necessary for the formation of a shadow :

• Opaque object
• Light source.

VI. Answer the following questions in detail

1. What are regular and irregular reflection? Explain with the help of diagrams

Regular reflection

• If the surface is smooth, and flat, all points on it have the normal in the same direction.
• Therefore a set of parallel rays striking the surface will be reflected at an angle, but the rays themselves will still remain parallel to each other.
• If the surface is smooth then we have specular reflection or regular reflection.

Irregular reflection

• Roughness of the wall means that each individual ray meets a surface which has a different orientation.
• If the surface is rough, then we have difused reflection. Light rays, after reflection go in many directions.

2. What are the difference between luminous and non-luminous objects? Give two examples of each.

3. Write about two everyday situations that tell you that light travels in a straight line.

• Torch is an example that proves light travels in a straight line.
• When we switch on torch, the light travels in a straight line.
• Light travels through the holes in cement grill.
• Light penetration through trees.
• Laser beam.

4. Differentiate between a reflection and a shadow

5. What are the characteristics of an image formed in a plane mirror?

Characteristics of an image formed in a plane mirror :

• The image is upright.
• The image and the object are the same size.
• The image is virtual.
• The image is laterally inverted.
• The distance of the object from the mirror is equal to the distance of the image from the mirror.

7. Define the following terms

a. Incident ray

The ray of light that falls on the surface of the reflection materials.

b. Reflected ray

The ray of light that comes from the point when the incident ray falls on the reflection material.

c. Normal

The perpendicular lines drawn from the point of incidence to the plane of reflecting surface is called normal.

d. Angle of incidence

The angle formed between the incident ray and the normal is called angle of incidence

7. Compare the images formed by plane mirror with that by pinhole camera.

Difference between the images formed in pinhole camera and plane mirror

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