Answer: (a) straight line
Explanation:
Light travels in straight line.

Answer: (b) shadow
Explanation:
A shadow is formed when an opaque object comes in the way of light.

Answer: (b) Opaque
Explanation:
Opaque objects do not allow the light to pass through them while translucent objects allow only a part of light to pass through them.

Answer: (a) A thin sheet of paper
Explanation:
A thin sheet of paper is transparent.

Answer: (a) Reflection
Explanation:
When a ray of light falls on a smooth and polished surface, whole of light is returned back in the same medium. It is called reflection of light.

Answer: (c) Right of the object appears left of the image
Explanation:
Right side of the object appears as left side in the image formed by a plane mirror.

Answer: (a) I, O, U
Explanation:
I, O, U will not show lateral inversion.

Answer: (b) Pinhole camera
Explanation:
Sun can be imaged by using a pinhole camera.

Answer: (a) Firefly (Jugnu)
Explanation:
Firefly (Jugnu) emits short bursts of light.

Answer: (c) Firefly (Jugnu)
Explanation:
Firefly (Jugnu) is a natural luminous body.

Answer:

Answer: luminous

Answer: screen

Answer: shadow

Answer: upside down

Answer: erect

Answer: lateral inversion

Important Questions

Question 1.
Define luminous body.
Answer:
The objects which emit light of their own are called luminous bodies, e.g., the sun, the stars, etc.

Question 2.
What are luminous and non-luminous objects?
Answer:
Luminous objects: Objects that give out or emit light of their own, e.g., sun, torch, light, bulbs, etc.
Non-luminous objects: Objects that do not give out light of their own, e.g., the moon, chair, shoe, pen, etc.

Question 3.
Define opaque objects with example.
Answer:
Those materials which do not allow light to pass through them, are called opaque objects. Examples wood, stone, etc.

Question 4.
What are translucent objects?
Answer:
Those objects which allow only a small part of the light rays to pass through them are called translucent objects. These are the objects through which one cannot see properly.

Question 5.
What are transparent objects?
Answer:
Those objects which allow light rays to pass through them completely are called transparent objects. One can see clearly through these objects.

Question 6.
Classify the following into transparent, opaque and translucent objects:
Wax, spectacles, a heap of salt, a stone, dense smoke, wood, skin, balloon, rubber, membrane of a tabla, blood and milk.
Answer:
Transparent objects are Spectacles.
Opaque objects are A stone, wood, a heap of salt, dense smoke, and membrane of tabla.
Translucent objects are Blood, milk, wax, skin, balloon, rubber.

Question 7.
Close your eyes while looking towards bright sunshine. Now, cover the eyes with your palm still keeping them closed. Do you notice any difference? On the basis of this experience, state whether your eyelids are transparent, translucent or opaque.
Answer:
On the basis of the experience, we observe our eyelids are opaque.

Question 8.
Sometimes, you are able to see the Sun or the Moon behind the clouds. What can you say about the ability of such clouds to transmit light? ‘
Answer:
We may say that clouds behave as translucent object.

Question 9.
How will you convert a transparent glass sheet into a translucent sheet? Suggest any two ways to do it.
Answer:

1. By smearing a thin layer of oil we may convert a transparent glass sheet into a translucent sheet.
2. By covering one side of the glass sheet by butter paper.

Question 10.
Name sources of light that are not hot.
Answer:
Tube light, jugnu.

Question 11.
List four natural sources of light.
Answer:

1. Sun
2. Stars
3. Fire
4. Jugnu (Firefly).

Question 12.
Write down four man-made sources of light.
Answer:

• Candle
• Oil lamp
• Electric bulb
• Torch.

Question 13.
Does the fire emit light?
Answer:
Yes, fire emits light.

Question 14.
On what factor does the proportion of light that enters an object depend?
Answer:
Optical nature of the object.

What Exactly Are Shadows?

Question 1.
What is an obstacle?
Answer:
An object which comes to the path of light is called an obstacle.

Question 2.
Give examples of two substances through which light does not pass?
Answer:
Wood and bricks are the substances through which light does not pass.

Question 3.
Whether a shadow is two-dimensional or three-dimensional?
Answer:
Shadow is not just the two-dimensional outline that you see on the ground. All the space behind the opaque object up to some distance behind it seems to be filled with the shadow

Question 4.
What do we need in order to see a shadow?
Answer:
We need

• a source of light,
• an opaque object in the way of light, and
• a screen;

Question 5.
Does the direction of shadow change during the day? Does the length of shadow also change from season to season?
Answer:
Yes, the direction of shadow changes as the sun changes its position during the day. The length of the shadow also changes from season to season.

Question 6.
What is a shadow? How is it formed? How does the colour of an opaque object affect the colour of the shadow?
Answer:
A shadow is a dark outline or image cast by an opaque object that blocks light coming from a source of light. It is formed when light hits the opaque object which does not let the light pass through. Everywhere else around the opaque object, the light continues in a straight path until it bounces off the ground or wall behind the object. The wall or ground behind the opaque object is the screen. On this screen is a dark patch, or shadow, with the same outline as the object surrounded by light. The colour of the opaque object does not affect the colour of the shadow that is formed.

Question 7.
What is the difference between image and shadow?
Answer:

Question 8.
Define screen.
Answer:
Screen is a surface on which images are formed.

Question 9.
What is umbra and penumbra?
Answer:
Umbra is the dark region behind object facing light which does not receive light at all.
Penumbra is the less dark part of shadow. It is the outer part of shadow.

Activity 4
Place a chair in the school ground on a sunny day. What do you observe from the shadow of the chair?
Does the shadow give an accurate picture of the shape of the chair? If the chair is turned around a little, how does the shape of the shadow change?
No, accurate shape of chair is not shown by the shadow. When turned around a little, the shape of the shadow will change because now different portions of the chair will obstruct light and make shadow.

Take a thin notebook and look at its shadow. Then, take a rectangular box and look at its shadow. Do the two shadows seem to have a similar shape?
Yes, the two shadows seem to have a similar shape.

Take flowers or other objects of different colours and look at their shadows. A red rose and a yellow rose, for instance. Do the shadows look different in colour, when the colours of the objects are different?
No, the shadows of the different coloured objects are not different.

Take a long box and look at its shadow on the ground. When you move the box around, you may see that the size of the shadow changes. When is the shadow of the box the shortest, when the long side of the box is pointed towards the Sun or when the short side is pointing towards the Sun?
Shadow of the box is the shortest when the short side is pinting towards the Sun.

A Pinhole Camera

Question 1.
Define pinhole camera.
Answer:
It is a device which forms a photograph like image of a bright object on a screen.

Question 2.
Can you think of a situation where we can see the path of the light?
Answer:
In a dark room, we can see with torch light which goes straight. Similarly, dust particles become visible when light enters the room through fine hole.
All these examples indicate that light travels in straight line.

Question 3.
Why is the image formed in a pinhole camera inverted?
Answer:
In a pinhole camera, image formed is inverted because the object is between radius of curvature and focus.

Question 4.
What is the path of light?
Answer:
Straight line.

Mirrors and Reflections

Question 1.
What is reflection?
Answer:
When a ray of light falls on a smooth and shiny surface, the whole of light is sent back in the same medium. It is called reflection. Mirrors do not allow even a small amount of light to pass through them. Mirrors show regular and complete reflection.

Question 2.
Define ‘Mirror’.
Answer:
A smooth shining surface, which rebounds the light back in same or in different direction is called a mirror.

Question 3.
Why is silvered glass used as a mirror?
Answer:
The silvered glass has a smooth surface and the smoothness helps in forming clear image. Silvering makes it shiny and the shiny surface helps in reducing the absorption.