Light Book Back Questions 7th Science Lesson 14

7th Science Lesson 14

14] Light

Book Back Questions with Answer and Do You Know Box Content

Do You Know?

Light is the only source of energy for plants. So, they entirely depend on light. People and animals derive energy from carbohydrates, protein and fat through their food. Plants produce food using the energy from Sun light, carbon-di-oxide and water by the process called as Photosynthesis. Sun light acts a vital role in the process of photosynthesis.

Is the moon a luminous object? The moon provides light as well, but it cannot produce light by its own. The light emitted by the Moon is the light of the Sun reflected towards the Earth. When we see the Moon, we see only the Moon’s lighted part. Thus, half of the moon is always facing the Sun and receiving light from it. Hence, we receive light from the moon.

We often use a kind of gas-discharge lamp that uses fluorescence to produce visible light. The electric current in the gas excites mercury vapour, which produces short-wave ultraviolet light that then causes a phosphor coating on the inside of the lamp to glow in visible light.

Al-hasan-Haytham was a scientific thinker who made important contribution to the understanding of vision, optics and light. He observed that light coming through a tiny hole travelled in straight lines and projected an image onto the opposite wall. Based on such experimentation, he concluded that vision is accomplished by rays coming from external luminous sources and entering the eye, rather than through rays emitted from the eye as was then commonly believed. He is the first one to experiment with light and found important properties like the rectilinear propagation of light.

Before the advancement of camera, Pinhole camera was used to photograph movement of the sun over a long period of time. This type of photography is known as solography and also be used for observing and recording solar eclipses. And it was also used to take photograph of stationary objects.

Optical fibre is a device that works on the principle of total internal reflection by which light signals (huge data) can be transmitted from one place to another pace with a negligible loss of energy in a very short time. It consists of a cable having one or more think flexible fibres with a glass core through which light signals can be sent. Optical fibre can be twisted and bent easily. When a light a ray of light is incident at one end of the core of optical fibre, it suffers total internal reflection at the many places inside the fibre and emerges at the other end with negligible loss of energy. The data or information in the form of pulses of light can be sent through bundles of optical fibres. Optical fibres have become very important in high-speed communications, such as cable TV and high-speed broadband services. Fibre optic cables are able to carry more signals than traditional copper cable telephone lines.

Why is the word “AMBULANCE” written backwards in ambulance vehicle? This 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.

Why danger lights in vehicles are red in colour? Red colour is scattered the least by air molecules. Red colour has the highest wavelength of all the other colours. So red colour is able to travel the longest distance through air, fog.

Choose the best answers:

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

(a) Curved line, shadows

(b) Straight line, shadows

(c) Straight line, reflection

(d) Curved line and then straight line, shadows

2. Light that hits a mirror gets ____________

(a) Transmitted

(b) Reflected

(c) Absorbed

(d) Refracted

3. _________________ surface reflects the light well.

(a) Water

(b) Compact disc

(c) Mirror

(d) Stone

4. Light is a form of _______________

(a) Matter

(b) Energy

(c) Medium

(d) Particle

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

(a) Regular reflection takes place in wooden table and irregular reflection in polished floor

(b) Regular reflection takes place in polished floor and irregular reflection in wooden table

(c) Regular reflection takes place in both polished floor and wooden table

(d) Irregular reflection takes place in both polished floor and wooden table

6. Choose the translucent substance from the following

(a) Glass

(b) Wood

(c) Water

(d) Clouds

7. Reflection occurs, when the light

(a) About to reach a surface

(b) Approaches a surface

(c) Passes through a surface

(d) None of these

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

(a) Plastic plate

(b) Plane mirror

(c) Wall

(d) Paper

9. Sivarajan placed a metre 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?

(a) There will be no shadow

(b) The shadow will be longer and on the opposite side as the sun

(c) The shadow will be shorter and on the same side as the sun

(d) The shadow will be shorter

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

(a) Light travels in straight lines

(b) Light rays become laterally inverted as they pass through a pinhole camera

(c) Light rays pass through the pinhole

(d) Light rays get reflected

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

(a) Light travels in straight lines

(b) Opaque bodies do not allow light to pass through them

(c) Reflection occurs at smooth surfaces like mirrors

(d) Lateral inversions happens

(i) Both A and B

(ii) Both A and D

(iii) Both B and C

(iv) Only A

Fill in the blanks:

1. A plane mirror produces an __________________ image.

2. A ____________ reflection helps us to see the objects.

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

4. Sunlight is a blend of _____________ colours.

5. The splitting of white light in to seven colours is called _______________.

6. The moon _______________ sun light.

7. The sunlight can be split into its constituent colours using ______________.

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

State true or false. If false, correct the statement:

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

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

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

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

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

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

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

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

9. A plane mirror is opaque.

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

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

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

Match the following:

1. Rectilinear propagation – Primary source of light

2. Plane Mirror – Non-luminous object

3. Fire fly – Periscope

4. The Moon – Pinhole camera

5. Wide light source – Spectrum of light

6. Regular reflection – Luminous object

7. The sun – Penumbra

8. Band of seven colours – Glossy surface

Answers:

Choose the best answers:

1. Straight line, reflection 2. Reflected 3. Mirror 4. Energy

5. Regular reflection takes place in polished floor and irregular reflection in wooden table

6. Clouds 7. Passes through a surface 8. Plane mirror 9. The shadow will be shorter

10. Light travels in straight line 11. Both A and B

Fill in the blanks:

1. Virtual 2. Light 3. Reflected 4. Seven 5. Dispersion 6. Reflects 7. Prism 8. Diffuse

State true or false:

1. True

2. True

3. True

4. True

5. True

6. True

7. False

Correct Statement: The image formed by the pin hole camera is always smaller than the object size.

8. False

Correct Statement: The image formed in a plane mirror is virtual image of same size.

9. True

10. False

Correct Statement: A Shadow is always formed on against the opposite side of the light source.

11. True

12. True

Match the following:

1. Penumbra 2. Pinhole camera 3. Non-luminous object 4. Primary source of light 5. Glossy surface 6. Periscope

7. Spectrum of light 8. Luminous object