Light

This chapter introduces the fundamental concept of light and its basic properties. 

• What is Light? Light is a form of energy that enables us to see things. It travels in straight lines. 
• Sources of Light:
  
  • Natural Sources: Light that comes from natural phenomena, like the Sun, stars, and fireflies.
  • Artificial Sources: Light produced by humans, like light bulbs, tubelights, and candles.
• How Light Travels (Rectilinear Propagation): A key property of light is that it travels in straight lines. This is called rectilinear propagation of light. This property explains why shadows are formed.
  
• Shadow Formation: When an opaque object (an object that does not allow light to pass through it) blocks the path of light, it casts a shadow.
  
  • Umbra: The dark, central part of the shadow where no light from the source reaches.
  • Penumbra: The lighter, outer part of the shadow where only some of the light from the source is blocked.
• Reflection of Light: When light falls on a shiny surface, it bounces back.
  
  • Mirrors: Smooth, shiny surfaces that reflect light well. Plane mirrors (flat mirrors) form images that are:
    • Upright
    • Virtual (cannot be projected onto a screen)
    • Laterally inverted (left and right appear reversed)
    • Same size as the object.
• Pin Hole Camera: This simple device demonstrates the rectilinear propagation of light and how images are formed. Light rays travel in straight lines from an object, pass through a small hole, and form an inverted (upside down) image on a screen placed behind the hole.
  

In essence, this chapter introduces light as a form of energy that travels in straight lines, explains how shadows are created when light is blocked, and explores the phenomenon of reflection using mirrors. It also touches upon how these principles can be used to form images, as seen in a pinhole camera.

Test yourself

A. Short Answer Questions

1. Write true or false for each statement

(a) The moon is a natural source of light.
Answer. False

(b) The moon is self luminous.
Answer. False

(c) We can see an object through an opaque medium.
Answer. False

(d) Light passes through glass.
Answer. True

(e) Light travels in a straight line path. 

Answer. True

(f) Image formed in a pinhole camera is real.
Answer. True

(g) The image in a pinhole camera gets blurred if the hole is made bigger.
Answer. True

(h) A shadow is formed because light travels in a straight line path.
Answer. True

(i) Solar eclipse occurs when the sun comes in between the earth and the moon.
Answer. False

(j) If the shadow of earth falls on the moon, the eclipse is called the lunar eclipse.
Answer. True

2. Fill in the blanks

(a) Light gives us the sensation of——–.

Ans : Vision
(b) The sun is a ——— source of light.

Ans : Natural
(c) A medium through which light cannot pass is called the———-.

Ans : Opaque medium
(d) A medium which allows light to pass through it easily is called the———.

Ans : Transparent medium.
(e) Moon is a ———-body.

Ans : Non-luminous
(f) Light travels in a ——–path.

Ans : Straight line
(g) In a pinhole camera, the image formed is———.

Ans : Inverted and real.
(h) The darkest portion of a shadow is called the——-.

Ans : Umbra

(i) The less dark portion of a shadow is called the———-.

Ans : Penumbra
(j) Lunar eclipse occurs when the ——–comes in between the ——and the sun.

Ans : Earth , Moon

3. Match the following columns

4. Select the correct alternative

(i) The natural source of light is

1. candle flame
2. electric lamp
3. sun
4. kerosene lamp

Answer : Sun

(ii) The formation of inverted image in a pinhole camera shows that

1. light enables us to see
2. light travels in a straight line path
3. light can pass through the pinhole
4. light does not pass through the pin hole

Answer : Light travels in a straight line path

(iii) The luminous body is

1. a lighted bulb
2. earth
3. noon
4. table

Answer : A lighted bulb

(iv) Umbra is a region of

1. complete darkness
2. partial darkness
3. complete brightness
4. partial brightness

Answer : Complete darkness

(v) Penumbra is a region of

1. complete darkness
2. complete brightness
3. partial brightness
4. none of the above

Answer : Partial brightness

(vi) Solar eclipse occurs on

1. every new moon’s day
2. certain new moon’s day
3. every full moon’s day
4. certain full moon’s day

Answer : Certain new moon’s day

(vii) Lunar eclipse occurs on

1. every full moon’s night
2. certain full moon’s night
3. every new moon’s day
4. certain new moon’s day

Answer : Certain full moon’s night

1)What is light ? Define it.
Answer : Light is the energy that allows us to see the world around us. It moves as waves and is actually just one type of energy in a big family called the electromagnetic spectrum, which also includes things like radio waves and X-rays. Simply put, light is what our eyes detect to make things visible.

2)How does light make an object visible ?

Answer : Some of these reflected light rays then travel straight from the book towards your eyes. Your eyes act like little collectors, catching this light. Inside your eyes, these light signals undergo a process and are then sent as messages to your brain. It’s your brain that takes these messages and figures out what you’re looking at – the book’s form, its color, and where it is.

So, to put it simply, light lets us see things through this process:

1. It starts from a light source (either the object itself if it glows, or another source like a lamp).
2. It then reflects or bounces off the surface of the object.
3. This reflected light travels into our eyes.
4. Our eyes and brain work together to understand this light, allowing us to “see” the object.

3)Name two natural sources of light.

Answer : Here are two natural sources of light:

1. The Sun: Our most important natural light source! It’s a giant star that produces a tremendous amount of light and heat through nuclear reactions. This light travels through space and illuminates our planet, making life possible.
   
2. Stars: Just like our Sun, all the other stars you see in the night sky are also natural sources of light. They are incredibly far away, which is why they appear as tiny, twinkling points of light. Each star is a massive ball of hot gas generating its own light.

4) List two artificial sources of light.

Answer : Humans have figured out ways to make their own light, and here are a couple of common examples:

1. Think about a light bulb. We use electricity to make a tiny wire inside get incredibly hot. This hot wire then starts to glow brightly, giving us light.
   
2. Another familiar way we create light is with a burning candle. When you light the wick, the wax melts and turns into a gas that catches fire. That flame is a source of light produced by a chemical reaction.

 5) Differentiate between the luminous and non-luminous bodies. Give two examples of each.

Answer : 

Feature	Luminous Bodies	Non-Luminous Bodies
Definition	Emit their own light.	Do not produce their own light; visible by reflection.
Light Source	They are the source of light.	They reflect light from other sources.
Visibility	Visible due to the light they produce.	Visible only when light falls on them and is reflected.
Energy Source	Have an internal energy source for light production.	No internal light production mechanism.
Example 1	The Sun	The Moon
Example 2	A burning candle	A book

6) Is the moon a luminous object ?

Answer : 

The Moon doesn’t have its own internal power source to generate light like the Sun does. Instead, it acts like a cosmic mirror, catching the sunlight that hits its surface and then bouncing some of that light back towards us here on Earth. That reflected sunlight is what we perceive as the Moon “shining” in the night sky.  

So, to reiterate and correct my previous answer:

Non-Luminous Bodies:

1. The Moon: As you correctly pointed out, it reflects sunlight.  
2. A book: It needs an external light source to be visible.

7) What do we call a body that shines on its own ?

Answer : A luminous body.

8)What do we call an electric bulb producing light ?

Answer:We call an electric bulb that produces light a luminous object or a source of light.

9)What is a transparent medium ? Give two examples.

Answer:Clear glass: The glass in windows, drinking glasses, and many other objects is designed to let light pass through so we can see what’s inside or on the other side.  You can usually see objects behind clear glass very distinctly

10)Explain the difference between a transparent, a translucent and an opaque medium. Give two examples of each.

Answer:Think of transparent materials as super friendly to light. They’re like open doors, inviting light to pass straight through without any fuss. That’s why you can see objects on the other side so clearly, just as if nothing were there. Clear glass in a window is a perfect example – you can easily see what’s outside. Pure water is another one; look into a clean lake, and you can often see right to the bottom.

Then you have translucent materials, which are a bit more selective. They let some light through, but it’s not a smooth, direct passage. It’s more like the light has to wiggle its way through, getting a little jumbled in the process. This scattering effect means you can see light coming through, but the view of anything on the other side is blurry or unclear. Frosted glass, like in some bathroom windows, lets light in but keeps things a bit private. Thin tissue paper also allows light to filter through, but you can’t really make out details on the other side.

Finally, there are opaque materials, which are like a firm “no entry” sign for light. They block light completely. When light hits an opaque object, it can’t pass through; it either gets absorbed or reflected back. This is why you can’t see through a brick wall or a solid wooden door. They create shadows because the light simply can’t get to the other side.

So, in a nutshell:

• Transparent: Light goes straight through, you can see clearly.
• Translucent: Some light gets through, but it’s scattered, so the view is blurry.
• Opaque: No light gets through at all, you can’t see through them.

11)What do we call a substance through which we cannot see light ? Give an example of such a substance.

Answer:When light bumps into certain materials, it just can’t pass through at all. We can’t see anything on the other side of these kinds of substances because the light gets completely blocked. We call a substance like this an opaque substance.

Think about a solid wall made of bricks. If you hold it up to your eye, you can’t see through it to the other side, right? That’s because the brick is an opaque material.

12)What do we call a substance through which light passes ? Give an example of such a substance.

Answer:When light can travel through a substance, we call that substance transparent. Think of it like a clear window – you can see right through it because the light passes through the glass.

A common example of a transparent substance is clear glass. You can easily see objects on the other side of a glass window or a glass of water because light can pass through it without being scattered or absorbed too much.

13)Can a transparent medium form an image ? Explain your answer.

Answer:   Yes, a transparent medium can form an image, but not in the same way a mirror does through reflection. Transparent mediums form images through refraction, which is the bending of light as it passes from one transparent medium to another.These are made of transparent materials like glass or plastic. They are carefully shaped to bend the light rays passing through them in a specific way. This bending of light causes the rays to converge or diverge, ultimately forming an image on our retina (in the case of eyeglasses) or on a sensor (in the case of a camera).   So, while you can’t see a direct reflection in a clear window (a transparent medium), the glass of a lens definitely manipulates light to create a focused image.  

14)How can you obtain a point source of light ?

Answer:To get a light source that’s as close to a single point as possible, you can use a small pinhole in an opaque (non-transparent) material. If you shine a larger light source behind this material, only the light that passes through the tiny pinhole will emerge on the other side, effectively acting like a point source.

15)Define the terms : a ray of light and a beam of light.

Answer:A ray of light is like a single, straight arrow showing the path light takes. It’s a simple way to draw and understand how light moves.A beam of light is like a whole bunch of those arrows traveling side-by-side in the same direction.

16)What do you mean by ‘rectilinear propagation of light’ ?

Answer : Rectilinear propagation of light means that light travels in straight lines. This is a fundamental property of light under normal conditions and in a uniform medium.  

Think of it like this: if you shine a torch in a clear, still room, the beam of light travels in a straight path from the torch to the wall or any object it hits. It doesn’t bend or curve on its own.

17)Describe an experiment to show that light travels in a straight line path.

Answer:   What you’ll need:

• Three cardboard pieces of the same size
• A small stand or support for each cardboard
• A candle or a torch 

How to do it:

1. Make a small hole in the center of each of the three cardboard pieces. 
2. Stand the cardboard pieces up in a straight line, one behind the other, using the supports. Place the lit candle or torch behind the last cardboard in the line.
3. You should be able to see the flame of the candle or the light from the torch.

What you’ll observe:

You can see the light because it’s traveling in a straight line right through all three aligned holes and into your eyes.  

What happens if you move one of the cardboards slightly out of alignment?

If you shift any one of the cardboard pieces so that its hole is no longer in the straight line with the other two, you will no longer be able to see the light source..  

Conclusion:

If it didn’t travel in a straight line, you would still be able to see the light even when the middle cardboard was slightly out of place!

18)In which of the following two arrangements (a) and (b) shown in the diagram, can you see the light of the bulb ? Explain Your answer

Answer: Here’s why:

In arrangement (a), the tube connecting the light bulb to the eye is straight. As we learned from the experiment we just discussed, light travels in a straight line. Therefore, the light rays coming from the bulb can travel directly through the straight tube and reach the eye, allowing the person to see the light.

In arrangement (b), however, the tube is bent. Since light travels in straight lines, the light rays coming from the bulb will travel straight until they hit the inside of the bent tube. They won’t be able to follow the curve of the tube to reach the eye at the other end. It’s like trying to throw a ball straight through a curved pipe – it will hit the sides.

19) Name a simple application of the rectilinear propagation of light

Answer : A simple application of the fact that light travels in a straight line is the formation of shadows. Because light can’t bend around opaque objects, when something blocks its path, a dark area in the shape of the object appears behind it. 

20) What is a pinhole camera ? Draw a neat and labelled diagram to show the formation of an image of a lighted candle by it.

Answer:Instead, it has a tiny hole (the pinhole) on one side. Light from an object passes through this tiny hole and projects an inverted (upside-down) image of the object onto a screen placed on the opposite side of the box.  

Think of it like this: each point on the object sends out light rays in all directions. Only the rays that pass through the very small pinhole can travel to the screen. Because the hole is so tiny, only one ray from each point on the object makes it through in a straight line. These rays then form an image on the screen.  

Here’s a simple diagram showing how it works for a lighted candle:

Key parts in the diagram:

• Lighted Candle: The object emitting light.
• Rays of light: Represent the path light takes from the candle.
• Pinhole: The tiny opening that allows only a few straight rays of light to pass through.  
• Screen: The surface where the image is formed (it could be tracing paper or a white sheet).  

So, a pinhole camera uses the principle that light travels in straight lines to create a simple image without needing any lenses. The smaller the pinhole, the sharper the image, but the dimmer it will be because less light gets through.

Light travels straight: Light rays travel in straight lines from every point on the flower.  

1. Passing through the pinhole: Only a very narrow beam of light from each point on the flower can pass through the tiny pinhole.  
2. Inverted image on the screen: These narrow beams of light continue traveling in straight lines inside the box and fall on the screen on the opposite side. The light ray from the top of the flower passes through the pinhole and hits the bottom part of the screen. Similarly, the light ray from the bottom of the flower hits the top part of the screen. This results in an inverted (upside-down) image of the flower being formed on the screen.  

Think of it like this: the pinhole acts as a selector, allowing only one straight path of light from each point on the object to reach a specific point on the screen. This creates a point-by-point mapping, forming the image.

The image formed in a pinhole camera is usually real (meaning you could project it onto a screen), inverted, and smaller than the actual object. The sharpness of the image depends on the size of the pinhole – a smaller pinhole makes the image sharper but dimmer.

22)State two factors which affect the size of an image formed in a pinhole camera.

Answer:The size of the picture you get with a pinhole camera depends mainly on two things:

• If you hold a piece of paper close to the opening, the spot of light will be small. But if you move the paper farther away, the light spot gets bigger. It’s the same with a pinhole camera – a greater distance between the pinhole and the surface where the image forms (like film or a digital sensor) results in a larger picture.
  
• How close the object is to the pinhole: Think about taking a picture of a tree. If you stand very close to the tree with your pinhole camera, the image on the screen will be quite large. However, if you walk far away from the same tree and take another picture, the image will be much smaller. Just like things look smaller when they’re far away in real life, the closer the object is to the pinhole, the bigger its image will be in the camera.

23)Is the image obtained in a pin hole camera erect or inverted ? Give reason for your answer.

Answer: This happens because light travels in straight lines. Similarly, light rays from the bottom of the object pass through the pinhole and travel straight to the top part of the screen. This crisscrossing of light rays through the tiny pinhole results in an upside-down image being formed on the screen

24)How is the image affected in a pinhole camera when another fine hole is made near the first pinhole ?

Answer:each tiny hole in a pinhole camera acts like its own little projector. It allows light rays from different points of the object to pass through and form an image on the screen.

If you have just one pinhole, all those single “projected” points line up to create one relatively clear image.

But if you add a second pinhole nearby, you essentially create a second set of these “projected” points, slightly shifted because the second pinhole is in a slightly different position. So, instead of one set of points forming a single image, you now have two very close but not perfectly aligned sets of points. This overlap of two slightly different images is what makes the picture blurry and appear doubled. Each pinhole does its job of forming an image, but having two so close together messes up the clarity of the overall picture.

25)State the effect on the image in a pinhole camera if

1. The hole is made bigger.
2. The luminous object is moved towards the pin hole.
3. The length of the pin hole camera is increased (le. the screen is moved away from the pin hole).

Answer:(a) If the hole is made bigger:

If you make the pinhole larger, instead of getting a sharp, clear image, you’ll get a brighter but blurrier image. This happens because each point on the object will send light rays through a larger opening, creating a small patch of light on the screen instead of a single point. These overlapping patches result in a fuzzy picture.

(b) If the luminous object is moved towards the pinhole :

When you bring the object closer to the pinhole, the image formed on the screen will become bigger. Think of it like casting a shadow – the closer your hand is to the light, the larger its shadow becomes. The brightness of the image might also increase slightly because more light is entering the camera.  

(c) If the length of the pinhole camera is increased (i.e., the screen is moved away from the pinhole):

If you make the pinhole camera longer by moving the screen further away from the pinhole, the image formed on the screen will become larger but also dimmer. The same amount of light is now spread over a larger area, making it appear less bright. The sharpness of the image might also change slightly depending on the exact distance

26) What is a shadow ? Give a reason for its formation.

Answer:

A shadow is a dark area or shape that is formed on a surface when an opaque object blocks the path of light from a light source. It’s essentially a region where light cannot directly reach because something is in the way.  

Reason for its formation:

This means that light travels in straight lines. When an opaque object is placed in the path of these straight rays of light:  

1. Light is blocked: The opaque object does not allow light to pass through it.  
2. Absence of light: The area behind the object, in the direction of the light source, does not receive these light rays.  
3. Dark area appears: This area where light is absent appears dark compared to the surrounding areas that are illuminated by light. This dark area is what we perceive as a shadow.

27)Draw a ray diagram to show the formation of shadow of an opaque object by a point source of light. How is the size of shadow affected if the screen is moved away from the object?

Answer:

Moving the screen farther away from the opaque object makes the shadow bigger.

Think of the object as casting a shadow that spreads out behind it. The closer the screen, the smaller the slice of that spread-out darkness you see. As you move the screen back, you’re catching a wider part of that dark area, making the shadow on the screen larger.

28) State two differences between an umbra and a penumbra.

Answer:

Feature	Umbra	Penumbra
Light Intensity	The darkest part of the shadow, where the light source is completely blocked.	The lighter, outer part of the shadow, where the light source is only partially blocked.
Formation	Formed by the central rays of light that are entirely obstructed by the opaque object.	Formed by the outer rays of light that are only partially obstructed by the opaque object.

29)Draw a ray diagram to show the formation of the umbra alone.

Answer:Here’s a simple way to picture it:

      Light Source (Small)

           *

          / \

         /   \

        /     \

       —–Object—–

      /                \

     /                  \

    /                    \

   ———————- Screen

           |||||||

           |||||||  <– Umbra (Dark Shadow)

           |||||||

In the diagram:

• The * represents a small light source.
• The —–Object—– is the opaque object blocking the light.
• The lines show the path of the outermost light rays that are blocked by the object.
• The area marked ||||||| on the screen is the umbra, where all direct light from the source is blocked, resulting in a completely dark shadow.

So, to see only the umbra, the light source has to be either a point source or smaller than the object.

30)Draw a ray diagram to show the formation of umbra and penumbra both. Label the parts umbra and penumbra in your diagram.

Answer:let’s draw a quick diagram to show how shadows with an umbra and penumbra are formed!

     Light Source (Larger)

          /   \

         /     \

        /_______\

       /_________\

      /___________\

     /_____________\

    /_______________\

   /_________________\

        \       /

         \     /

          \   /

           —– Opaque Object —–

                \     /

                 \   /

                  \ /

                   .

                  / \

                 /   \

                /_____\  Umbra (darkest part)

               /_______\

              /_________\ Penumbra (partially dark)

             /___________\

            /_____________\ Screen

Explanation of the parts:

• Light Source (Larger): This represents a light source that is bigger than the opaque object. For example, the Sun.
• Opaque Object: This is something that blocks the light. 
• Umbra: This is the darkest, central part of the shadow. 
• Penumbra: This is the lighter area surrounding the umbra. So, it’s only partially in shadow.
• Screen: This is the surface where the shadow is cast.

When the light source is larger than the object blocking it, you get both a completely dark central shadow (umbra) and a partially dark surrounding shadow (penumbra).

31)In each of the following diagrams, draw rays to form umbra and penumbra on the screen.

(a)

(b)

Answer:
(a)A’B’—umbra
Umbra alone is obtained on the screen when the opaque object is illuminated by a point source of light.

(b)

EF is Penumbra CD is umbra.

32)State the conditions when only the penumbra of an opaque object is obtained on the screen.

Answer:When a light source is bigger than the object blocking it, and the object is nearby, the shadow won’t have that really dark, central part (umbra). Instead, you’ll only see the softer, lighter outer shadow called the penumbra. It’s all about how the light gets partially blocked, creating that hazy edge instead of a sharp, dark core.

33)Why is it that the birds flying in the sky do not cast their shadow on the earth ?

Answer:1)Size and Distance: Birds are relatively small compared to the vastness of the Earth and the distance they are usually flying at. This makes their shadows very small and faint by the time they reach the ground, often too indistinct for us to notice.
2)Sun’s Size: The Sun is a very large light source. ¹ Because it’s not a pinpoint of light, the shadows it casts have blurry edges (umbra and penumbra). For objects far away, the completely dark part of the shadow (umbra) might not even reach the ground.  

Think of it like holding a small pebble high above a large sheet of paper in bright sunlight. It will cast a shadow, but if you hold it high enough, the shadow will become very small and might be hard to see clearly.

So, while the shadows are there in principle, they are usually too small and faint for our eyes to easily detect from the ground.

34)Why are shadows at noon shorter than in the morning or in the evening ?

Answer:At noon, the Sun is almost directly overhead. This means the sunlight is falling almost straight down on objects. Because the light rays are coming from almost directly above, the shadows cast are short, right underneath the objects.  

Imagine shining a flashlight sideways onto something – the shadow it casts will be much longer than if you shone the flashlight directly from above.

So, the lower angle of the sun in the morning and evening causes longer shadows, while the high angle of the sun at noon results in much shorter shadows

35)What is an eclipse ? Name the two types of eclipses.

Answer: It happens when one celestial body (like a planet or a moon) moves directly between another celestial body and a light source (like the Sun), blocking the light. This casts a shadow on the second celestial body.

There are two main types of eclipses that we commonly experience here on Earth:

1. Solar Eclipse: This happens when the Moon passes directly between the Sun and the Earth.  From our perspective on Earth, the Moon appears to cover all or part of the Sun, blocking its light. 

36)When does a lunar eclipse take place ? Does it occur on every full moon’s night ?

Answer: It doesn’t happen every full moon because the Moon’s path around Earth is a bit tilted, so usually the Earth’s shadow misses the Moon. Only when the Sun, Earth, and Moon line up just right can a lunar eclipse occur.

Question 37.

Draw a diagram to show the formation of lunar eclipse.

Answer:

38)When does a solar eclipse take place ? Does it occur on even’ new moon’s day ?

Answer:A solar eclipse takes place when the Moon passes directly between the Sun and the Earth, and the Moon casts its shadow on the Earth. This alignment of the Sun, Moon, and Earth can be total, partial, or annular, depending on how perfectly they line up and the apparent sizes of the Sun and Moon.  

A solar eclipse does not occur on every new moon’s day. This is because the Moon’s orbit around the Earth is tilted by about 5 degrees relative to the Earth’s orbit around the Sun (the ecliptic plane).

39)Draw a diagram to show the formation of solar eclipse.

Answer:

40)What is an annular solar eclipse ? Draw a labelled diagram to show its formation.

Answer:the Moon being a little farther away than usual during this celestial alignment. Because of this extra distance, our lunar companion appears a touch smaller in our sky.

So, when it glides right in front of the Sun, it doesn’t quite cover the entire solar disk. Instead of a total blackout, a brilliant circle of sunlight persists around the Moon’s dark form. This dazzling border of light is what we call a “ring of fire” – a truly spectacular sight!